Outbuilding advice please

New Topic
This topic has been archived, and won't accept reply postings.
 ben b 28 Nov 2022

I'm hoping the collective ukc van conversion and shed building experience can get us out of a hole...

Mum and dad live in a very small house, which they make up for by building various sheds and outbuildings. Dad seems to have CSBD - Compulsive Shed Building Disorder, I am showing worrying early signs of inheriting this  

An octagonal wooden building has been constructed on the front lawn (because we like a geometry challenge). It's made almost entirely of recycled and reclaimed materials and so the budget is not vast (so far, about 400 quid of total expenditure, mostly on roof flashings).

The roof is wooden, made from fencing panel timber in eight long triangles rising to a central point with a solid finial. The joins are heavily filled with caulking mastic and we have some quite smart flashing to keep the rain out fastened over the top. Which appears not to work. Beneath the timber there is building paper (overlapping) and then foam insulation panels. Water is running down the internal structure - it's hard to tell how much is condensation and how much is leak (but I suspect a lot of the latter). But it has been really, really wet here for the last week or two..

The aim on the inside was to have the lower part of the octagon roof to be cedar tongue and groove, with a central interior upper octagon being plasterboard which my very artistic sister was planning to paint on. Lighting will be via LED strips on a small architrave running round the interior circumference.

The size is the maximum size the planning officer has allowed, so it is all "size legal" I gather. 

So my question is - how do we fix this? Is condensation likely to be enough to explain the dampness inside (it was running off the builders paper so not insignificant)? Do we have to redo the roof from scratch? I was wondering about lifting the flashings and inserting more caulking to any visible gaps, and then possibly using some thin closed cell foam compressed beneath to try and fill in any gaps that might allow wind blown rain to come up under the flashing (e.g. a particularly squidgy thin karrimat type foam)? If it's all just condensation, do we need another layer of tyvek or similar under the foam insulation and a bone the cedar or plasterboard?

Thanks for your thoughts

cheers

b


 nniff 28 Nov 2022
In reply to ben b:

What would happen if you were to spray a hose under the flashing? It looks as if the slightest wind water will get pushed under there, and once on drop's found its way, more will track along its path...

OP ben b 28 Nov 2022
In reply to nniff:

I suspect so... which is why I wondered if the foam cell might fill most of the 'mass entry' bits. The internal builders paper goes over the corner spars, which we assumed would give some protection.

b

 jkarran 28 Nov 2022
In reply to ben b:

I'd guess it's leaking.

Hard to see the exact structure but I suspect the roofing boards don't overlap much, that combined with the gappy flashing won't keep splashes or driven rain off the membrane. You probably need to give more thought to where water can get past the top layer of the roof then what stops it getting further into the structure, where it goes next as liquid and how does what gets damp under the boards ever dry out.

Being a set of identical triangles it looks like it'd make a neat fiberglass project to me.

Assuming you want something more conventional, I'd probably sark it, membrane over that, fix batons on top of the joists with the number of fixings passing through the membrane minimised, ideally only top and bottom under the top flashing and outboard of the wall at the eaves so anything tracking down the screws doesn't drip into the room (but will still rot the joists). Further membrane strips over the batons to cover their fastenings then make sure the board fastenings only go into the batons, not through the lower membrane into the sarking. Water finds the slightest gap! I'd keep the flashing as is if you like the aesthetic or you could make it conform to the steps in the boards reducing splashed/driven ingress. Soft foam fillets in the gaps will probably just blow away or rot in time. Design in some ventilation above and below the sarking/membrane from eaves to a vent under the top flashing. Foam the gaps around the insulation boards (leaving the aforementioned ventilation gap above them) and aluminium tape the seams/joists.

edit: is it my eyes or does the flashing overlap the wrong way in some places, are you reliant on the adhesive backing to keep water from tracking under edges where pieces join? Also you seem to have screwed through it rather than stuck it over the screws.

jk

Post edited at 16:47
 NobleStone 28 Nov 2022
In reply to ben b:

I would avoid squirty foams, horrible stuff and makes the wood difficult to recycle (or burn) at the end of its life. Why not take off the 'flashing' and put roofing felt over the boards?

 Fraser 28 Nov 2022
In reply to ben b:

It's almost certainly leaking. As jkarran said, the gaps below the flashing are too open, it'll be wind driven rain that's penetrating the external shell. I too though the flashings were overlapping the wrong way,  ie. you'd worked from the top down, but I think what's happened is you've used a short, separate piece that overlaps two longer, abutting flashingsto cover the joints. Although it might look nicer done in your 'ship-lap' style, I'd have done that almost as a decorative covering that sat on top of the main waterproof layer. Unfortunately, building paper isn't a suitable membrane to provide adequate protection against water ingress, particularly when the joints in your boards are relatively exposed at each hip member.

OP ben b 28 Nov 2022
In reply to jkarran:

Good points, thanks all. Yes the flashings are slid under the roof cap, and screwed down x2 on each one. There is a joint in the middle which is identically shaped / coloured self adhesive material (without any perforations). I wasn't thinking of the godawful expanding foam, just something that would lie in a long strip under the flashing and fill in the gaps where it wasn't compressed.

I'm going to have to ponder the ventilation question - that's not going to be easy (now)...

Cheers

b


New Topic
This topic has been archived, and won't accept reply postings.
Loading Notifications...