Water ingress between shed base and wall

[Kempchi]

I’ve been having an issue with water ingress under my shed. It’s a 6m x 5m wooden garage, sitting on a concrete base that extends about 200mm past the edge of the shed on all sides. The shed walls sit on pressure-treated bearers, which are wrapped in DPC on three sides.

Initially, I assumed the problem was water hitting the base, so I added a sloped concrete cap to direct water away from the shed. Unfortunately, this hasn’t really helped. It’s very wet and windy where the shed is (Scottish Highlands), so I’m now realising that most of the water ingress seems to be coming from water running down the shed walls then creeping between the bearers and the concrete, as well as between the bearers and the shed walls.

I tried sealing it with Sikaflex construction sealant (in the middle of the night – during a rainstorm, which is my excuse for why it looks hideous), but that didn’t work either. So, I’m looking for advice on how to fix it properly once and for all. (Also, interested/embarrassed to find out how many layers of idiocy I’ve added to this situation)

 

[blup]

You've used a dpc to try to solve ingress of water. The perimeter of the shed needs to be tanked, using a sealant suitable for concrete and wood. You can seal on the inside as well for belt and braces. Maybe something like CT1 which is a hybrid sealant/polymer.

 

[Kempchi]

Thanks so much. Apologies for ignorance, when you say “tanked” I assume you mean just make sure there is a decent seal?

I used sikaflex construction sealant but it was during a very heavy rainstorm - unfortunately it was the only time I could do the job. If I follow your advice, I expect I’ll need to remove the existing sealant and apply a bead of CT1 where the concrete meets the wood?

Do you think flashing would help?

Thanks again

 

[Harry Bloomfield]

The usual way, is to have the timber raised up, front the base, on something impervious to moisture. The hut walls, on top of a low brick wall would do it. Additionally, the base sloping down, or better there being no base outside the walls (the 200mm) would help prevent water entering.

 

[Kempchi]

The usual way, is to have the timber raised up, front the base, on something impervious to moisture. The hut walls, on top of a low brick wall would do it. Additionally, the base sloping down, or better there being no base outside the walls would help prevent water entering.

Cheers Harry, the issue is that it's bolted onto the slab, and it's very heavy, too. The other thing is that it's not got a floor, the floor is just the concrete slab

 

[blup]

Thanks so much. Apologies for ignorance, when you say “tanked” I assume you mean just make sure there is a decent seal?

Yes

I used sikaflex construction sealant but it was during a very heavy rainstorm - unfortunately it was the only time I could do the job. If I follow your advice, I expect I’ll need to remove the existing sealant and apply a bead of CT1 where the concrete meets the wood?

There are many sealants and other treatments available. You need something that is designed to seal wood and concrete, CT1 should do this but will be expensive for the size of the job. A rubberised compound is another possibility

Do you think flashing would help?

Thanks again

It all depends on how it is let into the ground and the garage wall... and the Scottish weather.

 

[Kempchi]

CT1 should do this but will be expensive for the size of the job

It doesn't seem wildly expensive (especially compared to lead flashing which would be hundreds) Claimed coverage is 9.25m from a 290ml tube with a 6mm bead. I think I need about 45 meters or so - five tubes should do that. At £13.80 a tube from Toolstation that's less than £70, so it is pricey but doesn't seem too bad - unless I'm missing something?

 

[Harry Bloomfield]

Cheers Harry, the issue is that it's bolted onto the slab, and it's very heavy, too. The other thing is that it's not got a floor, the floor is just the concrete slab

Rather than raise it, could you cut away the base around it, or at least a channel, so water is more inclined to drain away from the slab?

What are the ground levels like, around the slab?

 

[Kempchi]

Rather than raise it, could you cut away the base around it, or at least a channel, so water is more inclined to drain away from the slab?

What are the ground levels like, around the slab?

On two sides the slab is held up by a retaining wall with a 1-4 foot drop, the slab is about 200mm deep and there's a trench of that depth around the other two sides where the formwork was.

My initial plan was to cut into the base, but I had some (bad) advice that I might want to build up the slab to make a slope rather than cut away the slab as it would be easier. Well, it was easy. And daft.