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UK diy (uk.d-i-y) For the discussion of all topics related to diy (do-it-yourself) in the UK. All levels of experience and proficency are welcome to join in to ask questions or offer solutions. |
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Damp under upstairs window. Porous sill?
My daughter's house has had a patch of damp appearing under the upstairs
windows since she bought the house 5 years ago from a builder (I did warn her!!) who had done it up to re-sale. There was no sign of damp then, it all being freshly decorated. The property is an 1890 end terrace on a high exposed location facing the prevailing wind, and rain does drive against this wall at times. The outside is rendered with small chips (spar?) in a yellowish base. There are a couple of hairline cracks in the render below the windows, and that area has been treated a couple of times recently with silicone fluid without any improvement to the damp patch. There are two adjacent rooms with windows in identical situations, and one has a large ugly damp patch below the sill, whilst the other shows only a very small patch about 3" diameter. The bad damp patch is about 3 ft wide by 1 ft high, starting 9" below the sill. I did the 'glass-slip plasticined to the wall' test to verify that it is damp and not condensation, and this was very clear with gross condensation on the inside glass surface. The builder had replaced the original sash windows with uPVC, and the sills are now concrete, and I suspect the sill heights have been increased. These sills are not 'bought' concrete ones but appear to have been cast locally, possibly in-situ. The finish is pretty poor and the top surface seems to be hand trowelled and not very flat. In fact there are some hollows that retain little puddles on the sill with the worst damp problem. I gave both sills a thorough soaking with silicone fluid a couple of weeks ago and this soaked into them very readily, so I am wondering if they are porous (or were, before siliconing). The damp patch has slowly dried back since the winter, and left ugly efflorescence marks behind. A test with my damp meter (a cheap squawk type) confirmed the surface to be almost dry compared to the winter wetness. Having now siliconed the sills, as well as the walls, I am hoping that further wet will be kept out, and so last week I decided to hack off the plaster with a view to making good the interior efflorescence, but I found some unexpected things. The plaster turned out to be only a skim of 3-4 mms on a sand/cement render. I assume that the builder did this to suppress the damp, but it is obviously ineffectual, so I hacked it all off - that turned out to quite a test for my cheapo Screwfix SDS+ drill, but was a dream compared to doing it by hand. I found the render to be covering a real mess of brickwork, but the main surprise was that the bricks were actually quite wet, the wettest being right under the sill. I surmise now that the problem IS that the sill is porous and water is percolating down from the sill becoming trapped between the (hopefully) waterproof cement render on the inside and outside surfaces. I suspect that the whole inner wall is rendered, at least up to the sill level rather than just over the damp area. Certainly the 4' x 1' area I hacked off did not reach the limit of the render. Incidentally, i could not detect any dampness on the outside rendered wall, perhaps not surprising as the weather has been dry and warm. I am leaving it for a few days to see how this wet dries out, now that it can via the exposed inner brick surface. What I need now is advice on is what to do to make good the inside wall. I am not convinced that re-rendering it is a good idea - I want to stop the damp at source, and the original render put on to contain it has not worked very well. Is a 'waterproof' rendering really waterproof? I suspect that under the pressure of a couple of feet of water inside the brickwork wet will seep through; or could it be that the builder didn't mix the render correctly? I am also worried about the poor quality of the sills, and suppose that we may have to replace them if wet continues to enter the wall in spite of the silicone treatment. In one sense, not rendering the inside will at least allow me to quickly see if wet is still entering from outside, whereas I imagine rendering it will delay the occurrence of future damp but not totally stop it. What is involved in replacing a sill should that be needed? Any advice or comments will be much appreciated. Phil The uk.d-i-y FAQ is at http://www.diyfaq.org.uk/ Remove NOSPAM from address to email me -- Phil Addison The uk.d-i-y FAQ is at http://www.diyfaq.org.uk/ Remove NOSPAM from address to email me |
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