Roof light aperture in a concrete roof?

Discussion in 'Engineers' Talk' started by Jen Lucas, Sep 20, 2018.

  1. Jen Lucas

    Jen Lucas New Member

    Hi - I am looking to install a rooflight in a concrete flat roof which is embedded with steel reinforcement. I understand this could get very £££££ expensive if I want to cut a large aperture which would sever the existing reinforcement. My question is.... how can I find out the span between the existing reinforcement to find out whether it's possible to cut one or more roof light apertures BETWEEN these? Is this even possible?? Thanks! :)
  2. KIAB

    KIAB Well-Known Member

    How big is concrete roof,& what's it's covering, what's the size of the openings
    Highly likely you would affect the integrity & safety of the concrete roof by cutting the rebar.

    Sometimes it's a lot easier to completely remove concrete & go for timber & a warm roof construction, if in a habitable room.
    Last edited: Sep 20, 2018
  3. Jen Lucas

    Jen Lucas New Member

    Hi @KIAB - thanks for the quick reply! The concrete roof is currently over a garage which is being converted into a kitchen. The concrete roof is approx 12"x12". We were looking at a single large aperture for a 1.5m x 1.5m roof light which would surely affect the structural integrity.... but now re-thinking to see whether there's a way to fit smaller openings in between the existing reinforcement.
  4. KIAB

    KIAB Well-Known Member

    I would get it removed, you could have timber joists & the roof light openings the size you desire,make sure it's a warm roof construction,11mm OSB3 then rigid insulation (not sure on present recommended thickness),then 18mm OSB3, covering GRP ideally, will last 40 years or more, with concrete roof there a risk of condensation.
    Last edited: Sep 20, 2018
  5. Isitreally

    Isitreally Well-Known Member

    I think,you meant 12' x 12' not ".

    As KIAB says, remove existing concrete roof and start from scratch.
    It would be far cheaper than trying to cut a hole in the existing roof.
    KIAB likes this.
  6. Jen Lucas

    Jen Lucas New Member

    Spot on - measurement unit fail! 12'x12' is correct ;-)
    KIAB likes this.
  7. Jen Lucas

    Jen Lucas New Member

    Thanks @KIAB and @Isitreally - was hoping for some clever way to punch smaller hols in the existing roof but it sounds like replacing the roof is the better option!
  8. KIAB

    KIAB Well-Known Member

    Not worth the hassle,& you have to consider the integrity/safety of the concrete roof,there is always the risk it could end up collasping into the kitchen below if you start cutting holes in it.
    Plus you have no idea how much rebar has be used,
    With a warm roof you could use 100mm rigid insulation outside, then add more insulation between the joists to bump up the thermal performance (W/mK), plus a warm roof doesn't need ventilation.
    Last edited: Sep 20, 2018
  9. Richard_

    Richard_ Active Member

    Switching to a new pitched timber roof could give you a nice high ceiling in the extension.

    The concrete is most likely to have bars at 100 or 150 centres.

    An alternative is that it might be possible to cut an opening in the middle with a steel frame around the opening because the slab will act monolithicly and the forces will work around the opening. You'd need an imaginative structural engineer to run an FE model to prove it.

    The steel could be in skylight upstand with through bolts to pick up the slab underneath. They would act in the same way as the reinforcement that would normally be used to trim an opening.

    If not then you'd need a couple of trimmer beams and they'll drop your headroom .... however these could also be on top of the slab.

    As for internal climate, a concrete ceiling would moderate the heat, giving you that cool feeling you get in an old cathedral with some vaults. Cover it (and the steel besms) with a really thick layer of insulation and you've got yourself the perfect heat store. I've done a few museums that used exposed slab soffit to reduce the temperature fluctuations for the exhibits. It'd be a lovely space.

    However acoustics could be noisy with hard surfaces on both the floor and ceiling so make sure you have thick curtains at the patio door and a roman blind on the windows. Also Amtico (or similar vinyl tiles) would be quieter than wood or ceramic.
    stevie22 and Jord86 like this.

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