Concrete Garage Roof Fix/Repair Help

Currently reading:
Concrete Garage Roof Fix/Repair Help

Not open for further replies.


Mar 4, 2022
United kingdom
Hi all,

I am trying to figure out the best way to repair/fix up my concrete garage roof. There are 2 issues as far as I can tell, and I have very little experience with concrete (especially outdoors).

1. It has a draining hole towards the back of the roof, but that hole doesnt seem to be the low point anymore. The roof is no holding water in spots, which seems to be making the walls saturated and never dry.

2. There used to be a raised concrete border that was ramped shaped to stop the water draining off the sides of the roof and make sure it went down the draining hole. The concrete has separated from the roof and I was able to lift off the "ramp" and it was wet underneath, so I think it actually helping keep the roof wet!

The concrete is really coarse, probably from age and erosion?

I am not planning on staying in this house for too long, but don't want to leave the buyers with a rotting garage. One idea was to build a timber flat roof on top of the concrete with a decent gradient and felt it. (about £350 for materials).

Do you think this is the best way to go, or is repairing the concrete in some way a cleaner, cost effective solution? (self leveller of some kind, re do the retaining ramp and treat with something? will the old concrete need prep work for a good bond etc??)

Any way, I am a bit stuck on how to proceed without eating in to my profits when I sell!

Any help would be greatly appreciated.
Father in law had his ‘concrete’ corrugated roof, not sure if you mean a slab roof…
Either way you can replace with ‘box profile’ roofing replacing the corrugated concrete or, as I did over a friends flat slab roof, overpanel it with box profile…you will need to support it at the recommended distances and go no less than 12 degree slope
Solid concrete -
Corrugated bitumen fibre lasts and lasts. You just have to make sure it's well fastened down. Screw pressure treated battens over the concrete and screw the corrugated bitumen over that. Profiles are available for ridges and eaves.

Corrugated concrete -
Remove and replace with OSB sheets over the rafters. Waterproof with felt or corrugated bitumen sheets.
Corrugated Bitumen 'Onduline' good but expensive

I didnt do my research.. I replace cracked 'asbestolux' with the Onduline.. direct to the original rafters ( no boards..)

Lovely and leaktight..and warm... until one hot summer the 'tar' melted and it lost its form

Just held water until it oozed through.. brown sticky water :-(
My parents had an old gargae with similar issues. It was judged too high risk to leve and RSJ's were installed to give extra support before the roof was injected with resin and waterproofed.. You certainly need to be very careful with this. The old concrete seems to have broken down and may be more unstable than at first seems the case, The uneven nature suggests possible failure of the reinforcement. Once a slab has gone rotten the only safe solution is demolition. If you are sure its still structurally sound then you can get products that will help seal the concrete. I have seen success with the application of fine cement screed washed into crumbling concre te,but you still have the problem of the unstable slab being a risk. If you want a quick waterproofing job Screed the roof with cement to get a level and then either felt the roof or fibreglass it. Your could also overbatten, bosrd and felt as suggested above. I have done a fibre glass roof and it was still good after 20 years If your selling its probably best to leave it as is and let the buyer decide how to proceed, They can assess the risk / cost benefits over their own time frame and act accordingly. Because of the potential risk from a failing slab roof I would be inclined not to cover it up just from a safety standpoint.
Lava 20 is supposed to be a good waterproof treatment for stone and concrete roofs but need primer and 2 top coats

Another option is 5o get a fibreglass roof put over it
I ilke fibreglass if the concrete is still strong. Its easy to apply and can be repaired and resurfaced in the future too. You can inject the concrete to help rebond it but this really needs an expert to approve its safe to do this and another epert with the right ger to inject. It would be cheaper than trying to loose the concrete slab. My parents garage was probably jerry built in the war or immediately after and the roof is stil there 10 years after they wnet to pastures new
Not open for further replies.