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rafter lateral fixing - bird nose top and bottom

Discussion in 'Building' started by CChris, 1 Nov 2018.

  1. CChris

    CChris

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    Hi,

    Im currently building a small(ish) garden building with pitched roof and am looking for advice or reassurance on how to tie rafters at the top.

    The roof has been designed what I understand to be built "traditionally" with single ridge steel and rafters.

    The beam has a bearer board bolted on top, and the rafters are bird nosed both to the bearer board and the wall plate.

    My understanding with the engineer is that spread is not an issue given the size of the ridge beam (it was specified that the celing would be vaulted without collar/rafter ties so the beam is quite large)

    My concern/question is how do I prevent the rafters movig latterally (pic attached), or is it just reliant on the loading of the rafters with the tiles etc.

    Would it be acceptable to use wind/hurricane ties between both the ridge bearer and the rafters, and again at the bottom between wall plate and rafters. Should the two rafters at the top be mechanically fixed to each other.

    I'm not a great fan of cross nailing as that resulted in the split of one rafter and having to redo. After that they were piloted and a single nail put through just to hold in place while I get an answer to this question on belt and braces fixing.

    Any advice greatly received.

    [​IMG]
     
  2. tony1851

    tony1851

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    With a (relatively) rigid steel ridge, and the rafters well-fixed at the top, there should not be a problem with rafter spread.
    What might happen is that the rafters will deflect slightly and this deflection will have the effect of pushing out on the wall plate (simply because the rafter assumes a curved shape).
    However, as your rafters look fairly large (8 x 2 ?), the deflection should be minimal (assuming the span is not great) and any movement at the wall plate is likely to be minimal and not a problem.
     
  3. CChris

    CChris

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    Hi, thanks for this. As part of the belt and braces, we went over on the rafters.

    Spec for the 28 degree pitch was C16 5x2 (47x125) at 600 centres with a span of 2.25m, we ended up using C24 6x2 (150x47), and 565 centres. I will probably tie across the tops of the rafters with some plate and a few twisted downstraps every other rafter at the wall side just for peace of mind.
     
  4. ^woody^

    ^woody^

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    Excuse me, how can that happen? The rafter can't get longer to push the wall out. o_O

    If anything it will pull the wall inwards, as the end points of a line that bends get closer together not further apart - but we are only talking about nanometres
     
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  5. ^woody^

    ^woody^

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    What you need to do is blunt the pointy end of the nail using the hammer, and this stops the nail spliting the timber.

    The horizontal seat cut stops the rafter moving sideways
     
  6. garyo

    garyo

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    I skew nailed mine to the top plate, but also put a single screw to hold them together just to stop them moving from the hammering.

    Like woody says, giving each nail a tap with a hammer to blunt it works wonders.
     
  7. tony1851

    tony1851

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    Excuse me, it can and does get longer on the underside. This is of course more pronounced in timber than in - say - steel or concrete, which have much higher modulii of elasticity than timber.
    It's just basic beam theory: when loaded, the beam deflects downwards, so the top shortens very slightly, and the bottom elongates slightly, while the centre of the depth of the beam - the 'neutral axis' - stays the same. See the lower diagram in the attached pic.
    This is the reason that raised tie roof trusses are usually fixed to the wall plate with two-part glide shoes; the rafter chord is only nailed firmly once the roof is loaded with tiles and the bend in the rafter has occurred.
    Scan0016.jpg
     
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  8. DIYnot Local

    DIYnot Local

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