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What to do with Rafters slightly bowed

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  1. #1
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    Default What to do with Rafters slightly bowed

    Hi guys,

    Progress is going well on my pergola. I have put my rafters onto my beams. All the ones which are very straight, I have nailed to the beams on both sides. However, there are a couple of rafters which have a slight bow in them (horizontal from left to right). The rafters are 4800mm from beam to beam - so fairly long. When I look from below, it is not very noticeable but I worry once the battens/roof go go it might show the bow more. I have only nailed them on one side at the moment while I figure out what to do - if anything.

    I was thinking that when I put my battens on, then I could slightly unbow the rafters before I screw them in to make them a bit more straight. If I had already nailed in both sides of the rafters then unbowing a little might be a bit harder as they may not be much wiggle room. It might also bit quite tricky to actually do this? I am thinking I would just get a bit of wood which is 470mm long and place it between the straight rafter and the bowed one. Or get a spreader etc. I guess I would do it to the middle batten first (which sits in the middle of the rafter and then nail in the other rafter end into the beam and all the rest just get screwed in?

    What are peoples thoughts on if this should be done or not?

    Cheers

  2. #2
    4K Club Member Marc's Avatar
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    Trying to straightening TP will probably prove be a futile exercise. If it is only one or two, I would just replace it and look carefully for a straight one. Does not mean it will stay straight though. Some will some won't unless you can add a support midway.
    Real knowledge is to know the extent of one's ignorance
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  3. #3
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    Default What to do with Rafters slightly bowed

    You could put a row of noggins through the centre using the measurements you have at the ends of your rafters. This should straighten them enough to look ok. Make sure you put temp brace to hold the outside rafters straight while you nog and leave them on until your cross bracing on the roof is done.

  4. #4
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    Very common 'issue' at those lengths and even shorter, whether it be building above your head or building a stud wall frame on the ground first, there is always some pieces that aren't quite right. I would add those 470 noggin/s midway (or whatever the exact distance is as measured at the fixed ends of your rafters), even temporarily, and when the battens are all on and fixed properly it will be all straight.

  5. #5
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    I would say pull it straight with your battens and screw fix them. If it's barely noticeable now it sounds like you don't need to pull it far.

    I had a 6m long beam (190x45) twist on me about 30 degrees end to end. The project was a 6x6 Dutch gable carport. I was in the country so couldn't get a replacement with out waiting for a truck delivery so I used it. I wrestled it into place with clamps and various bolts. Locked into 3 notched posts and rafters screwed into the sides it hasn't moved in 7 years. It was TP also. If there is any residual spring in it you wouldn't know, the carport is solid and straight.

  6. #6
    Senior Member webtubbs's Avatar
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    What they said, but I'd add that if you have 2 bowed rafters, place them so the bows are in opposite directions. That way they are assisting each other to remain straight and not impacting other rafters.

  7. #7
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    Quote Originally Posted by webtubbs View Post
    What they said, but I'd add that if you have 2 bowed rafters, place them so the bows are in opposite directions. That way they are assisting each other to remain straight and not impacting other rafters.
    +1

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    Hey Marc,
    I think you read it like me...thinking bowed up/down, it's actually side/side.

    Boggo, the way to do it is to get the first rafter absolutely straight, then work off that one with your accurately cut block idea, doing all the battens at the same time.
    I have a gut feeling that the rafters will not be all EXACTLY the same distance apart, so you may need to trim the block or a new block for each gap...use the fixed point at one end, or possibly both ends to make sure you are right...

  9. #9
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    Quote Originally Posted by webtubbs View Post
    What they said, but I'd add that if you have 2 bowed rafters, place them so the bows are in opposite directions. That way they are assisting each other to remain straight and not impacting other rafters.
    +1 again. A very good tip!

  10. #10
    4K Club Member Marc's Avatar
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    Quote Originally Posted by David.Elliott View Post
    Hey Marc,
    I think you read it like me...thinking bowed up/down, it's actually side/side.

    Boggo, the way to do it is to get the first rafter absolutely straight, then work off that one with your accurately cut block idea, doing all the battens at the same time.
    I have a gut feeling that the rafters will not be all EXACTLY the same distance apart, so you may need to trim the block or a new block for each gap...use the fixed point at one end, or possibly both ends to make sure you are right...
    Aaaah

    That's a non issue. If your rafters are all leveled and not up or down when you place a string over them, to straighten sideways is rather easy. Blocks is one way if you don't mind the blocks up there, or just pull them into line as you screw the battens down.
    You can do this with a spare batten and 3 F clamps. Clamp the batten down at both ends and with the third clamp fix the bent rafter. Use a hammer to tap the bent rafter into line, check with a string and then screw.
    If you don't have 3 F clamps, screw the spare batten half way at each end and use one F clamp. if you don't have one F clamp ... well buy one
    Real knowledge is to know the extent of one's ignorance
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  11. #11
    Community Moderator phild01's Avatar
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    Problem is if more bend one way than the other, then the whole lot will tend to do the same, best to balance the bends in an opposing way as already mentioned.

  12. #12
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    Thanks guys for your suggestions!

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