Flat Roof Repair – Modified Bitumen rolled roofing

Flat roof leaks and flat roof repair are often very different and require professional “look” at the roof leaks issue. For most homeowners, flat roof leaks are very frustrating, because only few roofing contractors have the skill and knowledge to properly diagnose and repair leaking flat roofs.

We often get questions from homeowners about their flat roof leaks and this time we were contacted by one such homeowner from London, UK about his roof leaks, and wanted to share this with you as it might be helpful for many other homeowners with flat roof leaks.

Below is a conversion with Ajay – the homeowner with leaking / damaged modified bitumen flat roof (we’ve attached roof pictures that came with each email):

Hi Leo,
Sorry for emailing you “out of the blue” like this but I found one of your posts on this website (Flat Roof Repair article on CoolFlatRoof.com) and thought I would seek your advice. If you don’t feel comfortable
giving advice to a stranger like this, then I fully understand!

The problem is that my neighbor is doing building work and one of his builder’s scaffolding planks fell into our flat roof and made a
dent/hole in the roof. It’s been two weeks now and they haven’t had the courtesy to fix it and I’m now suspecting there could be water damage as it has rained quite a bit over the past few days (I live in London).

I hope you’ll be able to offer some advice as to what needs to be done to do a proper professional repair. I was thinking of going with the
EPDM option – how should this be sealed? What do we do about the board underneath? Do we go with 2-ply 25mm waterproof marine plywood? I’ve attached photos so you can see exactly what the damage is.


Mr Ajay

image of leaking flat roof


My response:

Hi Ajay,

Judging from pictures, you have a rolled asphalt roofing OR granular surface modified bitumen (though I think it’s the first one), that is adhered directly over OSB – particle board.

I doubt there is much rot damage, though the in area right around the hole, the OSB composition has probably weakened. Do you have water stains inside?

I think you should talk to your neighbor to have this fixed OR have your insurance company go after your neighbor and the contractor who damaged the roof – this will force them to get the contractor to fix the damages … or something like that.

As for repair – you cannot use EPDM rubber to fix it. You will have to use same or similar roofing material and all seams will be done either with TAR or torch (if it is mod. bit. roof). Tar does not hold too well, but I guess it held so far.

You would need a whole section replaces – not just a patch, as patch will not hold long. Best bet, is to replace the whole thing if possible, as it looks like a “hack job” to me – the seam overlaps are in the wrong direction – they should be facing the roof slope – away from the building. Insetad the face the building, so all water running down, catches seams.

I don’t know about your local building codes, but 25mm (about 1 inch) waterproof marine plywood seem EXCESSIVE to say the least.

Typical roof deck here in US is 5/8 inch 4-ply regular plywood, and “overkill” is 3/4″ … that’s when your rafters are spaced far apart.

Bottom line – most hack roofers will will throw a bunch of tar on top of the hole, maybe a mesh and more tar. This would work for short time (maybe a year or two). The correct way to fix this would be to replace a section, which can tear the adjacent sections … hence I recommend the replacement if possible.

My reasoning is that they damaged your roof, and throwing tar on it will not make it “whole” or in the same condition as it was – it will be merely a temporary fix.

One more thing – you need a professional roofing company to fix this – not the contractor who caused the damage – they have no clue about flat roofing!

I believe I know a company in London … I’ve seen their work on YouTube and the seem to know what’s up.

Good luck.

Leo – Project Manager – http://www.CoolFlatRoof.com

Second email from Ajay:

Hi Leo,

Thank you so much for your detailed reply. It was most helpful! I’ve had a temporary fix job – a piece of plywood to plug the hole (which their roofer made bigger by cutting away for a closer inspection), then he put some kind of felt??? on top, then he covered the whole lot with some waterproof material(???) and sealed it with tar/bitumen by torch. I’ve attached the photo of the completed work. I’ve also arranged for a professional roofing company to do the permanent fix on Thursday.

If you wish, you may post the conversation on your website to avoid repeat questions and to help others.

Can I ask you another question please related to water leakage through small holes where felt on a flat roof has come away from the main
building wall? Is it best to use lead flashing to seal the felt to the building wall or can some other material be used i.e. the same material
that’s used to fix the actual roof? I was told that if some other material is used, it might start coming away from the wall and leave gaps.



Image of flat roof repair


image of Modified bitumen flat roof repair


My second response:


Hi Ajay,

As far as roof to wall flashing – you need the roofing material to run up the wall, and then use lead or aluminum counter-flashing. You may also seal the roofing material to the wall, but the counter flashing is the best way. Using both, is preferred – seal to the wall and use counter-flashing.

If you do seal to the wall, I would recommend using roofing grade caulk + water stop (also called water cut-off – a caulk than never cures) and a termination bar with masonry anchors to attach the roofing material to the wall firmly.

There are plenty of pictures on our site if you dig deep enough for you to see how this is typically done.

Good luck, and let me know how it goes, and send some finished pictures. I will post this on one of our sites soon.

Sincerely, Leo.

We will continue to post updates to this flat roof repair, as Ajay sends in the update on how things go, and more pictures. If you have flat roof leaks or roof repair issues that you want to discuss with us and get our help with, send in your questions and pictures. You can post them in the comment section below and we will try to help you out as much as possible.PS – the reason we are posting this email is to helps other homeowners with similar flat roof leak problems and with Ajay’s permission. Because many flat roofs have EPDM Rubber Roofing material installed on them, which often leaks, you can post your rubber roof repair questions on our Rubber Roofing page.

BONUS VIDEO: How To Repair a Tar Flat Roof:

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