Damp sheet on ground under suspended wooden floor -fix down?

[RoryP]

Hi, new to this DIY lark and would be v grateful for any help from the experts!

I recently bought a house which had wet rot damage to ground floor joists and floor boards. There is a two feet space beneath the floor and then it is just earth (mainly clay(. I have ripped the wood out and am trying to do the necessary to prevent it from happening again (more ventilation, etc.).

I have read that it is also worth putting damp sheet directly on the earth beneath the floor to prevent moisture in the earth evaporating into the sub-floor air space and then condensing on the wood.

I was originally going to lay it down, and weight down with bricks along the edges of the room. However, it dawned on me that the earth underneath would then be permanently damp, not being dried by the air flow, and might start to smell. So then I thought perhaps I should fix down the edges of the sheet to the brick work just above the earth and attempt to seal it properly.

On the other hand, I have read elsewhere that I should puncture this sheet in a few places in case of water ingress (flooding etc - I live in Gloucester so this is a risk!) so any water can drain into the earth.

Any guidance on the above would be much appreciated.

 

[JerryM]

i've had a right game with what sounds very familiar.

you really need to keep watch to establish if you get water or not as this has a biggish bearing on what to do. there are some existing posts on the subject.

if the floor is just getting damp then dpm covered with gravel will be fine. the ventilation as you've already identified is the 1st thing to start with in putting the problem right. i would also treat the new timber as a precaution and sandwich dpc between it and the brickwork if not using joist hangers.

if there is water then depends on how high and how often. may be able to introduce a soakaway at the worst end. i would try to avoid putting unnecessary holes in the dpm as the idea is to stop the dampness escaping from the soil. nb the dampness is also most likely to be coming from the brickwork

if water is high (say sort of >1" and often ie every time it rains) then may need to put in concrete on top of dpm or introduce pit & electric pump.

i would not expect any additional smell to be a problem as the extra ventilation you've created will keep the void ventilated.

once you've completed the work you ideally need to keep a check for a few months (winter, although nowadays it seems to rain as much in the summer) to be sure the wood is staying dry. wood will rot if over 21% saturated for more than 8 weeks.

 

[RoryP]

JerryM - thanks for the reply.

The water comes up from the clay soil. I have observed it after reasonable levels of rain (but not days of continuous torrential rain). The water puddles a tiny bit, but it mostly just turns muddy.

I have already bought some DPC to put between the joists and brick work supports. Was also planning to treat the wood with Cuprinol wet/dry rot/woodworm treatment at the places where it is supported by brickwork in addition to the DPM (belt and braces and all that).

It sounds like I should lay the DPM sheeting and just weight it down with gravel. I'll make sure the sheet comes a few inches up the wall so no water can get on top of the sheet. You don't mention anything about sealing the sheet against the walls.

Just a couple of questions remain (I have searched for existing posts but not found anything relevant):

- Should I treat all of the timber (joists and floor) - was planning to used tannised joists and moisture resistant chipboard/marine ply so didn't think ity would be worth treating all of it again, just where it sits on the brickwork.

- Should I seal the edges of the DPM ground sheeting or just ensure it comes up the brick a couple of bricks high.

Thanks again.

 

[JerryM]

existing posts

//www.diynot.com/forums/viewtopic.php?t=93168

Should I treat all of the timber (joists and floor)

No just treat the 1st 2ft from the wall (it's the most critical area as it's still absorbing moisture from the adjacent brick).

Should I seal the edges of the DPM ground sheeting or just ensure it comes up the brick a couple of bricks high.

YES. this is quite important as by putting the sheet down the water tries to surface elsewhere and the smaller the area the higher it's able to rise.

i've not found a good way of sealing the joints ie tape or contact adhesive will work to start with but i'm sceptical on how long.

i've put a chamfer of mortar all around the sheet on mine. i found once i'd put the dpm down (which i then put 2 to 3" concrete ontop - i had sort of 1 to 2" of water ponding in places before i started) the water then started rising up the gaps at the walls. only the mortar stop it.