Re-roofing an old detached garage

4 May 2021
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United Kingdom
Hey all,
I've searched the forums but couldn't quite get a specific answer to my question.

I have an existing detached single garage, single skin brick with piers, approx 4.8m x 2.6m and
I'm converting it to a hobby room (not habitable).

The existing roof is corrugated cement /asbestos sheets (which I'm arranging to have removed and disposed of properly) laid on top of 5 joists that span the width of the garage. The joist are 'embedded' in the brick.

The fall in the current roof has been created by the front of the garage being about 2-3 courses of brick higher than the back - so the height gradually decreases as you go front->back. Therefore it's not level lengthwise (but hopefully is width-wise!)

I'm trying to keep costs low, so have for the moment have discounted building up the the walls so they are level all round.

So, if I removed the existing roof (safely) - plan would be to;
- fix wall plates lengthwise on top of each side of the garage (on mortar bed if needed)
- fit new rafters width-wise at say 150mm at 400mm centres
- Insulate between the rafters up to 100mm (leaving a 50mm airgap)
- OSB (18mm) on top of the rafters, then torch on felt (or similar)

However, I can't figure out visualise what the front and rear elevations would look like given the rafters would be sitting at an angle?

Thanks in advance!
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Why not just add some new joists between the existing, board with OSB and finish with EPDM. Cheap, quick and easy DIY job. I don't know about your local authority but mine will accept a garages worth of asbestos cement roofing sheets from private householders free of charge if double wrapped and taped and it's safe enough to do if you follow some basic precautions.
Thanks for the reply and sorry for my slow response.

Two reasons I hadn't given that much thought - the existing joists are laid in with the top brick course (see my terrible pics attached) and secondly head space it a bit tight at the pedestrian entry door / low end of the garage so I'd hoped to find a way of gaining a couple of inches at least.

Hence why I thought about removing the existing joists, putting new wall plates on top of the brickwork - but they won't be level (front to back) perpendicular to the joists, which will run across the width of the garage. Is this ok?

If so, how do you deal with the first and last joists (at each end)

Any help much appreciated!


Sorry, can't see any pictures and I'm confused now. Are you proposing a pitched roof with a sloping ridge? You keep going between joists and rafters.
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You could double up the wall plates (treated 4x2 timber) which would give you extra headroom. Set them on a bed of mortar on the existing sloping walls ensuring you have at least 1:40 fall, ensure you strap the wall plates to the wall (Google holding down straps) and fix the rafters to the wall plate. The wall plates can be covered on the outside by uPVC fascia boards which need to be deep enough to weatherproof the join where wall plate meets brick. A joist size table will give you the correct joist sizes and spacing, OSB on top and I'd finish with EPDM - it's a cheap and easy DIY job.
That's brilliant - thanks so much.

Also meant to thank you for the heads up on disposing on asbestos roof panels - after a bit of digging and phoning around, it turns out there is one recycling site in my council area (South Lanarkshire) that does accept them - as long as they are shorter than 3m and double bagged and taped - Saved me about £500. Funny they don't make that service on the council website o_O

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