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Warm insulation over existing felt flat roof

Discussion in 'Roofing and Guttering' started by Custom45, 8 Jan 2020.

  1. Custom45

    Custom45

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    Hi
    The kitchen extension on the property I bought has a flat roof (I reckon 5-10° slope) and is uninsulated, so I want to insulate it. The felt is old and although doesn't leak could do with re-covering too.

    I don't want to replace the entire roof, due to time & budget restraints. Existing roof looks sound, I've pulled down a few ceiling boards, timbers all look ok.

    Can I fit new wall plate & rafters over the top of existing roof, lay 100mm Kingspan in between, 18mm osb over the top then EPDM?

    This way I can increase the pitch of the roof to say 15° and also get my insulation in without a complete rebuild.

    Is this a recipe for disaster? Am I talking out my arse?
     
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  3. geraldthehamster

    geraldthehamster

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    If you can identify where the existing joists are, you could fix the Kingspan directly to the existing roof, then screw OSB over that and fit the EPDM. I don't see the need to mess around with extra timbers unless you need to change the roof pitch. You'd need to give some attention to the eaves amd guttering detail, and perhaps fit a higher fascia
     
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  4. Custom45

    Custom45

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    Thanks. My only real reason for raising the pitch is so that it's not classed as a flat roof for the insurance, I read insurers don't like them.

    I guess it's not a major problem though, for the cost & effort involved it probably isn't worth it.

    I was thinking just fixing the Kingspan down with adhesive foam then laying EPDM directly on top. I can lay it without having to walk on the Kingspan, but that probably means not being able to walk on the roof in future.

    Think I'll go with your suggestion Gerald, I'll locate existing joists and fix the Kingspan & osb to those.

    100mm Kingspan (£235) or 120mm (£290)?

    11mm osb be ok? Or go with 18mm?
     
    Last edited: 8 Jan 2020
  5. catlad

    catlad

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    11mm will be fine but 18mm t&g will be easier to work with and will give a better finish. You don't need to find the joists if you don't mind the screws pocking through below.
     
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  6. Custom45

    Custom45

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    Cheers. I'll try and screw into joists if I can, it'll always make for a firmer hold than through ceiling boards and a neater job.

    I'll pull the rest of the ceiling plasterboard down & replace it all, do I need to worry about adding a vcl/vapour barrier or anything?
     
  7. catlad

    catlad

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    The old felt should act as vlc.
     
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