Leaking flat roof, could I pour liquid screed

Discussion in 'Builders' Talk' started by Flymore, Mar 14, 2023.

  1. Flymore

    Flymore New Member

    Evening all, I have a leaking flat roof roughly 3x3m. Looks like osb/plywood has sagged in couple of spots, more than likely Joists are not in ideal condition.

    Thing is I don’t want to spend money on a roof I plan on knocking down. I plan on building a large extension so need to a cheap temporary fix.

    would I be able to pva dipped areas and then pour a fibrous liquid floor screed over this. Maybe if required paint over acrypol type product


    corrugated plastic sheets an option.

    I need roof to last another 5 years but I need all the money for large extension. Builder wanted to charge 2/3k. To much when I plan to knock down this part of the house.

    suggestions welcome.
  2. Truckcab79

    Truckcab79 Screwfix Select

    Don’t add any weight would be my advice so no screed. Plastic sheet will do at minimal cost. A big roll of heavy gauge DPM is probably your best bet. If you want something less temporary looking then yes, corrugated sheet or onduline sheet will do.
    CGN and Flymore like this.
  3. Hausfix

    Hausfix Screwfix Select

    Pouring a self levelling compound on top of a failed mineral felt flat roof is totally bonkers!
    If the plan truly is to replace the roof in a few years time, all you need to do is go up there and liberally apply several coats of Acrypol to the entire surface.
    Pay particular attention to the areas that you report are sagging by adding a few extra coats. When that has had a week or so to dry, go up there and fill the low spots with a thin layer of 10mm pea shingle, which will help the Acropol from being degraded by the sun. Yes, this may mean water will pool in the low spots, but hopefully it won’t ingress and just sit there. This temporary measure should work for a few years at the very least.
    Attempting to add a layer of some other loose sheet material, be it dpm or corrugated plastic sheet etc, could only be marginally effective for a few weeks and as such, be a total waste of money and effort in this case.
    Last edited: Mar 15, 2023
  4. Truckcab79

    Truckcab79 Screwfix Select

    Mines had a layer of DPM on it for about three years now while I try to find time to replace it. :D Works fine. Costs about £30 and weighs nothing. Previous owner thought it was a good idea to board the roof in MDF which in time got damp and sagged by about 6 inches between joists. It’ll do the job perfectly for minimal costs and time spent.
  5. Flymore

    Flymore New Member

    I climbed on the flat roof yesterday to sort out the tarpaulin. It’s the front of the roof where their are a couple of 20cm sagging points.

    how about if I fill the sagging points with screed and then acrypol the whole lot.

    i was tempted to remove the whole lot and replace with 18mm osb3 but I’m sure I’ll uncover more and then to add flashing etc will make the job too big and costly
    Last edited: Mar 15, 2023
  6. Flymore

    Flymore New Member

    Did you literally plonk dpm on top? Glued down?

    where did you buy it for so cheap? I guess I could clip it down and weigh it down with couple of bricks
  7. Truckcab79

    Truckcab79 Screwfix Select

    Yep. Just laid on top. Screwfix sell rolls at sensible prices. I think even the big 25 x 4m is only about £45

    If you need to stop it flapping you can screw battens down through it into the joist. You’re obviously then making holes in it so make sure you slap a load of gutter sealant or other non-setting mastic under it first. Not silicone.

    it’ll get you through. If you want to fill the dips first a can of squirty foam is an easy bodge.

    None of this would I do for a client. It’s a total lash-up but it’ll get you through for about £60. Obviously the right answer is to replace it properly.
    Flymore likes this.
  8. Flymore

    Flymore New Member

    i think this is the way forward. I have some treated battens left over. Simple solution. I have render on the rest of the building sticking out couple cm above flat roof so I could fasten it down below that. Silicone the rest. Seems cheapest solution
  9. Flymore

    Flymore New Member

  10. Truckcab79

    Truckcab79 Screwfix Select

  11. mdmu

    mdmu New Member

    How did you seal the DPM edge to the roof and stop water seeping in where it meets the existing felt? Will Acrypol or Evercryl bond to it or something else?
  12. qwas123

    qwas123 Screwfix Select


    A similar question has been asked in the other trades talk forum.
    Pics showing the roof surface and the ceiling below might help?
    If your flat roof is over living accommodation then it needs serious attention.

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