Lean to shed queries

24 Oct 2018
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United Kingdom
Hi all,

so, sadly, I'd paid a local chippy to build my lean to shed - long story short he's a con man so i'm back to doing it myself....

at the moment, I have a concrete base, between the base and the house is a DPM. The base itself has no DPM.

I am intending to build a raised base from composite bearers. Then fix the wooden shed frame to those. Once the frame is on, I will then make a plywood floor to suit.

Now, do I need to put a dpm below the composite bearers or will the ventilation gap be sufficient?

I will be cladding the shed and will be insulating it, so is it also worth putting a breather membrane or vapaour barrier on the frame (external) before I put the cladding on?
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Should be ok as long as there no obstruction to the free flow of air into under and out of the base, and the ventilation gap is sufficient.

I believe the purpose of the vapour barrier is to prevent warm air leaving the inside of the structure (and condensing on the cold side) so it should be fitted on the warm side of the insulation.

A strip of DPC under each floor bearer, but if they are composite they may be water resistant anyway. You need to be careful with the perimeter detailing to ensure water runs off the frame and away from the floor and does not creep under and then up. Also ensure that the frame and cladding is high enough and protected from splashing.

Whether you fit a membrane to the external frame will depend on what its being clad with.

You don't need a vapour barrier on this type of timber structure, but that may depend again on how its being clad, and insulated, and used.
So, decided against the insulation etc. Reminding myself it's a shed, and will be building a garage later in the year anyway - which will be insulated.


I chose against composite bearers, cost was just going to be too high.

Put the DPM down, used some decking risers (I say some... I used 100) so the bearers are off the floor. Then covered them in 5 year stain (they were pre-treated but can't hurt)

floor covered in 18mm waterproof ply.



got the cladding up and roof on this weekend, I've got the odd little bit of light coming through which I plan to use a flexible sealer on (recommendations for this would be great) Boards were pretty good, but my frame wasn't perfect so there's a few bits that need a little filler (see top right of photo)

obviously you'll notice I haven't put cladding up to the roof, this is because i'm covering this from the outside anyway.

So just a door, the bit of remaining floor and the roof capping/fascia/bargeboard/whatever and staining and i'm done.

Then need the roofer to come put the rubber on.
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looking at this picture, should I be installing vertical rails on the outside to stop the boards warping/moving too much?

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