Battens to underside existing rafters, best solution?

Discussion in 'Builders' Talk' started by Powerelec, Oct 10, 2018.

  1. Powerelec

    Powerelec Member

    New roof etc , existing raftter on Victorian property, are only 60/70mm, .

    I need to allow for 50mm air gap and 40mm insulation between rafters and then 100mm insulation on top. Best solution to add to rafters for 40/50mm without loading rafters too much?
     
  2. Jord86

    Jord86 Well-Known Member

    Quick answer, counterbatten them. Better answer, look at using tri-Iso or alutherm foil quilt.
     
  3. b4xtr

    b4xtr Active Member

    i had same issue, ended up glueing & screwing 50 x 50 to underside of rafters, worked ok
     
  4. Powerelec

    Powerelec Member

    Thanks, I think I'm going to change rafters 6x2 or 8x2
     
  5. Jord86

    Jord86 Well-Known Member

    A bit extreme, and costly, but if you're set, leave the purlins in and use 6x2, you'll be grateful for the extra headroom.
     
  6. Richard_

    Richard_ Active Member

    Is the issue simply getting enough depth to fit the insulation? Then either batten on top or underneath. Either is a lot quicker and less disruptive than replacing the rafters.

    If you are worried about strength, the extra weight of the insulation is negligible compared to the other loads. For example 140mm of kingspan is just 5kg/m2. It's irrelevant.

    Your rafters have been there for a century without any problems so keep them. Of course if they are sagging, then now is a good time to replace them to get a nice straight roof and ceiling!

    ps - If you screw and glue (with PU) 50mm timber to the existing 70mm rafter it will make them 5 times stiffer for deflection, and 3 times stronger in bending.

    pps - I'm confused by your numbers. You say you need 40 insulation plus 50 air gap which adds up to 90mm. You suggest adding 50mm timber to a 70mm rafter which makes 120mm depth, ie 20mm more than you need. Now you are talking about 150 or 200 deep rafters ?
     
    Last edited: Oct 11, 2018
    Powerelec likes this.
  7. Powerelec

    Powerelec Member

    My confusion, it has to be 100mm between rafters and 50mm air gap so total 150 inside rafters, I think it will have to be new rafters , I'm replacing 2 purlins anyway so might as well replace rafters
     
  8. Richard_

    Richard_ Active Member

    Assuming the original 70mm rafters were strong enough, go for 100 rafters and put more of the insulation as a continuous layer across them to avoid cold bridges. That would also reduce air leakage.
     
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