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Another "flat roof, can't find leak" thread

Discussion in 'Roofing and Guttering' started by dave163, 20 Dec 2020.

  1. dave163

    dave163

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    Hi would really appreciate advice on finding the source of and repairing a roof leak. Setting - Victorian terraced house. At some point kitchen had a single storey extension out at the back through removal of part of downstairs rear wall (above which the brickwork is supported by RSJ). The extension roof has a pitch of about 5 degrees. It is around 3m wide front to back but supported along its length by another RSJ which rests on the other one. The reason for having a beam to shorten the span to 1.5m is because the extension carries the extra weight of a "green roof" consisting felt and sedum plants.

    A few years ago, just prior to laying the green roof we had the extension re-reroofed using roofing felt. Thick plastic liner is laid free floating on top of that and weighed down by the green roof (felt substrate and plants). Along one side and top of the extension is brickwork - on one side the upper floor of the house, and down the kitchen is our neighbour's kitchen extension - the top of their roof is a couple of feet higher than ours.

    The felt on our roof is made to come up the wall by a couple of inches or so. Then there is lead flashing all the way around that is cemented into the wall and comes down past where the felt goes up to and then is squashed down onto the felt for a few inches.

    Despite checking meticulously for gaps, and despite having put sealant in any visible cracks and crevices, whenever it rains heavily we get a little shower coming down in the kitchen, always at the same point which is halfway down the roof where the lengthways RSJ rests perpendicular on the earlier RSJ. There are no damp patches on the ceiling that would indicate water coming in at any point higher than the halfway point where the beam is. I have already squirted sealant all along the top of the lead flashing and along all joints in flashing. I have spent a couple of hours on a dry day pointing a hose at every point around where the edge of the roof meets either of the walls. I have saturated the green roof plants with the hose. Yet none of that resulted in any water coming into the kitchen.

    Photo of where the new beam meets the original beam (you can't see the steel as it's covered in fireproof cladding and an oak covering). On the right hand side below the wooden covering you can see a dark dribble of water where it meets the wall.
    Roofleak2.jpg

    Photo of outside of roof looking towards back of house (I have sealed extensively using a grey sealant over every joint and crevice): 20201219_140930.jpg

    Plan of back of house showing where water is coming in inside the kitchen:
    Roofleak.jpg

    I've had the original roofer back numerous times and he can't find any cause for the leak. He suggested applying roofing tape (and painting some sort of primer so it would stick) on the brickwork and the lead to cover the gap between brickwork and lead flashing, and then again on the lead and the felt to cover that gap. Does that sound reasonable, and has he missed anything? If two hours of hosing it down isn't able to find the leak what chance do I have of finding it any other way?

    One other comment - the previous occupants of the house told us they also had a leak in the same place for some years until a roofer fixed it somehow. So whilst I suppose it's possible the leak could be anywhere on the roof and is tracking along the oak cladding of the RSJ until it reaches the wall, that does seem unlikely given previous problems coming through at the same point and brand new felt applied recently.
     
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  3. datarebal

    datarebal

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    if that is flash band stuck to the wall is not ideal, it looks loose in one place, probably trapping water in which then finds its way in.

    If it is a true lead flashing, it is poorly let into the brickwork. In fact it should be a tray

    take a close look at the window, check it is actually sealed all round to the brickwork
     
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  4. Notch7

    Notch7

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    If your house is cavity construction, I recommend you investigate the cavity above the flat roof.

    If there is water getting into the cavity, it will run down until it hits an obstruction...i.e. The steel.

    The window above looks like it was originally a box sash and the stone cill is still there and a upvc window with face drainage has been fitted. It certainly looks like a potential culprit.

    If you are confident your roof isn't leaking, I would say consider having a cavity tray fitted.
     
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  5. tel765

    tel765

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    Looks like a solid wall - so, if it is solid, then there cant be a cavity tray.

    The abutments of the flat roof to main house or the parapet should have had angle fillets installed to slope the
    felt and pond liner up the brickwork by breaking the sharp 90 degree angle.
    The lead cover flashing should have then come down to cover the upstand - not to go below the liner as you sketch it.
    The cover flashing should remain short of the roof itself.

    From what i can see i'd say the cause of the leak was the incorrect flashing detail - all the sealants and whatevers are a waste of time and only obscure the problem.

    You could remove the flashing and clean up the brickwork - cut 20mm lead chases, and re-arrange the flashing itself?

    The whole flat roof installation looks pretty amateurish - look at the "capping" on the parapet & how the parapet cover flashing is probably introducing moisture into the roof.

    FWIW: just a heads up not an alarm but given there's been leaks over a long period then there's always the possibility of some kind of wood rot forming in the roof.
     
  6. denso13

    denso13

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    As mentioned above, give all round, including below, the window a good soaking and see what happens. I had something similar and sealing between the uPVC window and stone cill cured it.
     
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  7. catlad

    catlad

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    Try leaving a couple of inch gap between the wall and the seedum roof for drainage.
     
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  9. Nige F

    Nige F

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    That is mono pitch ridge tiles. There is a stepped flashing showing @ the top - down the roof. But is there a saddle ,and a flashing down the vertical side under a coat of white paint.?
     
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  10. dave163

    dave163

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    Resurrecting an old thread I started...

    Under the lead flashing, the felt of the felt roof comes up about two inches and is firmly sealed against the wall. In the heaviest of rain it remains absolutely bone dry. However, I pinned down the source of the leak by hosing the wall down along the line where the flashing went into and was cemented into the wall, about 6 inches higher than the roof line. In a few places the cement was starting to crack and crumble.

    The builders returned and (as the roof was under warranty) they said they had not pushed the lead flashing far enough into the wall. Remember this is a solid Victorian brick wall. And this was the second time that they had come back to cement the flashing into the wall.

    So they removed the flashing again, ran their chislling powertool thingummy along and pushed the flashing further back into the wall, perhaps an inch this time rather than half an inch. They then cemented it back in.

    Fast forward a year and the roof is leaking yet again, from exactly the same place. And the new cement they filled the gap between the brick channel and the lead is crumbling and cracking once more.

    Whilst I think I need to find some different builders this time, can anyone suggest what I should be looking for in a good job when cementing lead flashing into a wall? I presume the previous cement was the wrong type - lead obviously expands and contracts (and that particular stretch is south facing so gets the full heat of the sun in summer). So whats the correct technique for ensuring that water can't erode cement and penetrate behind lead flashing where it is embedded in a wall? Is that a particular type of cement that should be used? Or do you need some sort of sticky all weather-proof sealant of some sort, if it exists?
     
  11. RandomGrinch

    RandomGrinch

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    Hi,
    Just a diy'er, so I may be wrong!
    There is definitely lead sealant available:
    https://www.toolstation.com/lead-mate-sealant/p33704
    I'm just wondering if it can be remortared to a certain depth, and then finished with a flexible sealant, such as above?

    Although hopefully solving the problem, I wondered if your next door had a flat roof and any pooling water?
    I've had some major issues with water tracking in from my nextdoors dodgy roof! :)
     
  12. Notch7

    Notch7

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    Ive tended to use lead flashing clips to hold the lead firmly, then use lead seal not mortar.
     
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  13. catlad

    catlad

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    You need a brush and a blow torch to make sure its clean and dry before applying the lead seal and flashing clips, and as tel 765 mentioned cut a couple of inches off the lead as it will only help to wick the water up.
     
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