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Skylight in middle of square hip roof

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There should be no problem in designing this.  The framing around the 
skylight (all four sides) would be in compression and at the wall lines, 
horizontal members above the walls (all four sides) would act as tension 
ties.  The hip members would be in compression and bending.  The hip member 
support at the top would be *horizontal* only; the support at the wall would 
be vertical with the horizontal component taken out by the tension ties.

The trick is that this would have to be constructed as one would construct a 
shell, with centering that will hold the members *higher* than their final 
position or a method of transferring the loads to the members before removing 
the centering.  Remember, that as the load is transferred to the hip members, 
the hip members shorten, and the slope decreases, which causes a higher 
compression force in the hip member, which causes the hip member to shorten 
some more ... .  And, as all this happens, the tension force in the tension 
members gets greater, which causes them to elongate, and ... well, I think 
that you get the picture.

A. Roger Turk, P.E.(Structural)
Tucson, Arizona

Eddie Gonzalez wrote:

. > I've been asked a wood related question.  Being I'm a novice in this 
. > area, I thought someone on the list may have solution.
. > 
. > Q: Owner wants to build a square addition but would like a skylight 
. > (2'x2') in the middle of the roof.  In addition, he wants an inclined 
. > ceiling to match the slope of the rafters -- no horizontal members 
. > between walls.
. > 
. > He may tolerate two horizontal members, from wall to wall, in one 
. > direction, such that posts can be used to support the corners of the 
. > skylight (top ends of rafters).
. > 
. >