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Moving collar ties?

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  1. #1
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    Default Moving collar ties?

    I have some storage space in the ceiling of a recently built home in Perth. I'd like to raise 3 of the collar ties by around 30cm if possible. Naturally these are critical members so i want to get my brain around what options may be possible before i talk to an engineer.

    The roof is a simple pitched frame at 25 degrees. The total span is 11.8m with each set of rafters rising to meet at the crown 2.8m above the ceiling level. The rafters are 120x45mm MCP10 pine. The existing collar ties are 1.8m above the ceiling line.

    What structural options are available to supplement to job done by the collar ties?

  2. #2
    4K Club Member OBBob's Avatar
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    Default Moving collar ties?

    I'm not sure... but is it actually storage space or just room in the roof cavity? If you are opening it up to store more and it's not constructed for that purpose then there may be more than just the ties you need to look at.

  3. #3
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    Its actual storage. The area already has 22mm tongued flooring laid over 250mm lvl joists.

  4. #4
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    Are collar ties under compression or tension? I would have thought tension since i cant imagine long spans of 120x35 pine being suitable for compression.

  5. #5
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    Tension
    You raised an interesting question and I've been doing some reading.
    Apparently the true function of the collar tie is to hold the rafter to the roof beam, not to stop the walls falling outward, which job is done by the ceiling joists [ heavily reinforced by the flooring you have added I suggest] in which case you can move them as high as you like. Because you will probably be cutting them in half when getting the clearance you want you may be able to double up with a tie on each side of the rafter and for total peace of mind use 10mm bolts rather than or as well as nails
    "A big boy did it and ran away"

    Legal disclaimer denying responsibility to be inserted here.

  6. #6
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    The Mathematics of Rafter and Collar Ties | Math Encounters Blog

    Naturally you are going to double check with somebody in the industry
    "A big boy did it and ran away"

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  7. #7
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    Actually i've been thinking of using steel rope or rods in place of timber ties. Being in tension, a steel cable should be fine and the added bonus is that its less obtrusive.

  8. #8
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    12mm threaded rod and the brackets work out reasonably cost effective for short lengths but 19mm ply might be even cheaper
    "A big boy did it and ran away"

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