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Pete W's Tin Shed Project.
awemawson:
Pete,
Give some consideration to stopping condensation on the underside of your box sheeting or it will quickly rot out your T&G boarding. Perhaps also a membrane of some sort between them? Or maybe an air gap spacing the box sheeting on something that won't rot?
AWEM
Pete W.:
Hi there, Andrew,
Thank you for your prompt about the membrane.
Having given that aspect some consideration, I have the beginning of a scheme in my head. It'll need some exploration on paper and phoning suppliers of material but it goes something like this:
[As they say in the Military, 'No plan survives contact with the enemy!'] :zap: :zap: :zap:
1. Pray for dry and calm weather, repeat as necessary,
2. Procure some insulation (Kingspan or similar),
3. Procure some timber (aka 'lumber') matching the thickness of the insulation,
4. Strip off the many layers of old roofing felt,
5. Examine the existing T&G boards and make good where necessary (or, strip off old T&G boards and replace with shuttering ply),
6. Lay out a heavy gauge polythene DPC over the roof, stapling to existing structural timbers,
7. Position three new timbers as purlins at top edge, bottom edge and midway up the roof, secure to existing rafters, (timbers to be pre-treated with preservative),
8. Add framing timbers between purlins at suitable intervals to stabilise the geometry, skew-nailed or otherwise fixed so as to not intrude into spaces,
9. Cut insulation to fit spaces in frame, seal cut edges with adhesive aluminium tape and fit in position,
10. Position four 1 metre wide sheets of box section steel sheeting, one sheet overlapping two boxes to adjust overall width to match roof,
11. Install eaves filler strips top and bottom. Attach sheets to purlins. top, bottom and midway, using self-drilling fixings with integral seals,
12. Contrive and attach some form of flashing to direct rainwater into existing gutter. (The upper surface of roof will be some 4" higher than the old one.)
As already stated, 'No plan survives contact with the enemy!' :zap: :zap: :zap:
I'll continue in this thread as it's still tin and still a shed, even if not the same shed!! :worthless:
Doubtless, pix to follow, watch this space!
naffsharpe (Nathan):
Hi Pete,
Sounds like you,ve got some work ahead of you! It also sounds similar to my recent rebuild and suggest you might like to consider the following ideas.
1) Frame the roof with timber the same thickness as your insulation board (eaves/apex/facia edges) and across the roof at centres to match the width of your insulation.
2) Lay the insulation into this framework.
3) Fit your ply to the framing timbers.
4) Fit a breathable barrier (ie. Tyvek ) do not use polythene or felt unless you want to redo the job in a few years !! Using polythene or felt means that your condensing layer will, because it is in contact with the underlying timber, simply soak it and help it to rot.
5) Fit purlins from treated timber and use a non setting mastic on their underside to seal the fixings to the existing roof boards.
6) Use more purlins than the minimum.
7) Close the sheets at the verges but not at the apex or facia , the air flow will help to keep the roof condensation free.
8) At the facia/gutter edge lay a piece of roofing felt over the purlin and bent down into the gutter.
I went a bit further with mine and laid a water guide geotextile over the roof sheets and then covered it in a mix of bark chippings/compost and pea gravel before planting it with rockery type plants. This gives me a quiet roof during heavy rainfall (quite common here in N Wales !!) and adds another layer of insulation to the roof.
On my roof which is a single pitch 5m x 5m there is no sag visible even with 100mm of snow on it.
Hope you can use somerhing from the above !!
Nathan.
Pete W.:
Hi there, Nathan,
Thank you for your recommendations.
Your item #1 is covered by my item #3.
I don't understand your item #3. I should explain that the existing roof of the shed consists of T&G boards supported by rafters, surrounded by barge-boards and fascias, the whole topped with several layers of roofing felt. The shed, a nominal 8' by 12', has accompanied me through two house-moves. The wooden part of the roof, whose approximate dimensions are 8' 7" by 12' 4", is actually in two halves and at each house-move I've cut the roofing felt where the sections meet and fitted two layers of new felt overall on re-assembly. After the first house-move, I inserted two lengths of Dexion slotted angle, vertical web upwards, between the two sections, with their ends resting on the front and rear walls. I secured this sandwich with coach-bolts through the two relevant rafters picking up convenient holes in the slotted angle. I have heard this sort of sandwich described as a 'flitch beam', the wooden members prevent the vertical steel webs from buckling despite their being in compression. This flitch beam feature greatly reduces any tendency for the roof to sag in the middle.
I intend to retain all this existing structure except the roofing felt. If the T&G boards have deteriorated (there is a leak in the felt) only then will I replace them with the shuttering ply.
I take issue with your item #4. I plan to fit a vapour barrier of polythene sheet on the top of the T&G boards (or ply) and lay the purlins on that, spaced with noggins and with the insulation material cut to fit the spaces in the resulting frame (your item #1, my item #3). The purlins function is to transmit the load of the steel sheets, either wind induced lift or snow load, to the existing rafters. Because the upper surface of the insulation is flush with the upper surfaces of the purlins, the insulation will share some of the latter form of loading. With this arrangement, the notional condensation zone will be above the insulation, the T&G boards will be in the warm zone. I do not expect condensation to occur at that level, neither will it occur beneath the steel sheets because the vapor barrier will confine water vapour originating in the manned space to the manned space. Also, with this arrangement the T&G boards will support the polythene vapour barrier and prevent it from drooping into the manned space.
Regarding your item #6, see the preceding paragraph above regarding load bearing. I had originally planned to use three purlins, at the top & bottom edges and mid-span but the roof dimensions are such that a fourth purlin will reduce the total area of the spaces in the frame and avoid my having to buy a fourth 2.4 m by 1.2 m sheet of insulation.
Regarding your item #7, the manufacturers of the insulation material specify that it should be protected from the weather. The eaves filler will satisfy this requirement and condensation at the insulation/steel sheeting interface will be minimal because moisture vapour is excluded by the vapour barrier and the eaves filler.
Regarding your item #8, I intend the new roof to be totally free of roofing felt. The manufacturers of the steel sheets offer a matching flashing strip come fascia which I shall arrange to route the rain into the existing gutter.
For access to the central fixings of the box section steel sheets, I plan to attach battens to the underside of a scaffold board to lift it clear of the upstanding parts of the profile.
Regarding your closing remarks, my existing roof also has an organic topping but mine is of moss, acorns, oak apples and twigs. The shed is overhung by a mature oak to the East, a large willow to the West and a struggling apple tree to the North. I expect that these will soon apply a similar topping to the new roof!
This is my second attempt at this reply - I must have hit the wrong key just as I finished it and it seems to have got lost in cyberspace! If it reappears and this becomes a double post, I can only throw myself on the mercy of the Moderators.
Pete W.:
I had a phone call to say the Kingspan (seconds) will be delivered on Monday.
The guy also told me that I don't need to tape the cut edges.
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