On Sat, 7 May 2005, Wayne Whitney wrote:

Date: Sat, 07 May 2005 20:57:48 -0500
From: Wayne Whitney
Newsgroups: alt.home.repair
Subject: Spacing of roof rafters

On 2005-05-06, wrote:

I realize that 30 is pushing it even with 1" roof decking. 24" is
more common in these parts (Vermont). I am using 3x6 rafters which
are beefier than normal and putting in some 30" wide skylights.

As other posters mentioned, there are two question: are the rafters
strong enough for the roof load, and is the decking strong enough for
the span between rafters?

For the rafter strength question, it is impossible to say without
knowing the rafter span. However, it is worth noting that the load
per rafter is proportional to the rafter spacing, and the rafter
"strength" (shear, bending and deflection) is proportional to its
width. So if your rafters will be a full 3" thick at 30" spacing, their
strength is equivalent to 1.5" wide rafters at 15" spacing. Just
check a standard rafter sizing table for your load and span to verify
that nominal 2x6s at 16" o.c. will suffice.

[BTW, if the skylights are a full 30" wide, then you'd need your 3"
wide rafters 33" o.c. to have a 30" wide space between them.]

As to the strength of the roof decking, my quick calculations
suggested that a full 1" thick material would actually be OK at 30"
o.c. However, I'm not an engineer, so I'm not confident enough in my
calculations to post them, and you shouldn't believe me.

Cheers, Wayne




I didn't see the original post, but I'd probably be talking with the local
planning & zoning people. If its going to be in a house or commerical
facility, they will want to see plans. Ask them what is OK.