Damp rising through foundations

[brum_screwdriver]

My single skin garage is rendered, and painted with weatherproof masonry paint. However with heavy rain I get damp coming up around the edges of the floor. I think the breeze blocks are either wicking water up from the soil, or rain is being blown against them. The render stops at the DPC, and the floor inside is approximately level with the DPC (I can measure to confirm if it is slightly above or below). The builders told me the floor is plastic wrapped insulation boards (these are resting on soil), covered in self levelling screed of some sort.



I'd like to minimise the ingress ... would some kind of waterproof barrier in the blocks help? Or a french drain all the way around and a coating of water repellent? Thanks.

 

[Harry Bloomfield]

I'd like to minimise the ingress ... would some kind of waterproof barrier in the blocks help? Or a french drain all the way around and a coating of water repellent? Thanks.

I would give the water somewhere to go, as my first option, were I you - In the form of a French Drain, but the drain will need somewhere to discharge.

I built my precast 24x12 garage on a concrete slab, originally just above the surrounding garden, then later, added a large workshop onto the rear, stepping the floor level up. My garden slope is higher towards the rear. All was well for decades, then the garage floor began to flood very slightly - due to settlement, and rising soil levels, when ever there was lots of wet weather.

Three years ago, I dug a French Drain all the way round the rear and sides, refilling with coarse gravel, and installing lengths of 40mm plastic pipe, leading to a drain. As I installed the pipe, I drilled numerous 10mm holes in it. Since doing that, and despite all the rain we've had this year, I have not once suffered any flooded floor - my garage is dry as a bone.

 

[brum_screwdriver]

I would give the water somewhere to go, as my first option, were I you - In the form of a French Drain, but the drain will need somewhere to discharge.

I built my precast 24x12 garage on a concrete slab, originally just above the surrounding garden, then later, added a large workshop onto the rear, stepping the floor level up. My garden slope is higher towards the rear. All was well for decades, then the garage floor began to flood very slightly - due to settlement, and rising soil levels, when ever there was lots of wet weather.

Three years ago, I dug a French Drain all the way round the rear and sides, refilling with coarse gravel, and installing lengths of 40mm plastic pipe, leading to a drain. As I installed the pipe, I drilled numerous 10mm holes in it. Since doing that, and despite all the rain we've had this year, I have not once suffered any flooded floor - my garage is dry as a bone.

I might build a test one ... along one side of the garage, with an open end sending water on to the flowerbed or something. If that works I'll add the same to the remaining sides.

 

[Harry Bloomfield]

I might build a test one ... along one side of the garage, with an open end sending water on to the flowerbed or something. If that works I'll add the same to the remaining sides.

You can buy the ready-made leaky pipe, but cheaper to buy lengths of 40mm plastic, from B&Q, and drill your own holes in it.

 

[brum_screwdriver]

You can buy the ready-made leaky pipe, but cheaper to buy lengths of 40mm plastic, from B&Q, and drill your own holes in it.

Great, thank you.

 

[noseall]

My single skin garage is rendered, and painted with weatherproof masonry paint. However with heavy rain I get damp coming up around the edges of the floor. I think the breeze blocks are either wicking water up from the soil, or rain is being blown against them. The render stops at the DPC, and the floor inside is approximately level with the DPC (I can measure to confirm if it is slightly above or below). The builders told me the floor is plastic wrapped insulation boards (these are resting on soil), covered in self levelling screed of some sort.



I'd like to minimise the ingress ... would some kind of waterproof barrier in the blocks help? Or a french drain all the way around and a coating of water repellent? Thanks.

Drainage is important, but it looks as though yours is not only free draining (not concrete etc) but also somewhat sheltered from above. 3.5n blocks are not really suitable in that location in any case, so you will always have a battle with damp.

If the DPM has been lapped onto the DPC at render level (internally), and has been detailed correctly, then I fail to see where external water is coming in? Are you sure it's not condensation?

 

[brum_screwdriver]

Just to add it is heavy clay soil ... not sure if that matters? I did hear once of someone who criss-crossed their garden with french drains ... the clay dried out so much that their new extension subsided

 

[brum_screwdriver]

Drainage is important, but it looks as though yours is not only free draining (not concrete etc) but also somewhat sheltered from above. 3.5n blocks are not really suitable in that location in any case, so you will always have a battle with damp.

If the DPM has been lapped onto the DPC at render level (internally), and has been detailed correctly, then I fail to see where external water is coming in? Are you sure it's not condensation?

It's hard to know whether the dpm/dpc were installed correctly (builder rather than diy). The previous garage (other end of the garden) would have water appearing at ground level when it rained, but we never did find out where from. There are underground rivers up the road, so it wouldn't surprise me if the soil gets so wet that it forces it's way up through somehow.

 

[Harry Bloomfield]

Just to add it is heavy clay soil ... not sure if that matters? I did hear once of someone who criss-crossed their garden with french drains ... the clay dried out so much that their new extension subsided

So is mine, just a few inches down - which means any rain is retained, until it evaporates. Heavy rain, meant my garden could become a muddy lake. I also added some French Drains, to the lower part of my garden, where I knew the water collected. Now, it does still flood, but it quite quickly drains away. You are only draining the standing water, in the upper level of soil - the clay will still only dry out in long periods of hot, dry weather.

 

[noseall]

It's hard to know whether the dpm/dpc were installed correctly (builder rather than diy).

If they were and they are continuous and are at the internal level you say they are, it's hard to see how damp can get in. That's the whole point of the oversite (DPM) detail.

 

[brum_screwdriver]

If they were and they are continuous and are at the internal level you say they are, it's hard to see how damp can get in. That's the whole point of the oversite (DPM) detail.

I am baffled ... it coincides with wet weather, and is visible at floor level (but works it's way up). the rear and 2 sides are sheltered by a fence but still show signs of damp. I don't think it is condensation (I have a dehumidifier on 24/7)

 

[^woody^]

Are you sure it's not water running off the render and tracking back to the blockwork?

 

[brum_screwdriver]

Are you sure it's not water running off the render and tracking back to the blockwork?

Could be ... is there any way to prevent that ... e.g. fill the gap between render and dpc? (I'll take pics shortly) e.g. run silicone between them?

 

[^woody^]

Render should be finished in a way that causes water to run off and away from the wall below. Google "render bellcast"

Render should also finish at DPC level - not above and not below.

If water is running back to the wall and if this is the issue, you'll need to try and insert a vertical strip of DPC or lead behind the beading to prevent run-off getting to the wall at DPC level.

If the above is not the problem and water is coming up the blockwork, then you won't stop that and it's a quality issue from a poorly installed DPM under the floor slab