An experienced damp investigation surveyor will carry out an internal inspection of the property, and also an external inspection. On the external inspection survey they should be looking for possible problems relating to the internal problems and any other obvious defects. One of the most common problems is high external ground levels where the damp proof course has been bridged, or potential for rainwater to splash over the damp proof course.
The below diagram shows what bridging of the damp proof course looks like.
By simply lowering external ground levels, this will allow wall base evaporation, to help control rising dampness.
External ground levels ideally need to be lowered 150mm below the damp proof course – or the internal flooring. Mass ground excavation isn’t necessary, simply cutting out a channel and using terram to supress any weed growth and pebbles is a simple procedure.
Complete Preservation carry this type of work out normally using the typical drawing below.
Before any cutting into the ground is carried out it is imperative to use cable avoidance detection equipment.
We use the below Rigid SeekTech SR-20, along with our rigid drain cameras. Our drain cameras also have a sond, so we can be very precise when cutting or breaking up the ground.
We use specialist Hilti dust extraction systems connected to professional Hilti cutting equipment to minimise dust.
Geo-membrane is placed above the earth to stop weed growth and golden gravel is poured on top
The picture below shows the level of what the ground level used to be before we lowered it, the sub floor vent was blocked which we cleared out.
This is very effective in helping evaporation of rising dampness at the wall base.
Often people can presume that because the ground level is slightly high reducing this will fix the issue; however if hygroscopic salts are present in the plaster lowering the ground levels will not remove the hygroscopic salts. Gravimetric can confirm if moisture and/or hygroscopic salts are present; this is a service we also offer.
A damp wall takes time to dry out, as a general guide the drying rate is given as 1 month for every 25mm of wall thickness (BRE,”Drying out of buildings”) A 230mm thick wall will take approximately 9 months to dry. However, the drying processes depend on conditions, ventilation and the type of masonry, so drying may take considerably longer than anticipated.
When ground levels are lowered it is also likely that a small amount of lime pointing will also be necessary at the base of the wall.
Ground levels should be lowered by a competent person or you should take advice from a structural engineer in order to ensure the structural integrity of the building isn’t compromised.
If you would like a survey, and price to lower external ground levels if they are high, please contact us on firstname.lastname@example.org or on 01225 769215. We can also carry out the plaster sampling internally following the methodology in BRE DIGEST 245 to confirm if hygroscopic salts are present and the plaster will need replacing.