Flat roof fall

Discussion in 'Builders' Talk' started by diybsp, Oct 12, 2019.

  1. diybsp

    diybsp New Member

    Hi, hoping someone here can offer some advice.

    I'm building a flat roof extension with the help of a chippie to do the joists, firings etc. I'm just a bit concerned that the firings that he has fitted are too big.

    The joist span is about 3.4m and he has used 150mm joist ripped to nothing. I imagine that too much fall is better than not enough I'm just worried how its going to look down the side of the extension when I do the fascia etc, it looks a massive slope which I haven't seen on other flat roofs.

    Also there is a roof lantern, I was concerned about the water coming off the existing pitched roof onto the new flat roof behind the lantern and it just sitting there. Is this normal?

    Am I worrying about nothing and is this acceptable?

  2. Beeero

    Beeero Active Member

    That’s way to much fall 1:40 is the standard for flat roofing which is about 1.45 deg you can go as low as 1:80 in some cases
    On that span I would have said a 50mm firing would have been sufficient
    Last edited: Oct 12, 2019
  3. dobbie

    dobbie Screwfix Select

    Way too much fall, a recommended fall is 1 : 80 although I think 1 :60 is the ideal fall for a flat roof.
    He would be able to make an ideal firring from 3x2.
    Jord86 likes this.
  4. Jord86

    Jord86 Screwfix Select

    He’s opted for too much fall instead of too little, unfortunately it appears he knows little. Agree with dobbie, 3x2 would be the preferred choice for that span.
  5. diybsp

    diybsp New Member

    Thanks for the replies. What would the ratio for my firings be?

    The sub deck has been fitted onto the firings with paslode nails, I imagine it would be difficult to get these out to redo it? Another thing I'm not happy about is the joins of the boards being supported by a short bit of timber along the join with both boards screwed to it, instead of the boards being cut to both meet on the firings.
  6. Jord86

    Jord86 Screwfix Select

    You can remove the nails with a Japanese pry bar, but the deck will most probably be mullered, especially if he’s driven the nails in deep and not flush with the surface. If the short bit of timber(noggin) is fixed between each joist then it’s ok, but if it’s just floating then it’s not and he’s an idiot.
  7. diybsp

    diybsp New Member

    Thanks, will get a pry bar and try and remove them. Yes the noggins are just floating, thought it didn't look right!

    Regarding the fall behind the lantern, would it be normal to have a fall left to right behind it?
  8. Jord86

    Jord86 Screwfix Select

    Yes it would be usual to direct any rainwater away from the lantern, unless you have a secret gutter/box gutter system next to it.
    dobbie likes this.
  9. stevie22

    stevie22 Screwfix Select

    Perhaps you had a refugee from a fish shop because as others have said it's pretty poor. 1in80 is the standard fall for a flat roof and is fine on a rigid roof. With timber there will always be some deflection so we adopt 1in60 to avoid ponding
    dobbie likes this.
  10. ramseyman

    ramseyman Screwfix Select

    The reason for 1 in 80 or 1 in 60 was to prevent loose chippings to reflect heat from being washed off. If you aren’t going to have loose clippings I can’t see why you cant leave it as it is, anyone enlighten me please?
  11. Beeero

    Beeero Active Member

    Nothing wrong with it other than it will look ****
  12. diybsp

    diybsp New Member

    That was my main concern, it looking like ****

    The thing is I had the 150 joists sat on the block with a smaller ladder system around the outside so as to avoid a big fascia, but now by the time it gets to the pitched roof it will be enormous.
  13. Jimbo

    Jimbo Screwfix Select

    Why not leave it as it is and go for a GRP fibreglass finish.
  14. Tony Goddard

    Tony Goddard Well-Known Member

    My garage, built 1962 has a totally level roof, got up there with a big stabila level - it's a bit odd but seems fine!
  15. ramseyman

    ramseyman Screwfix Select

    Agree with Jimbo, that was my earlier point but it’s the aesthetics that are the problem for the OP
  16. diybsp

    diybsp New Member

    Yeah its just the aesthetics really, and i have to look at it every day, id rather sort it now rather do the rest of it then regret it.

    The span actually is 3.1m from steel to blocks if that makes a difference? Could I use 2x2 instead of 3x2?

    Regarding the fall left and right behind the lantern, it doesn't seem many people bother with it? Cant see any discussions on it anywhere. I guess some water would sit behind the lantern but does it really matter? EPDM finish btw.
  17. diybsp

    diybsp New Member

    Well he has told me that 6 inch firings are perfect and if I use 3 inch the roof will get ponding.
    Funny thing is he ordered 3 inch firings but said the merchant cocked up and ended up with 6 inch.
  18. Jimbo

    Jimbo Screwfix Select

    There are flat roofs with pitches from nothing to enough to put slate on. I wouldn't worry about it personally

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