Garage roof/workshop options

7 May 2014
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United Kingdom

I have just moved into a house that has a brick built garage and an additional workshop extension on the back of it. The roof is split level and both slope away from the middle joining wall.

At the moment, both roofs are flat felt and are leaking - the workshop excessively so. The garage roof has joists running across the garage roof and what appears to be tongue and groove planks making a solid wood base layer, whereas the workshop has chipboard (I know!) that has become very damp and smells really bad. The joists in this roof run parallel to the side walls and not across.

I would like some advice on how to go about replacing the roof. I'm a very competent DIY'er, but I'm thinking of moving away from a felt style and using corrugated fibre cement sheeting for the whole roof.

Is it ok to install this directly on top of the tongue and groove? Are we going to get problems with condensation or roof sweat if I do this? I was going to put OSB3 down on the workshop to replace the chipboard. Or is it worth simply screwing the fibre cement boards directly to the joists?

Is it worth insulating the whole roof between the joists and boarding the underside of the roof as well?

It's going to be a place where I store power tools/paint etc as well as the usual garage stuff. I'm not looking to spend a fortune, but at the same time I want something that's going to last and be watertight for a while to come.

Any suggestions /help is greatly appreciated :)

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what sort of photos are you looking for? I presume the standard external shots, but anything specific?
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Board the whole lot out as you suggest in osb3 and then I would staple shed felt on the boards with a 6" overlap, then I would use box steel profile roofing sheets to finish instead of cement board.
Here are some pictures that should help give an idea of what I want to do…

From above

Board the whole lot out as you suggest in osb3 and then I would staple shed felt on the boards with a 6" overlap, then I would use box steel profile roofing sheets to finish instead of cement board.

Thanks catlad. Will there be any planning / BCO implications if I use fibre cement or box profile sheets?
Sadly I cant see the images at work as the block photobucket and many others, but it depends how flat 'flat' is in terms of the current roof.

Cement and steel corrugated roofs work well at low pitchs, but the minimum pitch is usually higher than that of a felt roof as far as I have seen. I presume you are not too fussed about what it looks like from outside?

I am not an expert, but I expect if it was me I would take the old felt off, replace the chipboard with OSB or second hand floorboards, and then recover with felt or maybe epdm. I might well also chuck in 50mm of PU foam insulation board, even if I didnt plan to heat it, to make it cooler in summer and keep it warm if you want to spend a few days in there over the christmas break....

Thanks Daniel,

I like the sound of an EPDM and have looked at the Firestone Garage Kit - it looks a good option -

I'm not sure if we can afford to insulate and re-cover the whole thing in one go - I presume it would be ok to install some insulation at a later date between the joists and then board the underside of the roof with ply or something similar?

Or would it be better to install the insulation on top of the boards and then the EPDM on top of that?


You can get those box profile sheets for your garage in one length then screw them straight onto what is there now.
Thought I'd update this post - after researching around I decided to go with an EPDM from Ordered it one day and the whole lot turned up next day via courier. After waiting for the right weather I stripped the old chipboard roof and replaced it with 18mm OSB3 and also added an additional layer of 11mm OSB on top of the garage roof T+G boards to give a flat surface.

I must say after watching the instructional videos a few times, the whole EPDM process was really straightforward and I'm really happy with the results. Still have to put the edge trim and guttering trim on but it's watertight and that's the most important thing.

Here's a few photos for those that are interested…











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