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cross ventilation and issues with roof design with lantern

Discussion in 'Roofing and Guttering' started by martysmarty, 27 Jul 2020.

  1. martysmarty

    martysmarty

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    hi guys


    we got our groundwork/drainage passed this weekend so iv been looking at the roof, i know how do you go from ground to roof? im always looking wayyyy ahead.

    im miffed by the drawing my architect has come up with for my extension. We have plans for a flat roof with EPDM covering and a 1x1.5M roof lantern. there's a few issues i have picked out and i have questioned some of them already.

    i have attached a couple of snap shots of the building regs.

    i cannot see how you would get cross ventilation from either side with the lantern in place. the lantern cutout/framing would be blocking the crossflow.

    jump to drawing 2 picture. roof consists of 150mm timers, with 50mm timers going in the oposite direction with superfoil sandwiched between. 50mm airgap abouve superfoil achieved by the 50mm battens. so if im right, its this 50mm space that neds venting as that is where main difference in moists and temperature changes are going to be.

    soffit side venting would work up to the poiunt where the skylight comes out, then it would block direction from either side?

    we couldnt get a warm roof design due to our houses having slightly higher cielings and the whole permitted development sizes. i wish we went full PP and just went with a taller roof!

    there's a couple other defects but maybe i am missing something

    Details2 picture says joists to be 150mm in depth.

    Details1 says 200 x 50 C16 @ 400ctrs

    flitch beam is a195x50 c16, with an odd sized flitch plate of 170mm?

    it looks like it's computer calculated sizes, and nothings been matched up to equal beam sizes? so 195 would be rounded to 200?? 50mm would be 47mm?

    U beam with 8mm plate, IMO 10mm would be better in terms of jointing depth

    flat roof isnt really a flat roof, there's usually a run off?
     

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  3. martysmarty

    martysmarty

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    been trying to find an example of what i mean. found this picture on google and edited it with good old paint!!

    this is the same style timer frame i imagine it to be

    only thing i can think is not taking the 50mm batten's right up to the skylight upstand frame. maybe leave them 50mm short allowing air to move around the upstanding framework from one side to the other?
     

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    Last edited: 27 Jul 2020
  4. noseall

    noseall

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    Bummer. Would have made a lot more sense.:(


    Can you not ignore the multifoul spec' in those areas that are impossible to vent and just fill those voids completely with PIR board insulation?
     
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  5. Notch7

    Notch7

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    That's a rubbish design for an orangery. Using superfoil is over complicating it.

    You've correctly identified ventilation issues and there's a definite risk of condensation issue.
    Also the design seems like it won't allow downlighters.

    It needs redesigning to be a warm roof.

    The firings for that roof are quite complex with falls in 3 directions.

    What is the PD issue -I thought you could have 3 metres at eves. Maybe you could check what your highest adjacent ground level is and maybe you could squeeze a bit more for the roof.
     
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  6. martysmarty

    martysmarty

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    Hi guys thanks for the replys. I thought i was just being thick but yes i agree it should of been a warm roof to keep it simple.

    If i were to use pir in certain areas im not sure il get it past bc due to u value? I guess id need to work out how much to sell it to bco.

    Downlighters is something i didnt see a problem with but certainly is as we plan on using them. Oh cr*p

    Iv not had much luck with this process this is just a small part of things i have found wrong so far.

    So theres no real way to make it work then? Im gonna have to phone him. I thought it was 3m to heighest point? Il measure up. I know we have 2.55m ceiling height and ground is a little low as the building projects out towards the garden.
     
  7. martysmarty

    martysmarty

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    All information i am seeing suggests 3m max height to top of the roof from ground level where it meets the house. We are building with 2m of boundary
     
  8. Notch7

    Notch7

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    I'm sure you are correct, but I'm not seeing a 3 metre max height at house only at eves

    The PD rules are clear on eves height being max 3 metres for both pitched and flat roofs, but it's not clear whether it can be higher at other parts of the roof.

    https://planningjungle.com/wp-content/uploads/Part-1-of-the-GPDO-Visual-Guide-FREE-VERSION.pdf
     
  9. martysmarty

    martysmarty

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    I wonder if its worth asking my pointed bco? Best of it is i have paid for this design, and also got the superfoil stuff already and that was £400. Nuts

    Can anyone see a reason as to why the flitch beam would be 2x 195mmx50mm when trimmers t1 and t2 are 200? Is it just me or does that seem odd?
     
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  11. Notch7

    Notch7

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    Its just a typing error would be my guess -the majority of structural timber these days is regularised which ends up 195mm rather than 200mm

    I wonder if a treble joist would suffice rather than a flitch -the span is only 3.5M, although I appreciate that is a supporting beam taking some roof load
     
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  12. martysmarty

    martysmarty

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    I did ask him why some of the sizes are odd. Like 50mm width not nominal sizes like 47mm. He said its the software.

    So just to touch on the downlight issue. I take it the issue is the mineral wool is in the way, and once its cut free and clear of the spots, we now have cold spots? Or is it the physical heat the lights generate in the void causing condensation? I know you can get those pods to go inside but not sure how practical it is.

    Ok, other things then. 170mm flitch is odd from looking them up. 180 is nearest so probably have to over spec?

    Im going to throw a sketch together and post it on here just proposing the only way i can see cross flow working. Before i go complaining.
     
    Last edited: 28 Jul 2020
  13. ^woody^

    ^woody^

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    Use abutment vents on the lantern upstand. No problem.

    Or just fill the roof with insulation.
     
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  14. martysmarty

    martysmarty

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  15. martysmarty

    martysmarty

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    Ooooo i really didnt think of that. I know the type you mean.

    Can you elaberate a little on if downlights are an issue? I can see where Notch7 is coming from
     
  16. ^woody^

    ^woody^

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    Use hoods on the back, or the sealed type for bathrooms and suchlike.
     
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  17. Notch7

    Notch7

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    The difficulty is downlighters penetrate the vapour control layer - it can be done and be vapour tight but it must be done carefully.

    The issues are, risk of gaps in VCL and heat getting into the cold void.

    I've stripped down orangery roofs where downlighters have led to serious decay, admittedly from halogen hobbies not LED.
     
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