Damp problem

The main extension at the rear has a suspended timber floor and whilst checking everything was OK before laying the tiles inside I discovered a damp problem which needed to be resolved. This page will detail the steps taken to deal with it.


I was aware before laying the floor that the ground at the South end of the extension was very slow to dry out after any amount of rain so I made some of the boards quite small sections to leave me inspection panels to check it and to give access to the drains for final fitting before commiting to laying the floor tiles and sealing everything in for good.

When checking it was obvious that the walls were very wet and you can see the salts coming out of the bricks and blocks in this pic.


As you can see there was condensation on the inside of the DPC that I added to protect the end of the beams and the insulation. The insulation was wet and this was holding moisture against the beams.

Not a good situation and definitely not something that could be left. Having thought about the issue, two problems were apparent as the cause.

1. There was something causing the wall to remain wet and that must be excess water against the outside of the wall.
2. The ventilation was not removing moisture from within the floor void with the result that it was condensing onto the structure.


I started by having a look at the water problem and dug out the fill from against the wall in the dog run area I had backfilled previously.

The picture shows how wet the bottom of the wall was, but it does also show the DPC doing its job as there is a clear line above which the wall is nice and dry.


It had not rained for a few days, but within five minutes there was a pool of water forming at the bottom of the trench.

My supposition here is that the water is draining down through the fill in this area and when it hits the layer of clay at the bottom it is just forming an underground lake. This clay layer is level with the top of the foundation so the wall is constantly sitting in a puddle of water.

This area is also lower than the garden and next door's lawn so I expect it is getting some water from those areas too.


I continued the trench around the corner and along the back of the extension reomving the backfill I had placed there previously.

This should at least allow the water to drain away from the area for now and permit the wall to dry out.


The (hopefully) permanent solution I have decided on is to place a french drain with a pipe in the bottom all around the outside of the extension. This will drain the water away to a connection with the rainwater drains to eventually go to the soakaway at the bottom of the garden.

The tricky bit it is going to be getting the french drain pipe past this drain pipe from the toilet. I suspect the best way is going to be to go a little deeper and go under this pipe.

You can see the water collected here again after a day's rain.


The second aspect of the problem is to address the ventilation issue.

I believe the reason here is that although there are air bricks at the rear of the extension the floor in the existing house has not been excavated or the air bricks installed on the other side of the house, so there is no cross flow of air.

There's not a way of installing additional air bricks other than knocking some big holes into the walls.


Having made a suitably sized hole that extended down far enough to go under the floor joists I put the periscope vents into place.

There will be four new vents installed, two on each side of the extension. Coupled with the four already in the back wall this should give adequate cross ventilation until the vents are in the front and the floor removed in the existing house.


Here are the completed vents at the other end of the extension.

Care was taken to retain the integrity of the DPC and a new section was overlapped into the gap to make doubly sure.


At the North end where the walls are wettest and the problem greatest I have fitted adapters to the bottom of the vents.

These are intended to allow pipes to be connected to permit the extending of the vents to run under solid areas of floor. I will be using them to allow a couple of the cheapest extractor fans I could find to be attached and provide mechanical ventilation to help dry things out under the floor a bit quicker on those days when it is not very windy.

I will update this page with the work to complete the french drain system as it progresses.