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Thread: Sistering floor joists with a span of 11.5'

  1. #1
    Associate Engineer
    Join Date
    Jun 2016

    Sistering floor joists with a span of 11.5'

    Hi, I'm looking for some direction on sistering floor joices in a 90 year old home. Original joists are 2x6. They originally were not bearing any weight as it was an attic space. At some point, the walls were raised and a new roof added so the floor of the attic space became usable space, but the 2x6 joists were never supported. We want to sister them with 2x8s. The 2x 6s sit on the sill plate so I can only butt the 2x8s up against the esisting framing unless I notch them. Do you recommend joist hangers? I've been told that notching is not recommended. Also need to know if nailing and gluing is good enough or should we use lag bolts? Any pointers would be greatly appreciated. If minimum bearing length is 1.5" does that only apply to original joists, and not the sistered joists

  2. #2
    Principle Engineer
    Join Date
    May 2015
    The 1.5" bearing length sounds too small by half. You may need hangers.

    Confirm that your 2x8 is appropriate for the span using a span table, available on line, for the type of wood you're using. My reference suggests that it is going to work for about a 40 lbs/ft^2 live load but you should confirm for yourself or hire it done if you do not feel comfortable. My initial guess is that the 2x8's can do the jobs on their own, (based on a look at the joist span table in 'Building Construction Illustrated' by Ching, p4.5, ISBN 0-442-21532-0), so with 16" centers the method of attachment to the other joists
    shouldn't matter. Can the new 2x8's stand proud and support the floor on their own? It might be easier to offset the joist hangers a few inches and raise the floor by a couple inches.

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