Purlins needed, but no wall to support them


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Old 11-27-13, 10:53 PM
S
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Purlins needed, but no wall to support them

I have a master bedroom at the outside corner of a home. The bedroom measures 22' x 15' on the second floor of this 2-story home. Just outside this bedroom is a covered balcony that measures 8' wide (it sits along the 15' wall). The roof rafters run parallel to the long dimension of the bedroom and extend from the outer edge of the covered balcony, up to a ridge beam. The horizontal distance from the outside edge of the covered balcony to the ridge beam is 19'6".

The rafters are only 2x8 dimensional lumber, so clearly the rafter span from the outside edge to the ridge beam requires purlin support midspan. And yes, the ridge beam is an actual beam.

The requirement for a purlin is that it must be the same dimension as the rafter, and the strut that supports it must run to a wall, the struts must be spaced no further than 4' apart, and the angle of the strut to the ceiling joist below it cannot be less than 45 degrees. But I have a 15' wide room, and no supporting wall to run the struts to, so I need a floating beam to span the room that the struts can sit on.

I need recommendations on what kind of dimensional lumber I can use to create a beam to support the purlin struts? The beam has to span across the 15' wide room, and would need to support 3 purlin struts along the beam. The original builder, some 30 years ago, used a 2x12 as the purlin (overkill), and a 2x12 as a beam to span the room. But he didn't build the struts correctly, so the whole thing is now twisted out of shape, and the beam seems to have sagged over the years, causing a dip in the roof at the mid-point of the rafter span.

This is in central Texas, so there is no snow load to speak of, but I do have a lightweight concrete tile roof, and although it's called lightweight, it's really not, unless you're comparing it to solid concrete. It's a rather heavy roof in comparison to traditional shingles, adding 9.5 to 12 PSF, compared to even the heaviest of dimensional shingles at just 4 PSF.

Thanks, Jeffrey
 
  #2  
Old 11-28-13, 05:47 AM
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Can you add a picture to make it easier to visualize?
 
 

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