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Rafter top plate birdsmouth

Discussion in 'Building' started by 23vc, 21 Apr 2018.

  1. 23vc

    23vc

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    Pretty sure I’ve got this right but would really appreciate confirmation
    I’m starting my lean to single storey extension roof soon and was planning on birdsmouthing the top end of the rafter onto the top plate (possibly wrong name sorry) cutting out about a third of the plumb depth then just skew screwing /nailing a couple of fixings from each side down into the plate. Is that ok, or am I supposed to also nail straight into the brick it butts onto as well?
    Cheers
    John
     
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  3. ^woody^

    ^woody^

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    Draw a line parallel to the vertical angle cut, then another line horizontal from the bottom corner, and that's the seat cut - its not 1/3.

    You'll have a job nailing in to the brick, so skew nailing it is. Mind not to the split the rafters - flatten the nail points.
     
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  4. 23vc

    23vc

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    Cheers woody. So a third is too much to cut out? Just thought the deeper the cut the better fixing contact I’ll get. But from the ones I’ve seen it sounds like what you’re saying ie the cut out bit is pretty small
     
  5. catlad

    catlad

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    I normally cut out about a third and I pilot hole
    the timber to stop it splitting when skew nailing.
     
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  6. ^woody^

    ^woody^

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    The deeper the seat cut the less the rafter effective thickness.

    The is no reason to make it 1/3 as the bit of rafter butting up to the face of the bearing plate is not doing anything. It is the seat that matters, the 50mm bearing, so there is no advantage in making the seat cut deeper than the minimum possible to get that 50mm bearing.
     
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  7. 23vc

    23vc

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    Thanks woody.
    Got a related question, if it’s acceptable to post it in the same thread...
    At the bottom end, I’ve got a section over some patio doors where I’ll only have 150mm of masonary between the 4x2 wall plate and the lintel (for the usual roof pitch and lintel upstand reasons) whereas I’ve seen it quoted that you’re meant to have 225mm minimum. To do with maintaining the lintels structural rigidity I think.
    Is 150mm a complete no no? I’ll ask the bco in the week but would appreciate any thoughts. It’ll be cut celcons unless I’m better using something else.
    If it’s a no no, it’s not a massive issue but will probably mean some leadwork around an upstairs window or reducing the window depth.
     
  8. ^woody^

    ^woody^

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    The 225 includes the depth of the plate. If you used the more normal 75mm you would be there. 50mm is probably near enough though.

    If you are joining the wallplate, don't have the joint above the lintel.
     
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  9. 23vc

    23vc

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    Cheers. I think I may have a bigger problem now though, as I’ve been reading on here that for these kind of openings with the roof load above, you’re likely to need a heavy duty lintel as opposed to the standard duty stressline sl90 I’ve got. The opening span is 2.2m.
    Any ideas if I’ll get away with it? (I’ll check with bco in the week).
    The outer skin above the lintel will just be 2 courses of brick.
    I’ve also got a couple of 1.5m openings which may be less of an issue.
    I’ve already got the 3x lintels but I fear they may need to get swapped...
     
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  11. ^woody^

    ^woody^

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    Work out the m2 load from half the rafter span and compare to the lintel design specification.

    Otherwise use two bolted 50 X 200 timbers across the lintel instead.
     
    Last edited: 22 Apr 2018
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  12. CJRatch

    CJRatch

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    We did 1/3 birdsmouth cut on our mono pitched roof.

    I'm sure I read somewhere that theoretically it can be cut more than this and not effect the roof strength something to do with the wall plate would need to be pushed out for it to split at the bearing plate and with a birdsmouth cut top and bottom there are no forces to ever push the wall plate out. I'm sure a more technically minded person could explain it better.
     
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  13. 23vc

    23vc

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    Cheers. I’ll try and work that out and talk to the BCO.
     
  14. 23vc

    23vc

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    Not sure if this is right:
    Load of roof with concrete interlocking tiles 0.45kn/m2
    Half span of roof 1.2m
    Length of lintel 2.2m
    So load over that opening 0.45x1.2x2.2 = 1.19
    The swl figures for the lintel are way above that
    However I’m not overly confident that the above is correct :sneaky:
     
  15. ^woody^

    ^woody^

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    2.52 m2 of roof assuming 30° pitch, I get 3.7kN total live and dead load.
     
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  16. noseall

    noseall

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    You can rest the rafter onto the top plate, without a b/m cut, as long as you anchor down the ends with a truss clip.
    I prefer to do both.
    I know the image below is not a lean-to but it shows a similar top cut on the rafter ends.

    As Woody said - only need to chop out the minimum in order to achieve 50mm seat.
     
    Last edited: 23 Apr 2018
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  17. 23vc

    23vc

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    Thanks, not sure how I got to my figure to be honest. But the swa of the lintel is still way higher than your figure which I’m hoping means I may be ok.
     
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