Re-plastering Old Walls

TIME SCALE 4 Hours

SKILL LEVEL

If you've solved all other sources of dampness—water leaks have been fixed, your gutters have been waterproofed and the like—then the plaster in your old walls may simply be contaminated. Re-plastering may be your only option. You should first double-check, even triple-check, for leaking gutters and downpipes, high-ground levels, and water leaks first before proceeding.

Be warned though that many plasterers and renovators don't use the re-plastering method in most dampness cases. In fact, if your dampness woes are caused by salt contamination, symptoms of this condition will usually reappear through the use of most modern "lightweight" plasters, e.g. "browning" or "bonding".

You should approach re-plastering with due caution. Have a professional take a look at your wall to determine if it does need re-plastering before actually going through thorough the grueling process. In any case, here are the steps and tips needed for proper wall re-plastering:

1. Start removing the old plaster on the wall. Strip at least 300 millimetres off until the affected area is clear of all signs of dampness and salt, and at least 1 metre above ground floor level. Use a gauge to examine the walls and remove another 300 millimetres above the highest level where the readings can be acquired. This will guarantee that no residual salts will be able to creep into the new plaster in the future.

2. Scour the walls to strip off all remaining plaster residue, specifically around angle beads. This is because old plaster, even little bits of it, will probably have some salt left in it and cause moisture spots to appear on the wall in time regardless of how many re-plastering attempts you make. Only by removing every last bit of salt and old plaster will you be able to get the most out of the re-plastering process.

3. Fix all holes and deficient joints with sand and cement. Use a 4:1 ratio of washed, sharp, slightly gritty plastering sand (also known as rendering or screeding sand). Fine, unwashed sands are ineffective against re-plastering moisture-infected walls because they contain salt that could aggravate the problem and have too numerous fine particles to produce a strong, well-bonded render that's not prone to the easy passage of said salts.

4. Walls made of granite or blue brick may require the use of an adhesive. To be more precise, walls composed of non-porous stone or brick need adhesives to stick the render coat in the wall—the SBR concentrate is particularly recommended. Normal renders and plasters actually use the pores of porous, water-absorbent surfaces in order to stick onto the material via the power of suction. Naturally, non-porous surface will act like non-stick Teflon to the render, turning it into a loose and hollow layer once it dries. Ergo, an adhesive is the only solution for plastering such surfaces.

5. Moisten the walls lightly to reduce suction and apply a thin coat of render. The specifications for the above recommendation are as follows: Reducing suction via moisture will help avoid excessive drying and cracking of the plaster. The thin coat of render should be half an inch or 12.5 millimetres thin, and should consist of three parts dry, washed, sharp, and gritty plastering/screeding/rendering sand and one part of fresh, free-flowing, lump-free Portland Cement. This is because thick coats of render will disastrously slump the wall down during application, and break during the drying-out period.

6. Freely scrape the render surface with a trowel, nail board, metal float, or similar tool. The scratches will serve as yet another safeguard against fracturing, cracking, brittleness, and hollowness caused by natural drying shrinkage. Besides, the next coat will most likely draw the first coat off as it dries.

7. Use only Renderpruf Waterproofer. This plasticiser—when used in the water that the render is mixed with at a rate of forty parts water to one part Renderpruf—will help a lot in binding sand and cement together while doing its job at keeping water from passing through. Again, don't use fine, unwashed, or wet sands, because this will make the render ineffective. Wet sand weighs more than dry sand, so it will also make your mixture weak and useless.

Because it's a plasticiser, Renderpruf Waterproofer's plasticising effect of making the mixture stickier will also help hold the render together as well. Plus, you should not allow the plasterer to add plaster (usually "browning") or washing up liquid to the mixture; these ingredients will compromise the integrity of the render.

8. Once the render surface is hard enough (but not bone dry), apply a second coat of render using the same process. If the coat is bone dry or if you're working in hot weather conditions, you'll probably need to re-moisten the surface to slow the drying process. Depending on the thickness you desire, further coats of plaster may be needed; just don't forget to scratch each coat liberally for better top layer quality.

More to the point, too much drying out of the layer will increase the suction of the surface, which prevents coats from sticking to each other. Rapid drying will lead to shrinkage, which in turn leads to cracking and breaking of the current render layer of finish plaster.

9. Apply a skim coat of Board or Universal Finish while the surface is still wet. Do not add water or polish the surface afterwards. This added step will produce a glazed, glossy finish that looks great, but is prone to black mould growth, condensation, and poor drying; be warned.

10. Delay decoration of the surface for at least a month. For the time being, apply only a coat of breathable emulsion paint instead of heavy vinyl paint; don't repaint or wallpaper for at least three months, because the paper or paint will probably flake or fall off because of water vapour that's still trapped inside the new render.