Sanity check on outbuilding

Discussion in 'Builders' Talk' started by Villageidiotdan, Oct 28, 2019.

  1. Villageidiotdan

    Villageidiotdan New Member

    Can I just check my logic please. I've built a shed that I want to use through the winter as an extra room.

    I'm about to fix 18mm ply on roof. Am i right that the breather membrane can go directly on the ply itself and it will still be effective please? I plan to put shed felt down which will obviously be immediately on the breather membrane which just doesn't feel right.

    Thanks and sorry to ask.
  2. b4xtr

    b4xtr Active Member

    I reckon if you put shed felt directly on top of breather membrane, the breather membrane will not breath, so pointless
    rogerk101 likes this.
  3. Villageidiotdan

    Villageidiotdan New Member

    Ah, thanks B4xtr. I just can't see a way around it, as the alternative of having the moisture barrier on the rafters, then some battening, then the ply on top, then the felt is going to mean the ply surely gets soaked with the moisture the barrier is repelling
  4. Jimbo

    Jimbo Screwfix Select

    I don't follow the construction build-up of the roof here. Are you saying that the roof decking will be ply and the inside exposed i.e. a vaulted ceiling?
  5. Villageidiotdan

    Villageidiotdan New Member

    Sorry Jimbo, I hadn't really thought about the internal. So, thinking it through, from internal to external it will be some kind of plasterboard->a vapour barrier->the rafters->18mm ply->breathable membrane cloth->roof felt.

    I guess my question in short is should the breathable membrane be under the ply so that the roof felt can sit nicely on top of the ply or does that render it [the membrane] useless?
  6. masterdiy

    masterdiy Screwfix Select

    Dan, is this a flat roof.
    If so, who not pitch it .?
  7. JustPhil

    JustPhil Active Member

    Basically you don’t need a breathable membrane under felt. As you’ve realised, it won’t breathe.
    However, shed felt isn’t the most reliable roofing in the long run, particularly if there’s a plaster ceiling underneath which will quickly show any leaks up.
    If using felt, follow manufacturers guidance on buildup.
    Consider Epdm as an alternative.
  8. Villageidiotdan

    Villageidiotdan New Member

    Thanks Phil. I'd briefly considered Onduline but I don't really like the look of it.

    Ah, ok, will do re EPDM, I assume again it doesn't need the breathable membrane under it.

    I'd originally intended having a live roof but decided that I didn't trust my woodworking skills enough to put 3 tonne of soil over my head :)
  9. rogerk101

    rogerk101 Screwfix Select

    Where is the insulation in all of this?
    Without it you'll be wasting a huge amount of energy heating it in the winter.
    Whatever wood you have up there needs to be well ventilated so that condensation doesn't collect and destroy your plasterboard.
  10. Villageidiotdan

    Villageidiotdan New Member

    Sorry, missed your question. It's a single pitch roof of about 4° about 6m front to back
  11. Villageidiotdan

    Villageidiotdan New Member

    Ah, bl*£dy good point Roger! I planned on using celotex board in-between all the rafters.

    Hmm, Id kind of thought ventilation would be bad as it would let heat out? I think I'd better read up and ultimately put a could of vents in
  12. JustPhil

    JustPhil Active Member

    Get yerself a cuppa and watch this;

    in the next vid he goes into the warm roof. I suspect a cold roof will suit you better mind.
    Celotex in between rafters will be good so long as you leave sufficient ventilated space above, then ply, then epdm.
    I went with onduline because I like the agricultural look!(on a 16 deg pitch, don’t think it works on ‘flat’ proved)
  13. Villageidiotdan

    Villageidiotdan New Member

    Phil, so sorry, thought i'd replied. Thanks for this, really useful. I'm veering towards onduline also as they say 5 degree pitch is OK (did my sums & mine is between 5.2 degree and 5.7 degree...don't ask lol). More as a statement for someone to kick; I don't plan to use breathable membrane as I hear onduline acts as one anyway, I plan to put a vapour barrier (plastic sheeting) down directly on the ply before fixing the onduline.
  14. Hans_25

    Hans_25 Screwfix Select

    I was thinking you need a vapour barrier rather than breathable membrane.

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