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Flat roof design question

Discussion in 'Roofing and Guttering' started by ban-all-sheds, 14 Apr 2014.

  1. ban-all-sheds

    ban-all-sheds

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    Given a flat roof with these dimensions:

    [​IMG]

    I'd like the fall to run as shown, because where the red line is would be ideal for the guttering. It's a single-storey rear extension - the LHS is where the house is.

    I don't want the fall (of around 15cm??) to be seen from outside.

    So - pros/cons of a parapet wall vs an upstand as part of the roof? I don't mind seeing the roof membrane folding over the edge of a fascia, but I will mind seeing that if it's not the same height all the way along.
     
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  3. cotswoldbuilders

    cotswoldbuilders

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    Are you using rubber, ?

    Fit facia board level, then glue the rubber up and over, fit the trim.

    Should all look level from below.
     
  4. tony1851

    tony1851

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    Parapet wall;

    pros; looks much more attractive than fascia.
    cons; must be detailed carefully to get it right first time, and plenty of
    opportunity for bricklayer to get it wrong; more expensive.



    Upstand;

    pros; cheap.
    cons; difficult to stabilize a timber upstand which would probably need to
    be about 225 high overall; deep fascia which will not look good.
     
  5. ban-all-sheds

    ban-all-sheds

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    Probably - from what I've read EPDM seems favourite.

    But am open to alternative suggestions.


    Looking level is what I want,
     
  6. joe-90

    joe-90

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    I bet it's a home gym. :mrgreen:
     
  7. ban-all-sheds

    ban-all-sheds

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    Been looking at the detailing, and encountering potential pitfalls, so I need to look into it more.

    A thought occurred to me - could the roof rest on the inner leaf, and just the outer leaf continued up to form the parapet?
     
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  9. ban-all-sheds

    ban-all-sheds

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    And here I am back with another question.

    Finally getting to the short-strokes of drawings for the larger extension submission, and I'm trying to work out how high to say that the roof will be AGL.

    I know what ceiling height I want, and I can use the published tables to find the joist height, then add on the thickness of the deck, insulation, overlay etc.

    But I want a roof lantern in it, approx like this:

    [​IMG]

    Does the team think that the joists would need to be bigger than if there was no lantern?
     
  10. ban-all-sheds

    ban-all-sheds

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    PS - I should clarify that I'm not really looking for free structural design consultancy here - when it comes to BR submission I will use a SE if the roof light company cannot supply construction details.

    All I need right now is an idea of how deep the roof will be overall so that I can do the drawings for the consultation package. I'll be erring on the side of caution on the grounds that if the roof comes in lower than agreed nobody will get upset, but if it comes in higher.....
     
  11. theoldun

    theoldun

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    Feel you will have to agree that your post does not make sense.
    First you say that you are not looking for free design consultancy, Then you say you are looking for how high the roof will be above FGL , and all you are looking for now, is how high the roof will be overall.
    Is that not a free design consultancy?
    Any how no one can give you that information without knowing whether you want a 1in 40 or 1in 6o fall?
    What imposed load and dead load you require?.
    Is it warm deck or cold deck?
    Is it three layer hot bonded felt or EDPM?
    Weight of roof light to calculate trimmer sizes?
    Is roof light UPVC or timber, double glazed of treble glazed
    Apart from above it is bog standard flat lid with a couple of different ways of constructing.
    Regards oldun
     
  12. ban-all-sheds

    ban-all-sheds

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    Well - I guess I didn't explain myself clearly, but I'll also admit that I didn't think about some of the factors you mention. But basically I wasn't asking how deep the joists needed to be for the roof (although feel free ;) ) - it was more a case of "If they would be X x Y mm without the light, how much larger would they have to be with it?"

    Basically I've got to submit drawings to the jolly old council, showing a roof height. If when the proper design is done it comes in lower than that it will be fine, but if it comes in higher then I'll be ****er**.


    I'm assuming 1:40 -

    Am I right in thinking that for a flat roof, the dead load can only come from the roof itself? i.e. the deck + VCL + insulation + weatherproofing?

    Imposed loads will be snow, and I believe I can look those up for my location, occasional maintenance access and possibly a domestic split aircon unit.


    My apologies - warm.


    From what I've read EPDM seems favourite.


    Doh! - Now I should have realised that's a significant fact. :oops:


    Neither - my preference will be powder coated aluminium.


    Unless the difference is eye-watering, I'd go for triple, to make the overall thermal performance better. And even if it is a lot more £, it might still be worth it if it allows me to reuse my existing patio doors, as new ones of those wouldn't be cheap.




    I'm not averse to paying a professional to do a proper design - just trying to avoid that expense before I've got permission to build the thing.

    Worst case I can just show it as 3m.


    FYI - building related post here: http://www.diynot.com/diy/threads/loadbearing.411614/ which relates to the joist spans.
     
  13. theoldun

    theoldun

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    As you appear to be in a jovial mood, here goes. We will post the worst possible scenario.
    150mm above path level to finished floor level.
    2400 FFL to underside of joist or there abouts. You could cut this down to 2325 if you wish.
    If this was our roof we would go 1 in 40 = 0.025 per metre.
    Minimum fall is 1 in 80 = 0.0125 per metre or split the difference at 1 in 60.
    Snow load for London is 0.75kn/m2 and is included in joists size as is the dead load of joists.
    Dead loads are roof covering,18mm ply, insulation, plasterboard, skim and of course roof light. All straight forward.
    Just had a quick look at your post in the building section, and would appear that you have changed the footprint!!, consequently can not go any further with this post until you decide what way you are going, however to answer your question will not matter whether green wall is load bearing or not, just throw a light duty steel over the top or depending on width (not shown) a non composite concrete lintel. Await your decision.
    Regards oldun
     
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