Leaking new flat roof on recently completed extension Information, help and advice on maintaining, installing and repairing a flat roof. Post your questions and view answers here

[AndreaG]

Hi. We recently had an extension built (single storey, 13m x 3.5m) and it has a flat roof. It was completed in 2020. There are three vents in it with mushroom caps. The original builder used a modern material (not sure what it was called but it has a plastic appearance and seemed to be glued on). The roof was never really watertight, but got worse this year. The original builder has long gone. We contacted a specialist roofer, who we have used before for the flat roofs on our bay windows, and he did a good job on those. He recommended refelting it, which we paid him to do. It still leaks. I actually think its worse. He got very aggressive when we asked him to come back and fix it, and quite frankly scares me. I've done some DIY repairs where it looked like it could possibly be getting in (coping stones overhanging, around the vents) which has helped, but its still leaking. I've just been quoted £1500 by a leak detection company to definitely find the leak, but I'm not sure if we'd just be better off getting the whole thing taken off and redone (or even if thats possible?). I'd appreciate any advice as it is really getting me down. Thank you

[stoneyboy]

Hi andreag,
The success of leak detection on a flat roof is questionable, save your money for replacement or remedial works.
If you would like to post pictures of the roof we may be able to advise as to the most likely problem areas, please include details of the extent of the internal leaks.
Complete replacement is an option - at a cost!
Regards S

[AndreaG]

Thanks for the reply. I've attached a couple of photos. It's rained so you can see the pooling. I'm not sure how much is normal - the patio had dried from the rain when this photo was taken. I think your right about the leak detection - a lot to pay to be told the roof needs replacing! Inside, we took the vent cover off so the water would come straight out, as advised with a previous bathroom leak. The area around the vent feels spongy to walk on, and it does push the water out faster when you walk on it - the insulation is acting as a large sponge. It starts dripping a few hours after the rain starts, and continues for days after it stops. Thanks for your advice. Andrea

[stoneyboy]

Hi andreag,
I am really sorry for you, you are in a dreadful predicament.
I am not sure about the need for vents through the flat roof, these vents appear to be the main source of the water ingress. Clearly whatever flashing has been fitted around the vents they is "not fit for the purpose". If you have windows in the extension it would probably be best to remove the vents and make good the holes. This would involve interlayering patches within the existing roof layers. As a short-term repair you could try applying beads of silicone mastic along all joins you can get to around the vents.
Standing water on your roof is another issue because freeze-thaw action will seek out even the most minor fault and make them worse.
Replacement of the whole roof may be your best long-term option and you could consider increasing the roof slope within the existing parapet walls. Try to find a contractor for advice, who is a member of one of the flat roof trade associations but do check they are a member.
Regards S

[AndreaG]

Thanks - I've looked up the NFRC so I'll start ringing around the names on the list. The vents were part of the planning approval - one is in a downstairs toilet, one in the wetroom (which has no windows) and one above where a cooker could go (we were trying to future proof it to turn it into a flat for elderly relatives when required). We could probably take that one out but not sure on the legality?
I'm guessing the water should not be pooling at all? Looking at a neighbours extension which was just completed, the pitch does look to be more than what we have. There is about a 12 inch rise in the wall opposite the gutter so there would be plenty of scope to increase the pitch without infringing on the neighbouring property. Thanks for your help.
Andrea

[stoneyboy]

Hi Andrea,
Thanks for clarifying provision of roof vents. Suggest you relocate them so the outlets are in side walls and dispense with the ones through the roof.
Regards S

[stoneyboy]

Hi andreag,
One other thing you may have to address is whether the decking (the top boards of the flat roof structure) have degraded due to the water ingress, especially as you think the area around the vents feel spongy.
Moisture resistant boards should have been used and these would not become spongy.
Regards S