Badly laid Flat Roof felt in wet weather

11 Mar 2024
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United Kingdom

We’ve recently been having a replacement flat roof put on, removing all old felt, under-boards and fascias and replaced with new 18mm OSB and 2 layers of high tensile torch-on felt applied.

The process was held up for a number of days due to rain and damp weather. on one day the torcher had to stop laying as the felt was not adhering to the OSB. The roof was then covered by tarpaulins. After 4 days the team returned to finish laying the felt. The dry felting looked fine(?), however after the first rain it became obvious that there was a lot of rippling/ridging and pooling.

Our questions are what caused this, was it the damp or poor laying?

Also, what can be done to rescue the roof surface to ensure that the water flows towards the drain aperture you can see in the top left of the picture (cover to be fitted)?

What can we reasonably ask the contractor to redo?

Thanks for any advice!

before tarping.jpg
Finished-before rain.jpg
During rain.jpg
before tarping.jpg
Finished-before rain.jpg
During rain.jpg
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1. You could have had a warm roof .
2. Is there insulation between the joist bays?
3. Has a vapour barrier been installed?
4. Using a roof outlet instead of roof falls to a gutter is asking for trouble down the road with outlet blockages followed by leaks.
5. The felt has been badly laid, & poorly torched on - eg. ripples can be seen in the rain.
6. The ponding is due to lack of falls.
7. The drip edges are lacking "drip pieces" - a drip piece is necessary above any gutter if you install a gutter.
8. You dont appear to have raised edgings?
9. How has the felt been flashed at the house abutment? A pic please?
10. The original ply deck appeared to be in reuseable condition - so why was MDF substituted for the ply?
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Oh. So does that mean the laps are in the right direction or not?

Aren't kerbs and drips fashionable anymore?
Ree, thanks for your reply.

Here’s the answers to your helpful points:

1. No. see next point.

2. Yes, 200mm Rock Wool inserted between joists, above ceilings and below OSB boards.

3. No vapour barrier

4. The ‘roof outlet’ was the existing system. Awaiting fitting of cover and leaf guard. See photo of original roof. Not able to have gutter/drainpipe in existing location.

5. There were 2 layers, laid at right angles. Can another layer be laid, somehow, to ‘iron out’ the rippling?

6. If another layer were to be laid over existing could this be used to re-align the falls towards the roof outlet system? Or would all the felting have to be taken up, the falls aligned and then the felt re-applied?

7. Not able to have gutter/drainpipe in existing location.

8. Correct. Not sure how this is reversible.

9. See new photos of ‘Flashing’

10. The OSB board shown in the first photo is the replacement board. The original board and fascia boards were rotten and required replacing. (See new photo of original roofing.)

Once again thanks for your help. Much appreciated.


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Your contractor has removed the kerbs! He should have built falls into the new roof, installed a drip detail and gutter.
The insulation set up will not work.
Google cold vented flat roof or warm flat roof to give you a better idea.
Your deck will rot very quickly with your existing hash up.

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