My house employs purlins to support the roofing rafters. (this video explains almost perfectly what I have in the house). There are two purlins, about 6 feet from each other and the top ridge of the roof. The purlins are 2x8 and are supported by struts at various points.

One of the purlins is in the way of a potential attic access. I keep bumping my head on it. I'd like to move it a foot or maybe 16" away from the other purlin and closer to the wall supporting the rafters. This purlin spans over a room for about 11 feet. At one end it is connected to an exterior wall, at the other end a strut (as depicted in the video).

Ok.. the question: What is the formula/table required that determines the appropriate span and spacing for the purlins? I'd like to use that to figure out how much flexibility there is in relocating the purlin up or down the rafters or replacing the one purlin with two purlins that split the difference.

Update: Some measurements (rounded): The rafters are supporting a single layer of 50-year asphalt roofing. From the ridge to the exterior supporting wall, rafters measure 203 inches. From the ridge to the first purlin measures 25" oc From the first purlin to the second purlin measures 65" oc From the second purlin to the exterior wall is 114" oc Both purlins are independently supported. At the exterior wall, a 2x4 support nailed to 2 2x4 studs that rest upon the top plate of the exterior wall, the purlins rest on the 2x4 in both cases (to be clear, not the same 2x4!) Approximately 11 feet away is the next support, attached to the top plate of an interior wall using a 2x4 strut exactly as represented in the video. Roof is a 4 / 12 pitch roof. The entire structure is on a concrete slab.

Side view of room

  • So you only have 2? What is the pitch of the roof and the length from the ridge to the exterior load bearing wall? If you only have 2 sets that sounds a bit undersized, can you add a set and move the set that is in the way, I say set because I was taught that they needed to be located together for maximum strength one on one side and the other on the other side not staggered but have seen staggered so it may have just been my Dad's preference.
    – Ed Beal
    Jul 2, 2018 at 19:28
  • two seemed like a lot to me.. The pitch is 4/12 the distance.. um I'm not at home right now but it's probably 20 feet from ridge to exterior wall top plate. Rafters are 2x6", the purlins are 2x8" supporting the rafters. When you say "the other side" you mean the other side of the house? I hadn't considered move those at all.. in fact I'm not sure they are there - a previous owner added a large dormer to that side of the house so they probably got eliminated.. otherwise, I'm not following what "other side" means.
    – Eric Horne
    Jul 2, 2018 at 21:08
  • I need to understand the existing supports, 2 at 6' apart is what I would use for a garage. the roof line has been changed we need a simple drawing to be able to provide an answer I never mentioned moving the other set only 2 sets in even a 900 sf home is a bit light if they are needed, is the 20' horizontal or the hypotinuse it makes a difference since you did not provide the height. And the size of anything but the perlins, if you want assistance we need more information, many on this site are professionals with many years of experiance and we can't help other than guess with your info.
    – Ed Beal
    Jul 2, 2018 at 22:07
  • Thanks Ed Beal, I appreciate you are trying to help. I can give you more exact measurements. We might be talking about two different things. The video shows the exact construction of the rafters, purlins, purlin supports and joists, except I have 2 purlins on one side. I can get the ridge to P1 to P2 to ext wall distances, a pic of the purlin supports and the distance between purlin supports. What else is needed? So that I know I'm using the term correctly.. the purlin is attached under and perpendicular to the rafters, supported at load bearing walls at various points.
    – Eric Horne
    Jul 2, 2018 at 22:44
  • oh I misread your question on the distance I gave.. 20' is an estimate, it is on the hyp; ridge to exterior wall. I will get you a more precise measurement later tonight.
    – Eric Horne
    Jul 2, 2018 at 22:51

1 Answer 1


You’re thinking of the “tributary area” on a beam.

Just take half the distance to the next support and that is your “load” (live and dead load) on the beam. They don’t have to be uniformly spaced.

  • That makes sense. Moving the beam away from the other beam will increase it's tributary area.. Now how can I know when I've exceeded the beam's load capacity? I'm having trouble finding information for 2x8's used as a beam spanning these kinds of distances.
    – Eric Horne
    Jul 2, 2018 at 21:16
  • @EricHorne What is the span of the beam you’re trying to calculate, and 2) how far away is the next beam/joist/truss on each side of the beam you’re trying to calculate, and 3) what is on the “tributary area”? (Is it roof, floor, deck, waterbed, etc.? ) Try to be specific. Is there a post sitting on the beam you’re calculating or on the tributary area?
    – Lee Sam
    Jul 3, 2018 at 4:44
  • The clear span of the single 2x8 beam is 11 feet between "posts" on the section I'm concerned about (the purlin spans the entire length of the house, supported at various other points I didn't measure). There is another 2x8 purlin 63 inches to one side and the exterior wall upon which the supported rafters rest is 113 inches away on the other side. It is supporting an asphalt based single roof. As far as I can tell, its purpose is to strengthen the rafters near the top of the roof.
    – Eric Horne
    Jul 3, 2018 at 14:29
  • @EricHorne I’m confused...you mean 65” and 114”, right?
    – Lee Sam
    Jul 4, 2018 at 1:52
  • Yes. As I was making the drawing above, I realized 63" and 113" were inner face to outer face, not on center. So I adjusted and rounded for the drawing.
    – Eric Horne
    Jul 4, 2018 at 13:23

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