New Garage roof options

Help Support

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.


Established Member
4 Mar 2017
Reaction score
Hi All,

Been off the forum for a few years with work commitments, but happy to say that I've recently moved to a new house with a modest lean-to garage to use as a workshop. Currently the main barrier for me is that It's covered with a failing asbestos cement roof. I'd like to insulate it eventually so a new watertight condensation free roof is essential. Weirdly, the roof is not pitched away from the house. It is pitched down front to back (parallel to the side of the house) so the edge that joins the wall is 90 degrees (and probably why it's leaking)

Footprint is approx 8.5x2.5m

I was wondering if anyone has any experiences or warnings with regards to DIY garage roofs? I think I want to keep the shape the same to avoid planning permission issues. I don't think a steel roof would be appropriate as I don't want the same leaking issues at the joint (even with the best flashing in the world). I am trying to decide between EPDM and GRP as the material. Leaning towards EPDM at the moment as it seems to be a little quicker, but really like the GRP as it can be fixed to the brickwork of the joining wall (and presumably I could flash over the top).

Any advice would be greatly appreciated. I'd love to pay for the pro's to do it, but don't have 5k spare from budgeting for the rest of the house.

I'm looking forward to getting back to woodworking ASAP. Happy to keep everyone posted on the workshop build if that would be of interest.

We had an EPDM roofing system installed by a professional on a similar roof. It was a commercial system and all joints were welded using a hot air gun.
It has been absolutly bullet proof for the last 10 years.
The all in one solution would be insulated metal sandwich sheets. Quite expensive but solves the insulation and watreproofing in one simple layer, also nice and white inside for easy lighting.
EDMP, fully glued to roof and wall, with a upvc cap on wall seam, would work.

But, i assume your removing the asbesto roof, what will you replace it with, if its rafters, insulation and sheet boards then a call planning to check won't go amiss.
You could then see about doing a pent roof draining away from the house.
Normally they only worry about front elevation, so if it's an issue on front elevation, then depending on side height is the side wall. You could leave front face and pitch it from house down to the side behind the front face, so looks same from external view, just trim side wall level.
Having used the insulated metal sandwich roofing recently I wouldn't hesitate to recomend it. 8.5 M might be a bit heavy on it's own but you could split that into two lengths with an overlap and you would be able to fit in a day assuming timber underneath is suitable, and then apply appropriate flashing at the house wall. A traditional lead flashing could be used or speak to the suppliers about bespoke folded metal flashing at the correct angles so you can stagger down the brickwork. Watertight, quick, insulated, white internal ceiling, available in your choice of color and thickness of insulation depending on span supports. Front side and rear trims made to your request.
I replaced the roof of my own garage last year. Stripped it, replaced the firrings end to end to correct the incompetence of the original housebuilders and give a decent fall, reboarded with OSB3. I wanted a stiff roof and couldn't source 25mm OSB3 so this is boarded in 18mm and overboarded in 11mm, screwed and glued with cascamite. The membrane is Resitrix EDPM from rubber4roofs which is solvent glued down in 1m wide strips and hot welded seams. I did the 1 day course to learn how to do the welds and bought a proper barrel style heat gun to do the job.
It was time consuming but the cost was about £1000 for the rubber, a few hundreds for the OSB, few hundred more for the timber to cut all the firrings and screws and glue, few hundred more to replace the UPVC boarding all round and guttering along the long drain edge.
It took several days and the detail work around the edges dragged out over a few weeks into autumn waiting for the weather. Don't leave it until August to start like I did !

Essential tools :
Tracksaw with double track to cut 3m firrings and trim the OSB sheet
Laser to work out all the levels so that the 16 firrings could all be custom cut such that the finished roof would be flat with an even fall even though the roof joists are anything but.
Big prybar to demolish the existing roof - that was a hard day for 2 of us
Hot air gun, nozzle and roller

Tarps and clamps (to secure them around the edges) to keep it dry until fully weathersealed
My recommendation would be GRP on T&G OSB and particularly Flexitec 2000. As the name suggests it's more flexible than standard GRP and is extremely durable - suitable for decks and walkways. It's very much within the capability of a competent DIYer.
The system includes flashings for your situation and I would then track the wall with a saw and lead flash over as a belt & braces approach.
I doubt you’ll get a firm answer of grp or epdm. Both have pros and cons but both would be suitable. I’m epdm biased, having done 3 roofs with it. I’d be using a single piece to avoid the welding need. I did a 6x4 in a single piece, more area than you have, it was heavy but manageable.

If you go the epdm route then think about a warm roof design, it’s a lot easier to install and has no thermal bridges. You have the roof deck, insulation glued to it, and epdm glued on the insulation. It ends up a bit higher than the old roof so that’s one to watch for.