Using Damp proof membrane under shed

MassiveJim · 20 May 2016 at 11:22

Quick I'm stuck in life, to the forums again!!

I am building a small shed (kids playhouse) and have already constructed the base see pictures below

However I am not familiar with the rules when using Damp proof membrane.
My intention is to lay the shed floor directly onto the shed base in the picture below using screws, if I lay the DPM on top of that base and under the floor how do I get around the problem that I am now going to be drilling holes in it to fix both the floor down and the shed walls once they are ready to go up ?

http://i.imgur.com/JpmTiBy.jpg

http://i.imgur.com/NyGgp8w.jpg

Burned_Alive · 20 May 2016 at 11:51

My understanding is that if you were to use DPM, it should've gone between the base and the ground, not the shed and the base. Presuming you've used tanalised (treated) wood for the base, then I would just put the shed straight on without DPM to allow the air to circulate to the shed floor.

DXP55 · 20 May 2016 at 14:20

If that was me at that stage I would lift the base out and lay a paving slab at each corner and one in middle on each long side - then put base on that - sitting in that dirt it will be rotten in a few years.

Do it now not later.

Bunka · 20 May 2016 at 14:28

^ What this guy said. That base will be rotten in no time.

I put DPM between ground and base, then another layer between base and shed. Probably overkill but it'll do the trick!

BigBoy · 20 May 2016 at 15:30

Got some.old engine oil knocking about? Paint the bearers and underside of the shed Base with engine oil and it will keep any water ingress at bay.

MassiveJim · 20 May 2016 at 21:36

the "structure" that you see in the picture is actually 4 posts put in the ground with postcrete
I then screwed a frame to the top of the posts.
this left a gap around the back end so I filled the gap with some wooden battens.

so I can't really lift the structure up at present as it is screwed/nailed to the wooden posts at each corner

so although it may look like its 3 rows of timber all the way round it isnt, so I suppose I don't really have a base and a shed.
also the wood is tanalised yes.

mjd · 20 May 2016 at 22:34

Providing the framework you have in place isn't in direct contact with the ground then a DPM isnt really necessary at all.
Clear some earth away from the underside of the frame to allow air to flow freely.
Consider adding some guttering to the finished article to direct water away from your posts. I would put a few more supporting posts in up the middle. Granted its only a kids playhouse, but will reduce any possibility of it distorting in the middle.

MassiveJim · 20 May 2016 at 22:53

i filled in underneath the wood so that there were no gaps underneath.

I have put a couple of bricks/bits of old concrete fence posts under some of the joists to support them.

I'll have to have a look over the weekend to see if there is a way I can put the dpm underneath it somehow.

could moisture travel up the post and transfer to the base that way ?

thenewoc · 20 May 2016 at 23:07

I think I would remove the frame and dig out a few inches of earth and replace it with some hard core and go over it with a smacker / compacter. The corner posts I would have sunk in concrete or a metal post spike to protect the wood.

jonzun · 20 May 2016 at 23:46

Just dig out the excess soil and get an airflow, far too much soil inside that as said above wouldn't really need dpm - it'll be fine for the kids

MassiveJim · 21 May 2016 at 13:21

thenewoc said:
I think I would remove the frame and dig out a few inches of earth and replace it with some hard core and go over it with a smacker / compacter. The corner posts I would have sunk in concrete or a metal post spike to protect the wood.

The corner posts have been set in postcrete.
Frame has already been screwed/nailed to the posts so will be difficult to remove now

jonzun said:
Just dig out the excess soil and get an airflow, far too much soil inside that as said above wouldn't really need dpm - it'll be fine for the kids

I threw the excess soil in there to save another couple of tip trips so it's mostly loose soil.
what I did before I threw the soil in there however was compact the soil that was underneath the outside frame as additional support for the frame
If I digout the soil that is directly touching the frame what can I replace it with that will prevent moisture/damp travelling through the wood ?
I also didn't want to leave a gap just to prevent small creatures getting underneath it. we have a field behind our house that is full of mice, or will the scents that the dog leaves everywhere stop them coming in ? (it doesn't seem to bother the birds that keep getting worms out of the soil)

Abyss · 21 May 2016 at 14:47

MassiveJim said:
The corner posts have been set in postcrete.
Frame has already been screwed/nailed to the posts so will be difficult to remove now

I threw the excess soil in there to save another couple of tip trips so it's mostly loose soil.
what I did before I threw the soil in there however was compact the soil that was underneath the outside frame as additional support for the frame
If I digout the soil that is directly touching the frame what can I replace it with that will prevent moisture/damp travelling through the wood ?
I also didn't want to leave a gap just to prevent small creatures getting underneath it. we have a field behind our house that is full of mice, or will the scents that the dog leaves everywhere stop them coming in ? (it doesn't seem to bother the birds that keep getting worms out of the soil)

You seem to be thinking about things backwards in terms of how damp travels. You want airflow underneath the base to keep it dry, anything physically in contact with the wood will promote damp. Just clear away enough soil to let you put a layer of weed membrane/pea shingle underneath and still not be in contact with any of the joists. The membrane/shingle is purely to stop weeds/growth.

Similar idea to this:
http://www.tidygardens.eu/wp-content/uploads/2014/08/new-decking-frame.jpg
and this:
http://www.pegdecking.com/wp-content/gallery/002-david-and-kay/002-sec-03-deck-frame-being-built.jpg

In both scenarios it will be weed suppression membrane underneath the pea shingle as opposed to a DPM.

If mice want to get into the building whether you have a void underneath or not will make little to no difference.

MassiveJim · 21 May 2016 at 15:36

Abyss said:
You seem to be thinking about things backwards in terms of how damp travels. You want airflow underneath the base to keep it dry, anything physically in contact with the wood will promote damp. Just clear away enough soil to let you put a layer of weed membrane/pea shingle underneath and still not be in contact with any of the joists. The membrane/shingle is purely to stop weeds/growth.

Similar idea to this:
http://www.tidygardens.eu/wp-content/uploads/2014/08/new-decking-frame.jpg
and this:
http://www.pegdecking.com/wp-content/gallery/002-david-and-kay/002-sec-03-deck-frame-being-built.jpg

In both scenarios it will be weed suppression membrane underneath the pea shingle as opposed to a DPM.

If mice want to get into the building whether you have a void underneath or not will make little to no difference.

ok looks like I need to do the following then
1 - remove excess soil
2 - install another couple of posts on each side of the frame to support it instead of using the ground like it is now
3 - clear space underneath the frame to allow airflow

at least I have a plan of action now

this has however opened a couple more questions up as it changes other things that I planned to do.
I was going to brush in a dry sand/cement mix inbetween the block pavers and the wooden battens to fill the small gap between them.
I'm assuming now that this will be a bad idea ?

mjd · 21 May 2016 at 19:04

The wood should not be in direct contact with anything other than wood wherever possible.
The posts are in now, so not a lot you can do with them, but providing you can divert the bulk of the water away from the base they will be fine.