Wednesday Morning - January 18th, 2012 @ 5:24am
I-Joist Used as Rafters
(http://www.mrmachinist.net/20x30/pics/ijoist-rafters.jpg)
Here's something I came across, using i joist as rafters. It's a pretty interesting article.
Here is some text from this article:
I-joists are used as roof rafters where high, open ceilings are desired, such as this 2 1/2-story room. The I-joists
are supported at the ridge by an LVL ridge beam. Since there are no ceiling joists to resist outward thrust, the
I-joists must be supported at both ends. The load on the ridge beam is carried by columns to the foundation.
End of selected article text
Seems like a pretty good deal if a person wants cathedral type ceiling and a stronger than normal roof.
I guess the main issue would be to make sure that the supports at the ends go to the foundation. Also,
using i-joists for rafters seem like it would be lighter and would be easier to manage.
How in the world would a person cut the birds-mouth where these would sit on top of the walls top plate?
Would the i joist sit directly on top of the wall without a birds mouth cut? It looks like those i-joists in the
picture are sitting directly on top of that ridge beam. Also, I guess it would be very bad if these things got wet
after a couple days of rain? You would have to put the 4x8 sheets on the roof right away so they won't get wet. Is
that correct?
Does anyone know of any builds here at countryplans where they used this method? Here's the link
to the article: http://www.woodaware.info/PDFs/I-joists.pdf (http://www.woodaware.info/PDFs/I-joists.pdf)
I've done it and wouldn't again unless there was real reason to, such as the required length. It is slower, more heavily detailed and tough to make as strong as a solid sawn lumber roof. Those hangers at the dormer header are ~$6 each. The peak either needs to bear on the ridge fully or it gets another $6 hanger. The tail cut needs to follow the limits in the literature. and the bottom ends get blocking or bridging. It took us another day to detail the roof after setting rafters.
Re: Birds mouth cut.
Cut diagrams for I joist are put out by manufacturers. Georgia pacific is a large one.
http://www.gp.com/build/DocumentViewer.aspx?repository=BP&elementid=4372
p. 24 of the .pdf.
"Bottom flange must have full bearing on plate."
The nice part is with 16" rafters it is easier to achieve the higher R value required in northern climates with Fiberglass or Cellulose insulation.
I had to look at this for my now defunct large house design, on the ridge I'd planned to use a triangle of wood above the beam and on the walls - simpson makes an adjustable I beam bracket.
I was spanning 18ft in a high snow load area and could not be done with normal rafters