Advice sought on re-roofing an outbuilding

Hi all,

I have a block-built, single-skinned outbuilding, which I am planning on converting into a workshop:

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roof is corrugated asbestos cement, with a strip of clear corrugated plastic down the middle to let in light. The supporting timbers are rotting away, and need replacing. The building is about 5m long, and 3m wide. The roof slope is 4.7 degrees.

I want to fit a new roof to the building. Ideally, the roof should satisfy the following criteria: it should let in some natural light (there is only one small window), it shouldn't overheat in summer, it should be bearable in winter, it should be possible to do it as a DIY job.

I think I have the following options, but I would appreciate some comments as to their feasibility:

- Twin/triple wall polycarbonate, supported on timber joists running the length of the building (apparently they can't be supported on beams running across the building as that wouldn't provide sufficient support, and having self-supporting beams would be too expensive). Would this get too hot in summer? Would it be warm enough in winter?

- Solid roof, covered in EPDM, with one or two skylights. I think that this would keep the workshop at a more stable temperature. Having looked at the way my garage roof is constructed (wood strips on top of the walls, joists on top of those, fascias around the edge, and plyboard on top) I am sure I could build something like that myself, but do the same basic principles apply to a roof on a 4.7 degree slope? Does anyone know of any guide on the web, or a book, that details how to construct such a roof?

With the second option, the only skylights I have found seem to stick up quite a long way (20cm or so) above the roof (the kerb), with a dome on top of that. Does anyone sell skylights which have a lower kerb?

Thanks,

dan.

Reply to
public
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One reason your roof structure is rotten is because its flat. Pitched roofs can survive all sorts of defects, many indefinitely, a flat roof can survive almost none. My first design decision would be to put a pitched roof on there if you possibly can.

4" block walls and warm in winter dont go together.

then you want roof insulation. You'll also want to avoid bare metal sheet roofing as it causes condensation.

If you use the clear plastic sheets on pitched woodwork, and fit the insulation below the sheeting with a gap between insulation and plastic, you could do a little trick. Add a fan to cycle the shed air thru that gap, and the fan has slats that shut off airflow when not wanted. Now, you can control your solar gain. In winter, with triplewall sheet you can run the fan when the air up there is warmer, and thus warm the place. In summer you leave the fan off and the insulation keeps the heat out. Triplewall is pricey though, you'd need to work out whether the payback would justify the outlay.

If you want it warm youre going to need to insulate the walls, 4" just doesnt equal warm.

You can also box round a couple of areas to give roof windows. Or better, a long strip all down the centre. Secondary glazing with this would improve insulation, even something very basic like polythene sheet would trap some air, while it lasted.

NT

Reply to
meow2222

It is pitched in the picture I saw ???

Reply to
SJP

Flat? - it's running down about 18 inches over 16 feet (5m)...two courses of blocks disappear along the angle of the pitch.

Reply to
Phil L

You are going to a lot of work and a lot of expense for a bit of daylight, which incidentally, won't be much use on dull days, which account for about

80% of winter days. - if it were mine I'd scrap the rooflight ideas altogether and have a solid roof and two light fittings installed, either fluorescent or wahtever takes your fancy - mixing roofing materials like this is asking foir trouble, especially if you want to DIY and not spend much money.

Solid roof would consist of 5m joists, supported halfway on a steel support. On top of these I would have OSB boards or exterior grade ply and hire or borrow a roofers blowtorch and fix torch-on roofing felt over the entire surface, 20 years it will last on a pitch of this angle.

Torch-on felt isn't expensive and neither are the boards, the biggest outlay would be for the joists and steel support.

Reply to
Phil L

Agreed! I'm going to dry line and insulate the walls.

I like the idea of a long strip - it has that at the moment, and it makes the whole thing very light. But how would you construct the joint between the clear sheet, and the solid EPDM covered parts so that it was watertight?

thanks,

dan.

Reply to
public

Hi Phil,

At the moment the joists run across the building, rather than down its length as you suggest. Is there a reason why the joists should run one way rather than the other?

How do I determine the required size of steel support, and what sort of price are we looking at?

thanks,

dan.

Reply to
public

If the joists run from side to side (across the shortest span)then that's better still, you don't need a steel support, you just need to replace whatever joists have perished. The reason they would normally run from top to bottom is because the angle of the timber facing upwards (to be roofed on) would be flat.....putting them side to side requires each one to be placed on an angle so that the roof materials are sitting on the full face - not bad when you only have a few (for asbestos), but doing it in timber sheets will require one every

400mm, meaning you will require about ten, and they'll all have to be on the same angle - incidentally, have the joists rotted away at the ends where they sit on the wall? - if they have, the new ones will probably do the same unless they are sealed in some way from the elements
Reply to
Phil L

The thing with a pitched roof is that joints dont need to be watertight, they just need to shed water in the right direction. So for triplewall all you need is an overlap. For corru plastic, there are foam end filler strips to keep the wind out.

If you try all this with the same almost flat roof structure as now, I think you'll just get the same problem down the line.

If you use black roofing you cant do the solar heating trick of course.

NT

Reply to
meow2222

Alternatively, if you want to recreate a flat roof, why not leave the roof as it is, just adding new wood joists underneath, and doing the minimum work necessary on top. That would buy you rather more life than a felt roof.

NT

Reply to
meow2222

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