Damp - Plaster & Re-plastering

Do I have to wait to replaster?

Q I am having a DPC done at the moment and I need to know how long after it has been done can I re-plaster the walls? Is there a specific amount of time I have to leave after it has been completed or can I plaster straight away?


A You may re-plaster the walls being treated straight away should you wish. The specification of the new render to be applied is very important and should be suitable for this type of situation. If you are thinking of waiting be aware that it may take 4 to 6 weeks for the injected DPC to cure (start to work) and then many months, even years for the wall to dry down but it will never fully dry due to salt contamination. So go ahead and re-plaster but only paint with a trade emulsion and regard initial decoration as temporary for the first twelve months. What do the people who are doing the DPC say? Please let us know. You should comply with their requirements in order not to invalidate any guarantee.


 

Salts and more salts! Help!

Q We just bought a 100 year old house, and had rising damp throughout the rear and side of the property. After obtaining numerous quotes we employed a PCA backed company to carry out the remedial works recommended. And paid extra to get the 30 year insurance backed guarantee. The walls were treated in November, which included the removal of existing plaster, injection of chemical DPC and re plastering etc. After a couple of weeks there was considerable presence of salts which came back and after a stressful haggle with our contractor, they agreed to replaster the dining room walls once again! The contractor admitted to the fact that they used incorrect materials in the first instance, and we allowed them to come back and rectify the fault. Although we had insisted that salts were apparent in other treated rooms, the contractor put this down to a combination of condensation and salts trapped by decorations (water based emulsion on the replastered walls) and not to the use of incorrect materials in this room! The contractor did not rectify this room on the second visit - just the works to the dining room.
These works were carried out only last week! Some 4 days after this work the salts have appeared again on this replastered wall. What can we do now? We totally feel that this contractor is clearly not very capable of doing the tasks we have already paid him for?
We simply cannot have this firm coming back into the house for a 3rd time? What are our rights? What can we do next? And what is going wrong here?! Thank you for your help, because indeed we need it.


A We are curious about what the contractor applied onto the wall originally which he later said was wrong? Do you have any more details? The first coat for the re-plastering following a remedial DPC should be 12 mm thick and comprise of 3 parts sharp washed sand to 1 part cement incorporating a waterproof additive in the gauging water. Second coat is the same but no additive and finally a skim finish. We do not understand how this can go wrong?

The salts that you are seeing are almost certainly sulphates which are present in all building materials, especially the skim coat plaster which is virtually neat calcium sulphate. As water used in the render and plaster evaporates off it causes the sulphates, previously held in solution, to crystallise on the surface forming a white sometimes fluffy deposit that is easily brushed off. Trapped under a paint film they can become slightly
hard. The formation of these salts merely indicates that evaporation is taking place. It is important that you do not confuse the sight of the sulphates with hygroscopic chloride and nitrate salts which are seldom seen as they are usually in a deliquescent (liquid) state The formation of sulphates is very common and virtually unavoidable so do not be alarmed unnecessarily. You state that you have painted the walls already. Any redecoration in the first 12 -18 months following treatment should be regarded as temporary and should be what is termed a trade emulsion i.e. one that does not contain vinyl.


To conclude is sounds like you may be concerned without reason since the formation of sulphates is unavoidable and normal. It can take over a year for a normal wall to dry down but this process will be very slow indeed during the cooler winter months. Basically you need to give it time but do not attempt to force dry the walls under any circumstances.

 

Plasterer wants to use dot and dab

Q We have a house that was built around 1880. The house exterior walls were damp, so we had a chemical injection to stop the damp.  We were advised to by the company who put in the chemical DPC to use 3 parts sand to cement followed by a skim coat.  We wanted to employ our own plasterer as other plastering work needed to be carried out. The plasterer who we have started to use is insisting that he uses dot and dab with plaster board, or a PVC membrane followed by king span and then foil backed plaster board followed by a skim.  My gut feeling is to do the 3-1 traditional method.


A It is our opinion that you should only use 3:1 sand cement with the appropriate additive as a first coat. Never ever use dot and dab plasterboard on a wall that has been affected by rising damp!! BS 6576 and the BWPDA (PCA) Code of Practice for remedial damp proof course systems both state that dot and dab is not appropriate. Your plasterer may be a good plasterer and get walls nice and smooth but understandably he may not be up to speed on technical issues concerned with rising damp and the reason why set specifications are necessary. Ideally the contractor who did the injection should also do the replastering in order to avoid 'split responsibility' should anything go wrong in the future - each blaming the other. Insist on 3:1.

 

Patches of salt on treated wall - again!

Q We had a damp proof course carried out on our cottage in April this year and were then unfortunately victim to the July floods.  The house was completely stripped out and dried with dehumidifiers for a month.  We got the damp proof people out to do another course of injections and the labourers said they would use waterproof render.  All work is now finished and having just moved back in we have discovered patches of salt on the same wall that the damp problem first presented itself. I have asked the damp proof company about it they have said that it is part of the drying out process in that it can take months for the plaster to dry properly and the wall is still contaminated with salts so it's coming through the plaster.  We have been advised to rub it off daily and use a dehumidifier in the room. Is this sound advice or does the plaster need redoing for a third time this year and is there anyway of stopping this keep happening or will it be a permanent problem?
What a brilliant website by the way thank you in anticipation of your help.


A The patches of salt that you are seeing are almost certainly sulphates which are present in most building materials especially finish plaster which is virtually neat calcium sulphate. It is important to realise that these salts do not come from the ground. Their continued formation on the surface indicates that there is a source of moisture but this should not be happening beyond three/four weeks after a new waterproof render has been applied. If it is then the underlying render is not waterproof as it is letting residual water in the wall to come through to the surface which it should be preventing. It could also allow the transmission of hygroscopic salts in the wall as a consequence of the previous rising damp through to the new plaster surface and in so doing will contaminate it. Who did the replastering and what specification was used? Is it happening all over or in circular patches?

 

Salts on newly plastered wall?

Q Newly plastered chimney breast, in a partial basement (1 metre from floor approx) was recently painted and within 3 months has flaky, powdery patches in the paint with white crystals underneath. Are these patches caused by hygroscopic salts or damp penetration?


A The salts you describe are almost certainly sulphates (non hygroscopic) which are present in most building materials. Their formation on the surface is indicative of moisture evaporating off which may be due to the recent plastering but if it continues then there is another moisture source. If you used a paint that contains vinyl what you describe is more likely to happen if a moisture source is present.

 

How soon can I decorate after skimming walls?

Q My question seems a bit simple compared to others but here goes...I am having some walls skimmed in my house and wanted to know how soon it is possible to paint and tile them? Any advice would be much appreciated. Thanks.

A As long as the walls are just being skimmed (no thick under coat plaster is being applied) you should be able to redecorate once the walls are visibly dry plus one week.


 

Is it OK to speed up the drying of plaster?

Q Is it OK to speed up the drying process of new plaster skim with aid of large fan?
A The hardening of plaster is a setting process unlike the curing of a cement based mix which is a chemical reaction. Both of these materials require moisture to complete their curing/setting process and accelerated evaporation of water from the mixes is likely to have a detrimental effect. You should not therefore accelerate the drying of any new building materials as this could upset the cure or setting process.

 

Needs a new DPC but walls have already been plastered

Q In a recent home buyer's survey, it is indicated that the property has rising damp and is in need of injected DPC and re-plaster. I understand the need for the re-plaster due to the salts absorbed from the dampness. However, the internal walls have all been recently been re-plastered less than 6 months ago. Is the re-plaster still necessary? - I assume it is, but just to check.
A Have the walls been re-plastered back to the brick or have they just been skim coated? It would also depend what material the walls have been re-plastered with and the extent of any rising damp that might be present. What was the reason for re-plastering the walls six months ago? Is it a property development deal? Have you had a good DPC company check it out yet? If they did not re-plaster for reasons to do with a new DPC the chances are you will have to re-plaster again as an incorrect specification is likely to have been used and thus would not contain any hygroscopic salt contamination likely to be present in the wall.




 

Is dot and dab finish OK

Q I bought a house recently which has a 30 year damp course guarantee.  It still has about 27 years to run. However, when carrying out some building work, I have discovered that the damp coursed walls have in fact been dot and dabbed, and then plastered.  Will this cause a problem for me later on, and with the guarantee?

A Dot and Dab is certainly not an advisable procedure to use on walls that are known to have suffered from rising damp.

Moisture is always likely to be present at low level and what might happen is that moisture is absorbed by the porous plaster dabs which then transmit moisture through the plasterboard to eventually show as damp 'circles' on the decorative finish. This could cause you problems with your guarantee but whose guarantee? If the contractor who put the DPC in also did the dot and dab then he would be liable. He might also be liable if he did not leave adequate instructions for others with regard to the importance of correct re-plastering and what the consequences would be if these instructions were not followed. If you are talking about a third party guarantee then you are in trouble as dot and dab is not a recommended procedure. See BS 6576 and BRE Digest 245.


 

Do I have to wait to re-plaster?

Q I am having a DPC done at the moment and I need to know how long after it has been done can I re-plaster the walls? Is there a specific amount of time I have to leave after it has been completed or can I plaster straight away?


A You may re-plaster the walls being treated straight away should you wish. The specification of the new render to be applied is very important and should be suitable for this type of situation. If you are thinking of waiting be aware that it may take 4 to 6 weeks for the injected DPC to cure (start to work) and then many months, even years for the wall to dry down but it will never fully dry due to salt contamination. So go ahead and re-plaster but only paint with a trade emulsion and regard initial decoration as temporary for
the first twelve months. What do the people who are doing the DPC say? Please let us know. You should comply with their requirements in order not to invalidate any guarantee.

 

Should I delay re-plastering?

Q I am will be having DPC carried out on a new home I am purchasing. The company quoting for the job is well established and members of the BWPDA.  I do believe the quote is overpriced. They say they must re-plaster immediately with "THEIR" specified plastering system immediately after DPC.  However another Company quotes from the Dept of Environment leaflet 58 recommending that replastering be delayed as long as possible to allow soluble salts to move from the brickwork. If I can wait that would be preferable. What do you think?

A We are very familiar with the thinking and tactics behind telling the client to delay re-plastering.


It takes many years for the concentration of hygroscopic salts deposited in wall plaster as a result of rising damp to build up to a level that they cause a detrimental problem to wall finishes. This is based upon a continious supply of ground water over many years. Therefore the amount of 'additional' salt migration into the plaster that will occur following the injection of a remedial DPC will be small and for this to happen will take a considerable time. Remember of course the supply of water has now been curtailed by the new DPC.

The leaflet that you mention is not familiar to us and may be an old BRE leaflet. A correctly formulated sand and cement render incorporating a waterproof additive in the gauging water is a chemical cure and more than capable of preventing the migration of salt and residual moisture to the new decorative surface. In our opinion it is not necessary to delay re-plastering. We are aware that some companies use the delay tactic as a ploy to 'cherry pick' the profitable element of the work i.e. the drilling and injection. They will then come back twelve months later and quote a huge price to re-plaster so be careful.

There is no reason to delay - so have it all carried out at once which will enable you to live normally more quickly rather than wait a year in limbo for no real gain. Make certain the same company carries out the DPC and replastering to avoid split responsibility and you should get about three quotes.

Good luck and let us know how you get on.


 

Do I have to wait before re-plastering?

Q I am having a DPC done at the moment and I need to know how long after it has been done can I re-plaster the walls? Is there a specific amount of time I have to leave after it has been completed or can I plaster straight away? 


A You may re-plaster the walls being treated straight away should you wish. The specification of the new render to be applied is very important and should be suitable for this type of situation. If you are thinking of waiting be aware that it may take 4 to 6 weeks for the injected DPC to cure (start to work) and then many months, even years for the wall to dry down but it will never fully dry due to salt contamination. So go ahead and re-plaster but only paint with a trade emulsion and regard initial decoration as temporary for the first twelve months. What do the people who are doing the DPC say? Please let us know. You should comply with their requirements in order not to invalidate any guarantee.

 

Damp being treated again under guarantee - when should we re-plaster, decorate etc?

Q We're in the process of buying a Victorian terrace. Damp on the internal wall was highlighted by the survey despite a 30 year damp proof course guarantee being in place. After a number of months the damp proof company have finally agreed to go in and carry out the necessary work 8 days before our completion date. The whole room, lounge through dining room needs re-plastering anyway and that will be our first job. How soon can we re-plaster after the damp proof work has been carried out, and following that how soon can we paint once the plaster has dried? Are there any specific products we need to use for the re-plastering?

A There is most certainly a correct specification that you should use for re-plastering! Provided you adhere to the correct re-plastering specification there is no reason why you should not carryout this work immediately. I am concerned however, that the damp proof company have not offered you any advice (perhaps they have) with regard to re-plastering. Did they not re-plaster when the original work was undertaken? Have they now offered any further advice with regard to the re-plastering? What was the content of their report following their re-inspection and agreeing to go back and re-inject? You should follow what they advise in order not to invalidate your guarantee.

We would advise that you re-plaster using a first coat of three parts sharp washed sand (needs to be the correct zone type) to one part cement incorporating a waterproof additive in the gauging water. A second floating coat should be 3:1 sand:cement with no additive and then a skim finish. Do not apply vinyl wall coverings or vinyl 'type' emulsion paints.

We hope this information is of use, but you really should go back to the contractor and get him to put the answers to all of your queries in writing in order to avoid any potential guarantee problems in the future.



 

Walls have been dot and dabbed - Is this a problem?

Q I have recently bought a house, and was provided with a damp course guarantee for 30 years (with about 26 years left on it). The walls though have not been rendered and plastered, they have been dot and dabbed and plastered over. Only just realised when I hacked off a wall to open up a closed fireplace. Is this going to cause me problems, with damp etc, or even if I sell on. A friend has told me this should have been picked up by surveyor??


A Dot and Dab plaster board finish is not recommended for use on walls that have suffered from rising damp. To the best of our knowledge it does not form part of the re-plastering specification stipulated by any chemical DPC product manufacturers. It could cause damp 'circles' to appear on the decorative surface at low level in time. We suggest that you read the report and specification of work that should have been produced by the company that did the damp course work four years ago and if necessary seek their opinion. If they did the dot and dab then BS 6576 advises against the use of such a procedure on walls affected by rising damp.

 

Sounds like dot and dab - again!

Q We have recently redecorated our lounge room and noticed that some of the (acrylic) paint was not drying. It is a Victorian terrace and the wall in question is load bearing, double leaf and is part external and part internal (due to a kitchen extension to the rear.
On closer inspection the plaster was quite soft and the skirting board was removed which showed evidence of rot in the past (How old? - we are unsure). The damp spots are clearly evident on the part of the wall that is internal and then completely absent on the external section.To add more confusion, the other side of the wall in the now kitchen is dry with no evidence of marking at all. The damp has only penetrated the Plasterboard where it was affixed with bonding cement. I have removed the two damaged pieces of plasterboard (up to approx 1m) and would appreciate any advice on how to continue.


A We think we understand what you are describing. Are you saying that the lounge side of the internal section of the main rear elevation wall, which separates the lounge from the (back addition) kitchen is showing signs of damp? Are you also saying that this section of wall has been plasterboard lined - using a dot and dab technique and the damp patches are only showing where the plaster dabs are?

Assuming the answers to the above are yes then the length of wall in question is usually between 3 to 4 metres long. Whatever is happening 'dampwise' in one part of such a wall is almost certainly happening along its entire length. The fact that it is not showing beneath the window on the external section or on the kitchen side of the internal section probably means that some other type of damp protection may be present on the surface
of these sections of wall - may be they are rendered?

Plasterboard dot and dab is not an advisable wall finish system especially is there is, or has been a damp issue as it will eventually show through where the dabs are. The long term solution is to apply two coats of sand and cement render at a ratio of 3:1 incorporating a waterproof additive in the gauging water mix. This should be applied to a height of 4-500 mm past the height of any dampness present. It may be skim finished. Fix back skirting boards with physical protection between them and the wall such as plastic DPC.
Hope this is of use but if you have any further queries please ask again.

 

Do I have to re-plaster?

Q I have a problem with damp in my newly bought house. I have had many quotes for sorting out this problem of various prices. I received one quote which informed me that my plaster did not need replacing just injecting. My question is "is it always necessary to re-plaster the internal wall after a damp proof course has been done?"


A If rising damp has been correctly diagnosed in a wall then in order to achieve the desired long term dry decorative surface it is essential, we repeat essential to have the correct re-plastering carried out preferably by the same company that is going to inject the DPC in order to avoid 'split responsibility' in the future. If someone is proposing to inject only they are either cherry picking or just do not know what they are doing. It is probable that they are charging a hefty sum just to inject - would this be true? Be careful!


 

Is it essential to re-plaster?

Q Is it always absolutely necessary to re-plaster if the original plaster is in good condition (following chemical injection). I do understand that hygroscopic salts can contaminate the plaster but does that always occur? Are there any other tasks you can consider, i.e. using a dehumidifier for a period before deciding whether or not to re-plaster?


A Provided rising damp has been correctly diagnosed a remedial chemical damp course system is a two part process. One part is the injection of the chemical damp course system and the second part is correct re-plastering of walls affected by rising damp. There is no way around this. If a wall has been diagnosed with rising damp then the wall plaster will have ground derived salts (chlorides and nitrates) present regardless of how sound and firm the wall plaster is. No amount of dehumidification, fans, heat etc will remove or dilute salt contamination or diminish their hygroscopic ability. Bite the bullet and renew the plaster to the correct specification otherwise problems will occur in the future especially when you try to sell.

 

Salts forming on new plaster?

Q I have recently had a damp course done on my 200 year old cottage, and as instructed taken everything off back to the stone 1 metre high. This has been replaced by sand and cement then skimmed over the top. The problem I am having is what seems to be patches of salt coming through on the sides of the reveals which cannot be painted over as it just bubbles the paint and flakes it off then just rebuilds the salt layer. The new walls have been up one year now and the problem seems to have got worse with the cold weather. How can I cure this problem?

A The salts that you see forming are almost certainly sulphates. However the fact that they continue to form are an indication that moisture is evaporating from the point where they form. A common reason for this is a 'weak spot' in the underlying render as it is failing to contain residual moisture still in the wall. It could be 1) a weak mix 2) near to an angle and the angle beads have been fixed with carlite bonding before the render was applied or 3) an underlying slightly proud stone and insufficient render covering it. Whatever the cause the cure will be to remove the affected plaster back to the stone and re-render correctly using the correct sand/cement ratios. There is nothing that you would be able to apply onto the surface as the salts would push it off. Who did the plastering after the DPC? Have you had the damp proofing company back for their opinion? - If not you should.



 

How long should I wait before re-plastering?

Q I am having a DPC done at the moment and I need to know how long after it has been done can I re-plaster the walls? Is there a specific amount of time I have to leave after it has been completed or can I plaster straight away?


A You may re-plaster the walls being treated straight away should you wish. The specification of the new render to be applied is very important and should be suitable for this type of situation. If you are thinking of waiting be aware that it may take 4 to 6 weeks for the injected DPC to cure (start to work) and then months, even years for the wall to dry down but it will never fully dry due to salt contamination. So go ahead and re-plaster but only paint the new plaster with a trade emulsion and regard initial decoration as temporary for the first twelve months. What do the people who are doing the DPC say? We are concerned that they have not informed you what to do, what to expect, what not to do etc. Please let us know. You should comply with their requirements in order not to invalidate any guarantee.

 

Can I use sand and cement?

Q I have recently had a damp course done in my house and have been told I can use sand and cement instead of renovating plaster - is this true? And do I need to add anything else to the sand and cement?
A We would ALWAYS advise the use of sand and cement at the correct ratios with a waterproof additive on walls known to have been affected by rising damp. We would NEVER recommend using a renovating plaster as experience has shown that they are not up to the job in the long term. Who did the DPC? Surely they told you what sand and cement specification to use?
We would recommend a first coat render comprised of three parts clean washed sand (correct zone) to one part cement incorporating a waterproof additive in the mix at the prescribed rate. It is ESSENTIAL to get the sand and cement ratios correct. Anything less and you will eventually experience problems. The correct sand is also important.
Why has the DPC contractor not undertaken the re-plastering? This has the potential to cause problems due to split responsibility in the future.


 

Damp company wants to delay re-plastering?

Q I will be having a DPC carried out on a new home I am purchasing. The company quoting for the job is well established and members of the BWPDA (PCA). I do believe the quote is overpriced. They say they must re-plaster immediately with "THEIR" specified plastering system immediately after DPC. However another Company quotes from the Dept of Environment leaflet 58 recommending that replastering be delayed as long as possible to allow soluble salts to move from the brickwork. If I can wait that would be preferable. What do you think?
A We are very familiar with the thinking behind delaying re-plastering - be careful!

It takes many years for the concentration of hygroscopic salts deposited in wall plaster as a result of rising damp to build up to a level that they cause a detrimental problem to wall finishes. Therefore it is likely that the greater amount of salt migration into the existing plaster has already happened and any that will occur following the injection of a remedial DPC will be small and for this to happen will take a considerable time.

The leaflet that you mention is not familiar to us and we believe it may be an old BRE leaflet. A correctly formulated sand and cement render incorporating a waterproof additive in the gauging water is a chemical cure and more than capable of preventing the migration of salt and residual moisture to the new decorative surface. In our opinion it is not necessary to delay re-plastering. We are aware that some companies use the delay tactic as a ploy to 'cherry pick' the profitable element of the work i.e. the drilling and injection. They will then come back twelve months later and quote a huge price to re-plaster so be careful.

There is no reason to delay - so have it all carried out at once which will enable you to live normally more quickly rather than wait a year in limbo for no real gain. Make certain the same company carries out the DPC and replastering to avoid split responsibility and you should get about three quotes. Good luck and let us know how you get on.



 

Crumbling 'squidgy' plaster?

Q We have a 1930's semi that was completely refurbished by the previous owners including the installation of a new damp proof course (evidence of holes along render and re-plastering to waist level). Although our survey showed no damp, eleven years on, we have crumbling "squidgy" plaster under the heavy wallpaper and black stains behind the lighter wallpaper. The damp appears to span both the hallway and part of the sitting room wall. It also appears to be about 1.5 feet along an interior wall between the two rooms. We have suspended floors in that area; the walls are cavity and have been insulated. We fixed a dodgy gutter and sewer pipe earlier this year in the area hoping it may have been that which caused the damp but to no avail. Any advice?
A It is difficult to answer your type of question without actually seeing the problem as we all have our own different ways of describing a similar problem. We may therefore be misinterpreting what you have described. "Squidgy" plaster may be sulphate attack - the formation of sulphate crystals at or near the surface causing it to disintegrate. Sulphate is naturally present in building materials and is transported to the surface in solution (water) and reverts to a crystal when the water evaporates. It does indicate a source of moisture. Do you have a guarantee for the DPC work that you are able to see? If so get that company back for their opinion. Is there any damp present or is it just bad plaster and some discolouration?



 
Contact Peter: Email - info@dampdecay.co.uk - Telephone: 015242 71794