Log cabin wet under floor

[Johnny - Oxford]

I am building a log cabin from a kit.

I have put down 4 inches of concrete and the base, which is a framework of 70mm pressure treated wooden posts. The cabin floor which is tongue and groove boards, sits on top of this frame.

In my wisdom I decided to put down a layer of the silver backed bubble film to insulate under the floor before i screwed the floorboards down.

I then laid carpet tiles on top and was happy.

Today I lifted the corner tile to put a cable under it, and there is a damp feel to the floowboards.

I cant get underneath any more, but I drilled a 30mm inspection hole and cut through the foil to try to find out what is happening.

It feels like there is water on the underside of the foil, closest to the ground, but it is dry under the floorboards.

My guess is that with all the rain, there is bound to be some water get under the shed, and possibly some sweating out of the concrete?

Can anyone advise on what I should do.

 

[JohnD]

was the concrete base laid on a DPM?

Is the base above ground level?

Are the edges of the base exposed to falling rain?

Was the base laid with a fall to run-off water?

Is the void between the base and the floor ventilated?

 

[Johnny - Oxford]

was the concrete base laid on a DPM? We put down sheets of polythene over the hardcore. The sheet was in sections and was not joined, but it did cover the whole area.

Is the base above ground level? YES

Are the edges of the base exposed to falling rain? YES, there is about 6 inches all round.

Was the base laid with a fall to run-off water? No not really, but I could do something all round the shed now it is in place to achieve this, perhaps with some kind of tiles raised at the one edge.

Is the void between the base and the floor ventilated?
Not really, the floor frame is 70mm square sections, and is a large 4m x 4m square with about 7 cross members in one direction, these are the ones the floorboards are screwed to.

We have had a hell of a lot of rain recently.

Another idea I had was to drill a series of large ventilation holes from the outside through the 70mm bearers.

Thanks for your help, look forward to your replies.

 

[JohnD]

ventilate the void on all sides to get a through-flow of air. use something like air bricks to keep rodents out

find a way to stop rainwater falling on the slab edges

 

[Johnny - Oxford]

Great, many thanks.

 

[big-all]

your structure should not be in direct contact with the concrete unless it fully covers the concrete and overlaps slightly to keep it dry!!!

the normal is concrete slabs/blocks/bricks topped with felt or other waterproof layer then the structure thus seperating the wet concrete from the wood

unfortunatly your extra concrete is the problem the rain gets blown against the side of your shed runs down to the concrete and pushed underneath the same with the the water off the roof if you don't have gutters

 

[Johnny - Oxford]

Thanks

I suppose if I can cover that 6 inch overlap with some kind of tiles or similar, tilted away to run the water off of the edge of the slab, this would sort it out?

I guess I will need to make sure I get a waterproof seal on the edge of these against the side of the wooden base.

Appreciate all the help.

 

[Thermo]

if its only 4 inch thick concrete then why not trim it off with a stihl saw?