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nicholas bonham-carter
nicholas bonham-carter Member Posts: 8,576
in THE MAIN WALL
in our 22 ft. by 122 ft building, while bleeding the air out of the rads, i noticed a very small roof leak. a trip to the flat roof revealed a sag of about 21 by 15 ft.

looking through a ceiling access panel, i can see the first of several broken 22ft. by2" by 10" rafters, which are 1 ft. above the ceiling joists. 

my question is is it better to replace rafters completely, or is a "scab" or "sister" repair/operation just as good [talking to the insurance co. today].--nbc 

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  • Mark Eatherton
    Mark Eatherton Member Posts: 5,853
    edited November 2010
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    Sistering is OK...

    provided you get a good solid base support on the bottom of the structural element.



    The dead load capacity of the load bearing members should not be significantly affected by the addition of the sistern joists. There is usually a lot of room due to potential snow/wind loading factors.



    Best bet is to have a structural engineer check it out and specify a fix.



    Bummer... Must have gotten hit by some serious, wet heavy snow eh...



    ME
    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • Jamie Hall
    Jamie Hall Member Posts: 23,284
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    Oh dear...

    A sistering of the rafters CAN work.  Provided it is done correctly, and the sister rafters key in to sound material properly.  This really isn't a job for a carpenter -- I would very much recommend finding a structural or civil engineer who is familiar with timber construction (they're a little thin on the ground -- I know, I am a retired version of same!) to evaluate what you have, and what the best repair is.



    You need to provide for all three major considerations -- shear, moment capacity, and perhaps most of all on a flat roof, bending.  Major considerations are not only the characteristics of the sister itself (dimensions, material) but also the connection details of the sister to the existing rafter and the remaining characteristics of the remaining rafter.



    You should be very concerned on a flat roof.  Once the bending capacity is compromised, it is quite possible (in fact, dismayingly likely) to have a situation where ponding can occur; if the depth of the pond is great enough (and it doesn't have to be much) and the bending capacity of the rafter too small, the rafter will bend under the load, and make the pond deeper... which will make the rafter bend more... which will make the pond deeper... which will... there's no good end to that one.  I've seen it happen.



    DO something before winter, my friend!
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • icesailor
    icesailor Member Posts: 7,265
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    Long Spans

    That is a VERY long span for a 2X10 rafter joist. If it is a flat roof, the snow load and now water load is very high. There's no mid span girt shortening up the span?

    You need a structural engineer to go over this.

    I would think a solution like structural wood I- Joists spaced in between, after you push the sag out and sister up the broken joists. Maybe double up all the joists with I- joists. I'll bet those joists are douglas fir. That's one of the few joists you can get in that length with that structural rating.

    You must live in the snow belt and had a lot of wet snow on the roof and then it rained. It takes a lot of weight to break that many joists. The next one will break the roof and cause a collapse.
  • nicholas bonham-carter
    nicholas bonham-carter Member Posts: 8,576
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    snow belt

    snow belt [omaha ne.]--yes! we had 48 in. last winter, and most of it stayed until spring. luckily that roof, apart from this, has been in excellent shape.

    this is a C.1900 bldg. with that 21/22  feet span as clear with no columns. the wood then as you know was a real size, and probably was douglas fir.

    i don't go up ladders any more, or i would have caught it sooner. there are probably ties between the ceiling structure and the roof structure to help it span that distance.

    i suppose the sister pieces should be on both sides of the rafters, and "glued 'n screwed" into place? the bridging in the ceiling structure is going to make it difficult to do this work from underneath, so perhaps some roof cutting will be needed.

    good advice jamie, i'll look for a structural engineer. thanks to all.--nbc
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