Damp proof membrane and shed floor

9 Feb 2009
Reaction score
United Kingdom
Dear all,

I have purchased an ex-display shed. It was re-assembled in my garden and, following advice on this forum, I wanted a damp proof membrane placed over the tops of the tanalised wooden bearers and the underneath of the shed floor to protect the flooring from rot. The shed was delivered in sections and the wooden bearers were already attached to the floor. So the damp proof membrane was placed on the ground surface, the wooden bearers and floor on top of the membrane and the membrane was then lapped up inside the wall panels of the shed. So the void under the shed is essentially enclosed in a damp proof membrane envelope.

My question is this; will this arrangement lead to increased levels of moisture / condensation / rot / fungus etc that will hasten the demise of the wooden bearers and flooring?

Many thanks, I advance, for your advice.

Mutley56 :D
Sponsored Links
Ventilation is critical to stop rot. I would advise laying some joists down in the opposite direction to the ones on the floor, membrane over that, then shed on top... but that's just me :)
I agree with i_am_fubar about ventilation being critical. If I understand the OP (Mutley56) correctly, he has created an unventilated envelope under the shed, which might trap moisture and encourage rot.

To start at the bottom; what is the base: earth or slabs or concrete? If earth or slabs, is there a weed-suppressing layer (weeds channel moisture and cut ventilation)?

Next, as fubar said, a layer of bearers/joists at right-angles to those already fixed to the shed floor, provided that raising the shed an extra 2" is not a problem. Where I would disagree with both fubar & the OP is that I would put the membrane UNDER the bearers, so that the bearers are protected from moisture rising from the ground.

I would think of using something like http://www.screwfix.com/prods/33975/Building/Damp-Proof/Damp-Proof-Course-150mm as DPM, and attach it to 2" X 2" bearers by folding it up the sides of the bearers and stapling it at the sides. Doing this first might be easier than trying to fix it after the bearers are in place. I am assuming that if you can't move the shed far, you will be able to rock it a bit to get bearers in at each side.
Dear i_am_fubar and MickBee,

Thanks for your ideas. As the shed was an ex-display model the bearers were already attached to the shed floor and so it was not possible to have the damp proof membrane sandwiched just between the bearers and the shed floor. The dpm was laid directly onto earth, the bearers annd flooring on top of that and the rest of the shed constructed from there.

The shed is a 9' by 9' corner one and pretty heavy. I wonder whether I can cut holes through the membrane to allow air to circulate around the sub floor void area?

Thanks again for your advice.


Sponsored Links
Cut some holes in the sides all the way round and poke some holes in the base of the membane to allow some drainage, otherwise you'll end up with a swimming pool under the shed. Dig a channel around the shed and fill with pea shingle to avoid heavy rain from splashing up from the ground and wetting the lower part of the shed. Also fit mini guttering that drains into a butt or well away from the shed.
Many thanks for your advice Deluks. All received so far shoudl prolong the life of my shed.

Thanks again.

Mutley56 :D

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local