… why not underneath where the damp course is?

Because you “bridge” the damp in theory. However various scenarios do exist such as something that is below ground. What we talking here?

My workshop/shed. Its an unsealed breeze block built affair. I’m sealing it with Thompsons water seal but the tin says don’t apply below the damp course.

As (last winter) the wet crept through to the inside to give me damp walls, this would suggest that if I don’t do it, it might continue to creep in a ground level?

Put a waterproof layer on the floor?

My workshop is solid concrete blocks, but I have a vapour barrier on the inside, which keeps it dry as a bone inside, but wet under the floor insulation.

I only noticed the damp when repairing the damage caused by dropping a barbell through the floor:

Slightly damp under this bit by brf, on Flickr

Okay…. Theory time.

1) Thomson appears to be an external application (I’ve used it internally on my garage brickwork – don’t tell them)
2) Sealing below DPC could potentially trap any rising damp and as the external face is sealed, potentially send it back into the building.
3) Below DPC is a more aggressive environment and I suspect it may not be “man” enough.

That said, and if it was me, and I wasn’t sleeping in the garage I wouldn’t have an issue applying it internally to the blockwork below DPC.

Those footflaps are the issues we were talking about when you first started your project. After all the great work you did I still wish you’d done a proper detail at floor level.
Kryton does your internal floor level match up with the dpc and does the dpc actually exist?

Yes it does, its about 5″ above floor level.

I should add – I don’t get puddles of water in the shed – it just seeps through in the really continuous and heavy rain we had at the beginning of the year to give me damp walls inside. It’s single layer of breeze blocks.

Those footflaps are the issues we were talking about when you first started your project. After all the great work you did I still wish you’d done a proper detail at floor level.

The only difference is I’ve put the waterproof layer on top of the concrete rather than under screed. Either way there’s no ingress into the interior bit.

Those footflaps are the issues we were talking about when you first started your project. After all the great work you did I still wish you’d done a proper detail at floor level.

This^

The only difference is I’ve put the waterproof layer on top of the concrete rather than under screed. Either way there’s no ingress into the interior bit

Yet! Give it time, then there will be that damp stagnant smell that lingers.

Are you really going to use Thompson when there’s a concrete waterproofer named after you?? http://www.kryton.com

Yet! Give it time, then there will be that damp stagnant smell that lingers.

That’s what the vapour barrier is for….

It’s just wrong and bad practice. As I said before I don’t want to take anything away from the build because you worked bloody hard and turned round what was a disastrous start into a great space.
Anyway back on topic.
Kryton, wasn’t sure which bit was yes? The floor level matching up to dpc or dpc being 5inch above floor level?

It’s just wrong and bad practice.

You’re starting to sound religious….

I’ll agree it’s unconventional but unless you’re saying water can penetrate 100mm of closed cell foam and cross a foil membrane, it’s going to keep damp out of the habitable area.

Sigh.

Right. – the DPC is 5″ above floor level, the workshop walls are breeze blocks, the Thompson instructions says dont apply the sealant below the DPC.

What I want to know is why. Surely the damp/ rain would continue to seep through under the Dpc?

Thought I explained it?

Yeah pjm explained it then, and I’ve been doing it wrong for the last 20 yrs.
The one area where you won’t stop it is the joint between floor and wall. I should just lash it on and see how it goes…

I’m guessing they don’t want the wall below the DPC sealed as you’re potentially sealing damp in. The paint itself won’t bridge the DPC as it won’t be water permeable.

Ok, so this is slightly late as you’ve already bought some proofer, but you might be better off applying a waterproof ‘coat’ rather than the Thompson type stuff which is a bit more water repellant so to speak (if it’s the clear very thin waterproofer I’m thinking of?). Sika make a lot of good products related to this which I think you can get from Screwfix? They’re more like thick paints and work really well.

You are correct that you shouldn’t bridge the DPC however this material is hydroscopic so in effect it an barrier to water (subject to its pressure). If he was laying a bit of carpet across it would be an no!

Okay having just picked Thomson technical department “brains” the concern is with breathable walls. Sealing internally will slow down the natural drying process of the wall and therefore makes it more susceptible to the freeze / thaw cycle. Obviously below DPC if applied eternally will trap any rising damp.

Excellent. When the rain stops, I’ll carry on and include the area down below.