How to replaster old, damp walls

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Replastering old walls. Damp walls - direct replastering method - Membrane not used.

We explain the reasons for the replastering specification. If you want to avoid this type of plastering use one of our Membranes, then you can safely use modern, lightweight plasters or fit plasterboard.

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Indoors specification, the professional standard method in the UK.

1. Hack off all old plaster to at least 300mm clear of all signs of damp or salt and at least 1 metre above ground floor level - use a meter to test the walls and remove plaster at least 300mm above the highest level that readings can be obtained.

Reason - This is to ensure that the residual 'salts' do not climb above the new plaster in the future.

2. Brush the walls to remove all plaster residue, particularly around angle beads.

Reason - old plaster will have 'salt' in it and will cause damp spots to appear in time.

3. Repair any holes or poor joints with sand and cement (4:1) using washed, sharp (means slightly gritty) plastering sand (sometimes called screeding or rendering sand). Do not use fine, unwashed sands.

Reason - fine, unwashed sands often contain salt and the fine particles are too numerous to find enough cement particles to bond together tightly - these two problems make for a weak render, prone to the easy passage of 'salts'.

4. Special Note - walls made with non-porous stone or brick, like granite or blue brick, will require an adhesive to help stick the render coat to the wall - use BondAcryl concentrate)

Reason - normal renders and plasters stick to the wall by suction - the surfaces need to absorb some water. Without this suction the render or plaster will not grab the wall and may become loose and hollow as it dries.

5. Damp the walls lightly (to reduce 'suction', which can cause excessive drying and cracking) and apply a thin coat (maximum 1/2 inch, 12.5mm) of render consisting of 3 parts sand - dry, washed, sharp (means slightly gritty) plastering sand (sometimes called screeding or rendering sand) with 1 part of fresh (free flowing - no lumps) Portland Cement.

Reason - thick coats are more likely to slump down the wall during application and crack during drying out.

6. Scratch the render surface liberally all over with a nail board, trowel, metal float or similar object.

Reason - without these scratches the natural drying shrinkage will cause cracking, crazing and hollowness to develop - the next coat will probably pull the first coat off as it dries.

7. Use only Renderproof water proofer/plasticiser in the water that the render is mixed with, at the rate of 1 part Renderproof to 40 parts of water. Do not use fine, unwashed or wet sands.

Reasons - Renderproof binds the sand to the cement and prevents liquid water passing through. It also makes the mix stickier (plasticising) which help to hold the render together. Wet sand weighs more than dry, so it will make your mix weak. Fine sands produce a weak, powdery render, which will not resist 'salts'. Do not allow the plasterer to add plaster (usually 'browning') or washing up liquid to the mix.

8. When the render surface is firm enough (but not bone dry, or it will need re-wetting) apply a second coat to exactly the same specification - if further coats will be needed to reach the desired thickness don't forget to scratch liberally. In hot weather spray the render surface with water to slow the drying process.

Reasons - excessive drying out increases the suction and can prevent one coat sticking to another. Rapid drying always increases shrinkage, which gives rise to cracking of the render or finish plaster.

9. Whilst still damp (or re-wet again) apply a skim coat of Universal or Board Finish. Do not polish or add water.

Reason - this will produce a shiny, glazed finish which looks good, but is prone to condensation, black mould growth and poor drying.

10.. Delay any decoration for at least one month and then only apply a thin coat of breathable emulsion paint, not a heavy vinyl. Our Replastering Paint is the best option because it breathes and allows the wall to dry out naturally. Do not repaint or wallpaper for at least three months.

Reason - the paint or paper will fall off the wall due to the water vapour that will be trapped underneath in the new render.

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Site written by: David Moore

David Moore, B.A. (Hons.), C.T.I.S., C.R.D.S. Technical Author

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