Damp below Damp Proof Course
Causes of damp just above ground level
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Damp explained in plain English
Damp below damp proof course level - ground floor, a quick summary checklist:
Tip 1 - if a concrete floor is damp all over it is hard to isolate the moisture source. As it dries out the damp retreats slowly back to the points of ingress, so if you are not able to see any isolated damp areas either ventilate and heat or shut all the doors and windows and use a dehumidifier.
Tip 2 - always check the relative level of your floor to the outside ground level and damp proof course. If the floor is below the damp proof course you will need to 'tank' the overlap and seal the wall to floor joint, to prevent laterally penetrating dampness.
Symptoms and Remedies
Symptom - isolated damp spots on a concrete floor, not touching the edges. This is probably due to small holes in the plastic membrane (Visqueen 1200 gauge is normally used) laid onto sharp hard core without a sufficient sand cover. A water leak will add to the problem, so always use our Leak Testing Check List (see Water Leak Testing Project).
Remedy - either dig up the floor and re-lay, or clean the surface and apply two coats of an epoxy membrane (see www.epoxy-info.co.uk) or fit an Air Gap Membrane (see www.drywallandfloor.co.uk). Membrane of this type can be screeded (minimum 2 inches, 50mm) or a chipboard floor can be directly laid (a 'floating' floor, 18mm board).
Symptom - damp areas on a concrete floor, touching and emerging from the edges. This is probably due to a lack of joint material where the plastic membrane butts up to the wall edge. A water leak will add to the problem, so always use our Leak Testing Check List (see Water Leak Testing Project).
Remedy - clean out the floor to wall joint until you can find the plastic membrane all the way around the floor, chisel out a U shaped channel in the floor at least 25mm x 25mm (1 inch x 1 inch), vacuum and then fill with sand and cement (4:1 sand to cement) incorporating SBR liquid, both in the sand and cement and applied as a liquid primer to all the faces of the prepared 'slot'.
Symptom - white, fluffy salts, plaster possibly 'blowing' off the wall - 'efflorescence' - you have a water source which needs to be found and eliminated. Use our Water Leak Checklist (click here).
Remedy - Salt Neutraliser might work if you can find and stop the leak, or the plaster may be too damaged and need replacement to our Specification.
Symptom - damp patches (at low level,l right down to the skirting board - sometimes the skirting board will be rotten) that may come and go - you have 'hygroscopic' salts that are attracting moisture from the air.
Remedy - your damp proof course may be 'bridged' by high outside ground, plaster on the inside or debris in the cavity (cavity walls only). Clean out the floor to wall joint until you can find the plastic membrane all the way around the floor, chisel out a U shaped channel in the floor at least 25mm x 25mm (1 inch x 1 inch), vacuum out all debris and dust. Rectify any defects and 'tank' the wall area below the correct damp proof course level (6 inches or 150mm above outside ground level) into the floor to wall notch and then fill with sand and cement (4:1 sand to cement) incorporating SBR liquid, both in the sand and cement and applied as a liquid primer to all the faces of the prepared 'slot'. Inject Quick Cream at the correct level to form a new Damp Proof Course. The plaster will need replacement to our Specification.
Symptom - damp patches on or around the base of chimney breasts - sometimes showing yellow or brown stains in wallpaper or paint - you have 'hygroscopic' chimney salts that are attracting moisture from the air, which may include tars and other chemicals from burning wood or coal.
Remedy - these 'salts' are very difficult to hold back in the wall, even with the replastering to our Specification carried out to perfection. We recommend using Mesh Membrane - a type of dry lining - to allow you to replaster using normal, lightweight plaster, with no risk of further 'salting'.
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Site written by: David Moore
David Moore, B.A. (Hons.), C.T.I.S., C.R.D.S. Technical Author