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walkereng, Consultant
Category: Structural Engineering
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Experience:  Over 30 years of Structural Engineering experience.
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I'm wondering how to calculate roof purlin contribution to

Customer Question

Hi, I'm wondering how to calculate roof purlin contribution to beam load
JA: OK. Tell me a bit more about what's going on so we can help you best.
Customer: I want to remove a bearing wall and bury a beam in the ceiling to support the weight. Single story and purlins on the roof rafters mid-span transfer load to the bearing wall I want to remove. What fraction of roof weight should be attributed to the exterior bearing fall vs the interior (transmitted through purlin bracing)?
JA: Is there anything else the Structural Engineer should be aware of?
Customer: 20 foot span. Walls on either side are supporting purlins also and are ~12' away. I am not sure yet if joist are parallel or perpendicular to wall. But, perhaps I can get an answer on just the contribution of the load transferred to purlins to the interior wall. Say, if roof load is say 40 psf, should I count 20 psf as load through the purlin bracing (the rest going to the exterior walls that the rafters rest on)? If not, then how to think about it?
JA: OK. Got it. I'm sending you to a secure page on JustAnswer so you can place the $5 fully-refundable deposit now. While you're filling out that form, I'll tell the Structural Engineer about your situation and then connect you two.
Submitted: 4 months ago.
Category: Structural Engineering
Expert:  StructuralEng replied 4 months ago.


Expert:  StructuralEng replied 4 months ago.

When you say purlins, do you mean beams supporting the rafters that are parallel to the ridge and between the ridge and eave?

Customer: replied 4 months ago.
Exactly. There are angled brackets going from the purlins down to by bearing wall in question, and also to bearing walls 12' on either side.
Expert:  StructuralEng replied 4 months ago.

Great. So with that in mind, what exactly is the question? The bulk of the roof load appears to be carried by these purlins.

Customer: replied 4 months ago.
That is my question. What fraction of roof load should be considered transferred to the bearing walls through the purlin supports, versus transferred to the exterior walls that the rafters are sitting on? It is a gable roof in this area and the purlins are mid-span on the rafters on each side. What is accepted practice for sizing a beam to take up the purlin weight that was transferred to the wall to be removed? Or, is there another/better solution such as up-sizing the purlins to span more (24' vs 12').
Expert:  StructuralEng replied 4 months ago.

Based on your description, 12.5% goes to each exterior wall (25% total) and 37.5% to each purlin (75% total).

Customer: replied 4 months ago.
Thanks! What about the "beef up the purlin" question. As I said, I don't know yet if joists are parallel or perpendicular to the wall to be removed. But, assuming for the moment that they are parallel, it seems the only thing making it a bearing wall is the load transmitted through purlin bracing. In this case, would it be an acceptable solution beef up the (2x6) purlin in this area by sistering on additional beam to make it able to safely span 24' instead of 12' in between walls? If acceptable, what size and type beam might I be looking at (tell me if you need more info to specify). This would probably be easier that putting in a large beam above the wall area.Certainly, if the joists are perpendicular, I'd put a beam in the ceiling to support them as well as this purlin load. What size beam (double LVL I imagine) to support the 20' span?Side question: If beam in ceiling to support overlapping ceiling joists, are wood (say, 2x4) "droppers" screwed to the beam (with beam placed on top of joists) and joists with large structural screws acceptable, or do I need purpose-built joist hangers? It would be nice to use droppers if allowable since then I would not need to cut the ends off the overlapping joist ends in order to place them in joist hangers off the beam, and there seems to be ample clearance in the ceiling for the beam to go over the joists.Some additional info that might help: In my area, using 20 LL and 20 DL for roof, no snow. Ceiling load 10 LL and 5 DL (drywall and insulation only, no attic storage).
Expert:  StructuralEng replied 4 months ago.

The joists are typically perpendicular to the ridge (parallel to the rafters). If they are parallel, then you are correct, but that would be atypical.

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