Ask our Agony Uncles ...

John Bicknelll (London)

Mark Berthelemy (Bedfordshire)

SUBJECT: Does the build up of moss on my roof cause any damage
FROM: Robert Woolf (Cambridgeshire)
I have a substantial build up of moss in the north facing gulley on a 16th century (but re-done 20 years ago) peg tile roof. In some cases it could be more than 15 cms; other areas have normal build up. Should I remove both types and, if so, what would you suggest as the best method?

Robert Woolf

There are different types of moss and some can cause damage to roof coverings whereas most do not. The main problem is that when moss dies it rolls down the roof and can end in the gutter where it can cause blockage. I therefore recommend that you regularly check and clear your gutters. Although moss does not necessarily cause physical damage to the tiling, if the moss holds moisture against the tiles during frosty weather it can accelerate the freeze-thaw action that can cause deterioration of tiling. A small amount of moss is not generally a problem but a large build up is something that I would suggest you carefully remove.

Mechanical removal is not recommended as this can cause unnecessary damage to the tiling. Some form of chemical removal is perhaps more appropriate. I suggest a garden path cleaner (one that is environmentally sound).

Period Property UK would like to thank Stephen Boniface Associates for answering this question. Stephen Boniface can be contacted on 01279 421 500

SUBJECT: Can our cellar be converted into a damp-free living space
FROM: Terry Hall (Wimbledon, London)
We are currently converting the basement of this 110 year old property red clay & London yellow brick, 7 ft of which is under street level, with approx. 18inch thick walls, we have bad damp problems, so we have hacked all 6 rooms back to bare brick & are thinking of damp injecting & then applying 3 coats of waterproof render. Do you have any advice on this or a better solution?

Terry Hall

The basements of such properties were never intended to be part of the main living accommodation. They were sometimes used for the servants or daily staff and often acted as a sump to dampness in the building. The fact that they were damp was not usually a problem as there was usually good ventilation. Modern requirements have meant significant changes and this includes making sure that the basements are 'dry'. Unfortunately, it is difficult to achieve to provide one hundred percent protection with retrospective damp proofing.

There are two primary methods, one involves applying a waterproof render and the other involves creating a dry lining with a void behind. The problem with the former is that the render eventually breaks down and in my experience will often fail within ten to twenty years of its original application. It is therefore a system that will provide some protection for a limited period of time. It is generally acknowledged that a drained or ventilated cavity system (dry lining) tends to be more effective, but it cannot be guaranteed to deal with all moisture. The main drawback with this latter system is that it involves a greater loss in the floor area as the application to the walls is of greater depth than simple rendering. There are a number of different ways of creating a drained or ventilated cavity and there are some proprietary methods on the market.

Before you embark on this work I suggest you seek further advice from specialists experienced in dealing with this type of work in your area. As you are converting the basement it implies that you wish to create habitable rooms and my personal preference is for a drained and/or ventilated system, but you need to ensure that the system is tailored to the specific requirements of the property. One final word of warning - if moisture from surrounding ground is diverted by any system, it can later appear elsewhere in the property. This is always a drawback with inserting retrospective damp proofing. Any system therefore needs to take account of this possibility.

Period Property UK would like to thank Stephen Boniface Associates for answering this question. Stephen Boniface can be contacted on 01279 421 500

SUBJECT: Do I need listed building consent for numerous small jobs
FROM: Hilary Wymer (Nantwich, Cheshire)
My house is a Grade II listed property which I purchased two years ago. In the middle of the house is the kitchen which is covered with half flat roof half glass. At present this leaks and so I am looking into having it replaced, like for like. Do I need planning consent. Also I have an upstairs window, which is not an original window but replaced approx. 5 years ago which I would like to change to a pair of French doors so that I can walk onto the flat roof above the kitchen. This area is totally surrounded by mine or my neighbours roof so can not be viewed by anybody. Will I need planning consent to install the doors?

Hilary Wymer

Like for like replacement would not usually require planning consent or even listed building consent. Nevertheless, it is always advisable to keep the conservation officer informed of any major work to the building. If you wish to change the nature of the roof either in terms of its material or appearance then listed building consent may be necessary, although planning consent will probably not be required. Regarding the window, its replacement with a like for like window may not need consent but this is something about which you need to speak with the conservation officer. Repairing a window would not normally require consent but complete replacement sometimes does require consent. It is certainly the case that if you wish to change the window to French doors then this will definitely require listed building consent and may even require planning permission. The fact that it cannot be seen by anyone is irrelevant in terms of the legislation regarding an alteration to the character of a listed building. If planning permission is required for the French doors (and remember that planning permission will be separate from listed building consent) then this may take into account whether the property is overlooked or not. It sounds to me as if you should speak with the local authority officers before you undertake any of the work. Note - is the flat roof strong enough for use as a balcony?

Period Property UK would like to thank Stephen Boniface Associates for answering this question. Stephen Boniface can be contacted on 01279 421 500

SUBJECT: Churchyard wall bordering our property leads to damp
FROM: Alli Lucy (Banwell, Somerset)
Our converted coach house has one corner of the house where the ground outside (which runs partially along 2 walls) is around 1 metre against our outside walls. Along one wall, our house backs onto a churchyard and along the other a neighbour's garden. The grey stone walls are approx. 2 ft thick but the on investigating and digging out the mortar on the inside, it is very damp. How should we deal with this? We are unsure if the walls should be tanked and plastered, treated with something and repointed?

Alli Lucy

Although your situation is slightly different I refer you back to the reply given to Mr Hall about his basement in Wimbledon. Similar comments apply. In this instance I suggest that a drained or ventilated cavity system is again appropriate. You might be able to disguise this behind some form of dado panelling if it does not need to be the full height of the wall. Unfortunately, you are unable to lower the ground level externally as the land in question does not belong to you. Your only option is therefore to deal with the matter internally. Apart from the reply I gave to Mr Hall above there have been other similar requests on this site both within the Agony Uncle section and the Discussion Forum. I refer you to these.

Period Property UK would like to thank Stephen Boniface Associates for answering this question. Stephen Boniface can be contacted on 01279 421 500

SUBJECT: Will removal of cement render lead to reduced damp
FROM: James Millerchip (Walsall, West Midlands)
We have recently purchased a property built around 1840 which is fully rendered. However, there are significant problems with damp internally as the render is blowing in places and has moisture behind in others. Examining plans left to us by the previous owner it appears that this render was applied in the mid-1970's. The property is also located in a conservation area and within a more restricted zone.

We need advice on where to go from here! Can the render be removed completely and what are the pitfalls? Alternatively is repair feasible or should we go for complete replacement? Finally, is there a building federation for rendering from whom I could obtain a list of suitable contractors in my area?

James Millerchip

The fact that the render is clearly deteriorating means that work has to be undertaken. I am slightly confused about your comment that the property is located in a conservation area and within a more restricted zone. I assume from this that there is an Article 4 direction removing permitted development rights. You will therefore need to get consent for whatever work you undertake to the exterior.

The fact that you have evidence that the render was applied in the mid 70s should support an application to remove the render. The fact that you have problems and the render is deteriorating should be sufficient justification for removing the render. The real problem is what to do once the render is removed. Until it is removed it is difficult to know what condition the actual wall structure might be in and what you could or could not undertake thereafter. Photographs of the property prior to the application of the render might provide some guidance on what will be found under the render. If the render is blown you may be fortunate in finding that the original wall structure is still reasonably sound. However, if you have to physically remove the render it could cause damage to the surfaces and at that point you will need to consider how to deal with it. It is often the case that re-rendering is necessary in which instance it is best to use a lime based render on an older property. I am not aware of a building federation for rendering and the best I can do is suggest that you find contractors experienced in dealing with historic buildings. I would hope you should be able to find someone in the West Midlands from the Suppliers section of this website. An alternative would be to ask the local conservation officer who should know of builders experienced in dealing with historic buildings in the district.

Period Property UK would like to thank Stephen Boniface Associates for answering this question. Stephen Boniface can be contacted on 01279 421 500

SUBJECT: Insulation advice for clay lump barn
FROM: Paddy Burns (Deopham, Norfolk)
I am close to purchasing a clay lump barn with planning for conversion. It has a huge brick and flint plinth and the walls look sound, though some cement render may need to be removed and replaced with lime render. However, if I am to achieve the required U-values would it be better to insulate the walls between batons, either internally and/or externally and cover with plasterboard inside and weatherboard outside rather than re render in breathable lime render once the cement render is removed.

Paddy Burns

The most important thing is to carefully remove the inappropriate cement render so that the building can function properly. However clay lump does require some protective layer whether this be lime render or some other form of cladding. As the building appears to have been covered with cement render in the past I suggest that you take great care in removing it as the clay lump may have deteriorated behind. There are several methods of dealing with upgrading walls. I would prefer to see an insulation method on the outer face of the building just behind the cladding but of course you could install it internally. Whatever method you use must retain breathability and to this end I suggest that the insulation material itself should be a natural product such as lambs wool. However, my advice is that you have detailed discussions with the local conservation officer about the end appearance of the building and then decisions can made on how to achieve this and upgrade the building at the same time. A suitable supply for materials would be the Old House Store (see the suppliers section on this site) and will probably provide advice as well. You do not say whether you have an architect involved but your architect should be able to provide further guidance on some of these matters and should lead discussions with the various officers involved. Where building control is involved (I note that this is a conversion therefore building control will be involved) there will often be a trade off between the requirements of building control and the requirements of building conservation. However, it is essential that you discuss the matter with all appropriate officers. Since writing the above I have spoken with Conservation Officers from Norfolk who tell me that they have experience in such matters and many building control officers in the region take a relaxed view. You should therefore find discussion with the officers useful.

Period Property UK would like to thank Stephen Boniface Associates for answering this question. Stephen Boniface can be contacted on 01279 421 500

SUBJECT: Crumbling brick walls of property give cause for concern
FROM: Rob Tuplin (Scunthorpe, Lincolnshire)
I am in the process of putting an offer in on a detached house. One of the problems is the bricks are crumbling on the outside walls in various places and exuding a salty substance. It has also been highlighted that the property has no dpc and this seems to be a problem. Can you recommend a relevant survey and hazard a guess on what to do with the bricks and how long they might last?

Rob Tuplin

Older houses generally were built without damp proof courses and have survived remarkably well without any such protection. The problems of dampness usually arise from works subsequently undertaken or other changes. The problem you describe sounds fairly typical of a damp problem the salty substance being the crystallised salts left after moisture has evaporated away. There are a number of previous answers on the Agony Uncle section of this site and there are many discussions about damp issues on the discussion forum of this site that I would refer you to for further general information. In this instance however I suggest you obtain advice from a surveyor or architect specialising in repairing historic buildings. I would not recommend you to take advice from a high street timber and damp contractor, as they generally recommend treatment regardless and I do not know of any particular discerning specialist contractors in your particular area. It will be important to identify the likely cause for the moisture and to remedy the problem. This might involve lowering ground levels, improving ventilation, removing inappropriate materials and reverting to traditional materials, etc. As to how long the bricks might last, this will largely depend on how much damage exists at present. If the matter is dealt with soon, it is unlikely that there will be any serious problem. This case highlights the fact that if a property is neglected or inappropriate works are undertaken it can accelerate the deterioration of materials and the structure generally. If appropriate repairs etc are undertaken it will reduce the rate of deterioration. I would regard specialist timber or damp treatment as a last resort and from what you say you are not at that point.

Period Property UK would like to thank Stephen Boniface Associates for answering this question. Stephen Boniface can be contacted on 01279 421 500

SUBJECT: Crushed stone dust & lime provide suitable filler for stone house
FROM: Chris Robinson (Calne, Wiltshire)
I am renovating a Georgian Stone Property and would like to know how to create a mortar style filler for use between door/window frames and stone walls that will match the stone colour.

Chris Robinson

You should ensure that the mortar you use is a lime mortar. The aggregate you use mixed with the lime will determine the colour and nature of the mortar itself. It would be sensible to have a basic analysis of the original mortar undertaken to see if that can be matched. You might find that stone-dust or other aggregates were included in the original mortar mix and it would be sensible to repeat the exercise with similar materials. Although sand is the most common aggregate for mortar mixes there are many different types of sand and other materials were often added to mortar to create different textures, colours, or performance requirements etc. Several of the suppliers of lime for building mortars would be prepared to provide further advice on this matter. In this instance I am thinking of the Old House Store (based in Oxfordshire), but there are others that you will find through the suppliers section of this website.

Period Property UK would like to thank Stephen Boniface Associates for answering this question. Stephen Boniface can be contacted on 01279 421 500

SUBJECT: Should I bed my flagstones on a dpc or breathing floor?
FROM: Jamie Collier (Barnsley, Yorkshire)
I have recently bought a cottage that has a York stone floor under a layer of screed. I am hoping to expose the floor and am therefor keen to know the best way of insulate it against damp and cold.

Jamie Collier

If the York stone floor is already under a layer of a concrete screed there is a risk that the stone itself will not be salvageable. You will need to carefully expose and lift it. Without knowing your precise situation I cannot say that I would necessarily agree with trying to damp proof the floor. As a general rule of thumb, with older buildings it is better to allow them to breathe and with floors this may mean leaving out the damp proof membrane. In most other respects the floor can be constructed much the same as for a modern concrete floor but with the membrane omitted and with a lime/sand base on which to lay the stone flags. Of course, if you are in a low lying area where the water table is high or there is a risk of flooding you might have to reconsider this advice because simply allowing the floor to breathe might be inadequate if the floor is in a very wet situation normally.

The matter of insulating the floor is more difficult. If you are simply renovating the property you will not be required under building regulations to insulate the floor and therefore whether you insulate it is more a matter of personal choice. My preference would be to omit insulation. However, you might wish to consider under floor heating. This may resolve the matter of having a cold floor but whether the heating system would require a damp proof membrane will largely depend upon the manufacturers recommendations. You raise issues that are not straightforward to answer. You really need to obtain professional advise from someone experienced in dealing with old buildings but also someone who understands the need to upgrade old buildings in a sympathetic manner.

Period Property UK would like to thank Stephen Boniface Associates for answering this question. Stephen Boniface can be contacted on 01279 421 500

SUBJECT: Does rendered facade on timber framed property hide a hornets nest of problems
FROM: Peter Hall (Cheshire)
We are in the process of purchasing a period property thought to date back to mid-17th century. The survey report suggests evidence of a timber-framed building beneath a rendered facade. There are plenty of exposed beams within the property to substantiate this conclusion. My question is therefore what would be involved in removing this front covering and would I be potentially "unearthing" a major, expensive project.

Peter Hall

If the building is listed the removal of the façade would require listed building consent. There are many issues that this raises and it is unlikely that such consent would be obtained if the existing façade is of merit in its own right.

Many Georgian buildings do comprise a timber frame structure with a façade of a different material (brick, stone, cladding of some description etc). It is likely that the timber frame was never intended to be exposed and it would be inappropriate to expose it now. The only reason I would recommend exposing it is if there was a known or thought to be a serious problem with the frame itself that required exposing it for repair purposes. My general advice is that as work is gradually undertaken to a building and hidden parts of it are exposed assessments can be made along the way and further decisions taken as and when necessary. Targeted opening up of a structure might be appropriate if there is a suspected problem. Otherwise limited exposure provides only limited information. It may be that the part you expose is in good condition whereas the neighbouring section still hidden is rotting. The alternative can also apply. In any event, great care should be taken as you could potentially unearth a major expensive project unwittingly and perhaps unnecessarily. Close liaison with the conservation officer may be of benefit particularly as the officer may have a better understanding of the buildings and what you might find behind the façade. The officer may also be able to put you in touch with sympathetic builders who may be able to assist you in carefully and gradually opening up and repairing the property under a managed phased programme of works.

Period Property UK would like to thank Stephen Boniface Associates for answering this question. Stephen Boniface can be contacted on 01279 421 500

SUBJECT: Painted brick elevation hides badly eroded bricks
FROM: Natasha Doman (York)
I have recently purchased a two-storey terraced house in the Bedford Park area of Chiswick, London, which is designed in the style of May and Shaw as common to the area. However, the brickwork to the front elevation has significantly eroded (and has been painted by the current owner). My current thought is that the whole front elevation will need replacing to restore it to original condition. Can you give me a better understanding as to what would be involved in this process, or suggest any alternative?

Natasha Doman

If the building is listed you will need listed building consent for the work. The first stage would have to be careful removal of the paint to expose the actual condition of the brickwork beneath. Your question raises an issue that is quite common with buildings constructed of soft red brickwork and other material that has eroded behind a modern paint or other finish. It is of course impossible to restore the original finish and sometimes the deterioration is such that to leave the brickwork exposed mars the overall appearance of the building. However, to leave the existing paint finish could accelerate the rate of deterioration and is therefore also unacceptable. You could undertake some trial areas of removing the paint system using different methods. If these prove successful the paint can be removed from the elevation. Only then can you properly assess the brickwork. You might find that the amount of brickwork deteriorated is less than you thought and individual brick replacement might be appropriate. However, if the extent of deterioration is such that a significant part of the elevation needs to be replaced you need to consider very carefully whether this is appropriate. The cutting out and replacing of brickwork is a major operation. An alternative might be to install brick slips but this involves gluing thin pieces of brick to the existing structure. I am not convinced that it is appropriate for this type of property and is rarely appropriate for a historic building. A decision you might therefore have to face is whether the brickwork should be completely rendered over using a lime render. Although using sympathetic materials it will fundamentally change the nature of the appearance. My suggestion is that you obtain advise from specialist contractor regarding removal of the paint and have trial areas undertaken in the first instance. Once the brickwork has been exposed you should obtain advice from specialist contractors experienced in dealing with brick repointing and repair. Another alternative would be to expose the bricks, undertake limited repair and then to coat them with lime-wash. This would have a similar effect to the paint presently protecting the brickwork, but because lime-wash is breathable it would not cause acceleration in the deterioration of the bricks behind.

Period Property UK would like to thank Stephen Boniface Associates for answering this question. Stephen Boniface can be contacted on 01279 421 500

SUBJECT: Screw or glue new floor
FROM: Jonathan Baldwin (Rossett, Clwyd)
My friends and I have purchased a grade II listed building, about 400 years old. We intend to convert it to a wine bar. Another friend, who is an engineer, has given us a calculation for ensuring that the floor is structurally suitable. His solution involves gluing and screwing firrings to the original joists (there is an existing chipboard floor which we intend to remove). I don't want to glue, but my friend is insistent, can you advise? My view is that permanent gluing of new timber to old oak will be detrimental and that screwing will suffice.

Jonathan Baldwin

The works you mention are works that would require listed building consent. I am not convinced that gluing timbers in place would be appropriate. I think it would be sensible to get a second opinion from an engineer experienced in dealing with historic buildings. In my opinion if undertaken carefully and with long enough and frequently spaced screw, simply screwing the timbers in place should be sufficient. This does of course depend on precise circumstances. You should consult with the conservation officer before embarking on any such work.

Period Property UK would like to thank Stephen Boniface Associates for answering this question. Stephen Boniface can be contacted on 01279 421 500

SUBJECT: Should I clean by dis-coloured bricks & how?
FROM: Adam Haylett (Blackheath, London)
I would like to clean the exterior brick work of my Victorian House prior to repointing which over the years has become discoloured by pollution and leaching black dye from previous pointing. Presumably it rained shortly after the work was done. The side of the house is bricked in red rubbers and the rear of the house is London yellow stocks. My concern is that because red rubbers appear to be much softer than conventional bricks, they may need special care in cleaning. It also appears that in past, small areas of the brickwork have been cleaned with chemicals and an abrasive, possibly a sander. Can I use a steam cleaner, jest washer or chemicals?

Adam Haylett

As mentioned elsewhere when replying to questions about cleaning, there are numerous methods of cleaning surfaces. It is always best to use a couple of methods in a trial area that is not highly exposed. You can then work out what might be the best method for your particular situation. It is certainly the case that with soft bricks (typically red bricks) abrasive or mechanical means are less appropriate than chemical means whereas the yellow stocks are perhaps more robust and could withstand low pressure cleaning. This is something upon which you need specialist advice from a contractor experienced in cleaning buildings who can undertake a couple of trial areas. If you have your own steam cleaner etc you could carefully try this on areas that are concealed (in case it does not work). The one point about cleaning is that it can remove the patina of age and destroy some of the character that the colour and appearance gives a building. Further, in my experience a cleaned building does not remain clean for very long and quickly reverts to a dirty appearance (particularly in London). It is of course arguable that some of the accumulation of dirt etc provides a protective layer to the surface of the structure. I therefore suggest that you approach cleaning with great care. If the building is listed you will need to seek advice from the conservation officer.

Period Property UK would like to thank Stephen Boniface Associates for answering this question. Stephen Boniface can be contacted on 01279 421 500

SUBJECT: Are thatched roofs of greater risk of infestation by rodents & other so-called pests?
FROM: John Bicknell (London),

Do thatched roofs carry a greater risk of infestation or mice, etc, than other roofs? If so what measures could reduce this possibility? Are such problems reduced with newer thatched roofs?

John Bicknell

As you are based in London I am intrigued to note your query regarding a thatched roof but assume you may be thinking of purchasing a thatched property outside London. Thatch is something that provides a very good nesting material for vermin. For this reason it can indeed attract vermin such as mice, rats, squirrels etc. Birds will often also be found to pull away individual straws to take for nesting materials themselves. These are problems that apply to thatched buildings and thatchers are of course well aware of this. Reed thatched roofs that are relatively new provide a harder substance that is not so frequently attacked by vermin etc.

However, as the reed deteriorates with age it becomes more vulnerable to attack. You will therefore find that new reed roofs do not usually netting to them, but as they age netting is sometimes applied to reduce the risk of attack by vermin. Straw roofs are more vulnerable to attack from the outset and generally have a netting from new. Straw thatched roofs are vulnerable whether they are new or old. Reed thatched roofs are less vulnerable when new. The other problem is that sometimes vermin can be trapped within the roof when re-thatching takes place. Most thatchers will of course check and ensure that vermin are removed before re-thatching takes place. A thatcher may recommend that environmental health officers inspect and traps or poison can be laid to deal with vermin before any re-thatching takes place. Nevertheless, there have been examples of new thatched roofs trapping vermin within them and causing a problem. Unfortunately, it is not possible to guarantee that a thatched roof will be free of vermin. I recommend that before any work is undertaken the roof is thoroughly checked and measures taken to deal with any infestation that exists. When the work takes place care should be taken to reduce the risk of vermin entering the roof during the work. Upon completion a reed roof does not necessarily require netting but straw roofs will need netting straight away. Of course, I should return to your original question as to whether thatched roofs carry a greater risk and this implies that other roofs perhaps are not so vulnerable. I am not convinced that this is necessarily the case. Many houses in London suffer from mice infestation, pigeons in the roof space, etc. I have come across quite a few properties in London with cockroach infestation as well. I personally live in East London and local to me there are bats roosting somewhere in a nearby property. It would therefore be inappropriate to suggest that some of these infestations are limited to thatched or country buildings.

SUBJECT: Chimney problems lead to dampness
FROM: Mark Berthelemy (Kempston, Bedfordshire),

We live in a 1906 semi. The chimney that led up from the kitchen appears to be letting in water to the back bedroom - where the chimneybreast meets the ceiling. The wallpaper around this area has peeled off, and the walls feel slightly damp and the room smells of wet plaster when it rains. The base of the chimney is open at kitchen ceiling level. We can see a corrugated ducting tube starting from here. The top of the chimney has been capped with a cowl. We have had the rear wall of the house and the chimney repointed and sealed (with Screwfix and Wickes brick sealants). The flashing around the chimney has also been repaired. The plaster on the back wall of the bedroom had deteriorated and this has been replaced (over 6 months ago).There are white salts on the chimney brickwork in the small attic space above the back bedroom, but there is no damp appearing on this brickwork or anywhere in this space.

Mark Berthelemy

Damp ingress around chimneys is a common problem and sometimes quite difficult to resolve. I can only speak generally but the matters you will need to consider are as follows. You should check and ensure that pointing is in sound condition and in particularly you should check that the flaunchings around the chimney pots, cowlings etc are well maintained so that water cannot penetrate straight down into the chimney. If there are any cracks at high level water may penetrate down between the liner (the "corrugated ducting" you mention) and the original chimney flue. This may then leach into the brickwork and penetrate through at lower level. If the chimney is completely capped off there could be a problem of condensation within the chimney void and this might explain some of the dampness. However, a more likely cause of your problem is a present or past leak at the junction of the chimney with the roof. Although you say that flashings have been checked I suggest that these are the most likely cause of a problem in the area you describe. It is not only the flashings to the sides, the aprons of the lower face but also the back gutter to the upper face that will need to be checked. If these are found to be sound you will need to check the tiling around to ensure that water is not penetrating higher up the roof and running down to meet the chimney and then run down into the property.

If it is found that all external sources of moisture have been dealt with then it could simply be that the problem you are experiencing is one of residual damage. It could be that the plaster internally is simply "contaminated" and needs to be hacked off and renewed. It could be that a gypsum plaster has been used in the past and this has absorbed moisture. I would not recommend the use of gypsum plasters in this situation. It might be more sensible to consider a sand/ cement backing coat with a renovating plaster finish. The white salts you mention are the crystallised deposits of salts from where moisture has evaporated away. I note that you have attempted some repairs. My slight concern is that the application of modern materials etc might have served to trap residual moisture within the brickwork, which is now finding its way through. If this is the case hacking off the plaster and allowing the brickwork to dry out before re-plastering might resolve the problem.