Fixing Damp Issues Below a Damp Proof Course

Where the base of a wall resides below ground level is can become subject to many damp issues including the salts that are present in ground water and also also in colder conditions, freeze-thaw action where water and moisture freezes in tiny cracks and pits in the walls surface and forces it apart over time.

In this how-to guide you will learn all about diagnosing and fixing damp issues below a damp proof course.

Testing for Damp

If you would like to check for damp for yourself, click the below and purchase a Property Repair Systems Damp Meter. If you need any help with any type of Damp below dpc, timber repair or maintenance problems, Property Repair Systems will be pleased to give you completely free, no obligation advice on 01626 872 650. The information you need may be in the project below, if not, call them.

Damp Below Damp Proof Course Level

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.

Diagnosing Damp Causes and How to Fix Them

Isolated Damp Spots on Concrete Floor

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 a leak testing check list – one can be seen here).

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).

Damp Areas Coming From Edges of a Concrete Floor

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 a leak testing check list (see the one mentioned above).

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’.

Fluffy White Salts and Blowing Plaster

Symptom – white, fluffy salts, plaster possibly blowing off the wall – efflorescence – you have a water source which needs to be found and eliminated. Again, use the water leaks check list mentioned above to help you trace the source.

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.

Low Level Damp Patches Down to Skirting

Symptom – damp patches (at low level, 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. Click the image to the left to purchase Property Repair Services Hygrometer.

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 also.

Damp Patches Around Chimney Breast Base

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.

As mentioned at the top of this project page, if you still require some additional help and advice please contact Property Repair Systems free of charge on: 01626 872 650.