Should I add a damp proof sheet between shed base and paving slabs?

JRV · 11 May 2020

Hi all,

I'm about to begin a shed/out door occasionally used office.

I've designed the whole thing prior to beginning the work but wanted to pick peoples brains on what to do with the base in terms of weather/damp proofing. You can see the base below.

Base2

JRV
11 May 2020

I'm tempted to lay the base on a damp proof membrane before fastening the timber to the slabs. Such that the sheet will overlap each side enough to wrap round onto the top of the 100*100mm posts to sit underneath the stud work.

Base

JRV
11 May 2020

Is this a good idea? I'm worried about the wood sweating. The walls and roof will have insulation, a membrane and a 25mm air gap before the cladding/roofing materials are applied. (The inner walls will also be fitted with ply to encase the insulation).

Eventually I'll have a small oil heater to switch on for an hour each day, but maybe not until just before the winter.

Some other pictures to show the overall structure and finish to the building:

Structure

JRV
11 May 2020

Outside

JRV
11 May 2020

Thanks,
Jonny

conny · 11 May 2020

I would not use an oil heater/portable gas fire in a shed. Oil heaters give off fumes and portable gas fires give off tremendous water vapour.
Saying you will switch the heater on for an hour is not really a safe option. If you are working in there with the heater on you may be tempted to 'leave it on a little longer' if the weather is really cold.
If it is to be used as an office you will need a supply of electricity so maybe think about an alternative heat source. An oil filled radiator would be acceptable as they don't give out fumes/condensation.

JRV · 11 May 2020

conny said:
I would not use an oil heater/portable gas fire in a shed. Oil heaters give off fumes and portable gas fires give off tremendous water vapour.
Saying you will switch the heater on for an hour is not really a safe option. If you are working in there with the heater on you may be tempted to 'leave it on a little longer' if the weather is really cold.
If it is to be used as an office you will need a supply of electricity so maybe think about an alternative heat source. An oil filled radiator would be acceptable as they don't give out fumes/condensation.

Thanks Conny. I actually meant an oil filled radiator sorry rather than an oil heater. My bad.

lostinthelight · 11 May 2020

JRV said:
Thanks Conny. I actually meant an oil filled radiator sorry rather than an oil heater. My bad.

An oil filled radiator relies on being on for a long time to heat the oil up.
A fan heater will heat the air more quickly so could be a better bet if you only want it on for short periods

scbk · 11 May 2020

The sub floor timbers will need ventilation to help prevent rot

JRV · 11 May 2020

scbk said:
The sub floor timbers will need ventilation to help prevent rot

I've had a feeling i'd need to do this, i'll have a think and make sure they're not sat directly on the paving slabs. Could a damp proof sheet help too?

Bullet-Proof_Biscuit · 8 Mar 2021

Problem is your DPM sheet, wrapped around the timbers will act like a paddling pool - any water that does get in will never get out..

Ideally use thicker timber joists for the base lengths of your design, and have them raised on pedestals, these could be anything really such as engineering bricks, concrete bought/cast, or large steel bolts or similar, so long as there is never a 'wood sat in wet place' situation, with an air break / ventilation in the design..

JohnD · 8 Mar 2021

is the base a concrete slab?

if you are able to build dwarf brick walls on a slab, even 1 brick high, you can rest the shed sleepers on them with a ventilated void. you can flop dpc over the bricks (not as a big sheet) enough to prevent the timbers touching the damp bricks. The brick walls should be entirely under the footprint of the shed so that rain cannot fall on them or lie on the dpc.

daggermark · 10 Mar 2021

Assuming paving slabs aren't laid yet and are to be exactly the same footprint as the shed, you could put the dpm under the concrete/mortar bed, lay the slabs higher than the ground level, lap the DPM up the sides of the slabs and staple it to the side of the wall sole plate. Then just use the pavers as your shed floor. This should stop any rising moisture and ingress under the walls. I'm guessing you won't like this floor much as an office though - very cold.

To keep a timber subfloor, another option is to lay concrete or plastic posts on top of the pavers perpendicular to your floor bearers, and raise the whole shed up so it is ventilated underneath.

I'm assuming you don't want to pour concrete, hence the slabs?

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Should I add a damp proof sheet between shed base and paving slabs?

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