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Flat Roof Drainage And Slope

Discussion in 'Roofing and Guttering' started by Driftwood27, 16 Jul 2020.

  1. Driftwood27


    16 Jul 2020
    Thanks Received:
    United Kingdom
    Hi there!

    I am about to replace the flat roof above a bay window, removing the felt as it is starting to leak/deteriorate, and replacing it with a GRP one. I have a few questions and don’t know whether anyone can help?! I have attached a few photos of the existing roof.

    1) The roof has a water drainage outlet (1.5” lead pipe), difficult to see on the top of the roof. It frequently gets blocked as it is so narrow. The outlet pipe sits near the front edge And to one side of the roof. I think it needs to be replaced with something bigger as it would aid drainage although am concerned about joining plastic pipe to grp. Also the pipe can’t be too big as it needs to go through a narrow soffit. I have seen a solution where a niche is cut in the raised grp trim which allows water to ‘waterfall’ into a gutter running outlet below it but it looks unsightly.
    2) Do I need to slope such a small roof? I know ideally it should slope towards the outlet (1 in 60?). I presume it needs to slope towards the front away from the house, although that wouldn’t push water towards the outlet, does it need to slope from side to side as well? Also, would I use firring strips if it needs to slope? If it does slope side to side then it wouldn’t it affect the level of the grp trim across the bay?
    3) Even though concrete gutter would extend out over the grp upstand at the back of the roof, I presume I would still need a grp flashing?

    Thanks in advance for any tips/advice.

    Hopefully, roof will be better than photos-not sure how to turn second one round on iPad!
    E2E3F6CF-6E11-43B8-9F7F-6CA067B13F22.jpeg D1CE279C-0856-435A-8346-B3F18619DA30.jpeg
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  3. oldbutnotdead


    11 Jan 2013
    Thanks Received:
    United Kingdom
    For that surface area 40mm will be ample. Yes ideally you'll have a bit of a slope sideways towards the pipe, 1 in 100 will work (tbh 1 in 200 will work), the window is high enough that you'll not see any slope on the trim vs the windows
    If you could re engineer so the downpipe was near the wall you can set the front edge level & have slope back toward the wall and slope at the wall towards the pipe.
    Drainage- this is your friend https://www.roofingsuperstore.co.uk/browse/flat-roofing/roof-outlets-edge-trims/tpe-rainwater-outlets/240mm-high.html
    There's a thread on here from me in 2014 asking very similar questions which was what put me onto that source.
    Yes a cover flashing would be a good plan, have a look at what is there already (if lead and in good condition then prise it up carefully, do yr job then bring it back down to cover the upstand trim.

    EDIT That downpipe adaptor- you're best off using a router to set it into the deck before you laminate otherwise it'll be a high spot

    EDIT 2- you'll struggle getting a 100mm upstand in the gap between roof & gutter, never mind a cover flashing. See what is up there, my suggestion would be get the upstand flush to the wall, trimmed to just fit under the gutter & that'll do it- any water getting behind would be coming through the roof.
    Last edited: 17 Jul 2020
  4. ^woody^


    3 Sep 2006
    Thanks Received:
    West Mids
    United Kingdom
    Yes it needs to slope, but only slightly. You don't want standing water on a roof.

    Fit a leaf trap into the outlet, and make it wider than 1.5" on the surface

    Alter that nasty lead pipe with some plastic taken along the frame and down the wall
  5. DIYnot Local

    DIYnot Local

    3 Sep 2019
    United Kingdom

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