Spacing of roof rafters
 

I am building an enclosed porch using native lumber. The underside
of the roof will be visible from inside so instead of using 2x6
and plywood, I am using using rough sawn 3x6 rafters spaced at
30", and 1x12 pine smooth on one side for the roof decking.
My question is will the 1x12 ( 7/8 x 11 3/4) be sufficient
at 30" spacing? The roof will be covered with asphalt shingle.
Thanks,

--
Jeff

wrote in message
ups.com...
I am building an enclosed porch using native lumber. The underside
of the roof will be visible from inside so instead of using 2x6
and plywood, I am using using rough sawn 3x6 rafters spaced at
30", and 1x12 pine smooth on one side for the roof decking.
My question is will the 1x12 ( 7/8 x 11 3/4) be sufficient
at 30" spacing? The roof will be covered with asphalt shingle.
Thanks,

The 30" may be too far. If you do a google search you can find web sites
that can help with load calculations. My guess is that there is less of a
requirement in Florida than say, Ontario Canada with a snow load to figure
in.

set 2 2x4s 30" apart and span w/1x12

step on it

do you think that's sufficient for your roof?

I imagine what you could do is lay down the 1x12 underneath of a layer of
plywood (osb, whatever) sheathing but make damn sure you're nailing the ply
into rafters and not the 1x12. You'll need to add 3/4 in to the length of
the roofing nail you'd usually use as well.

Also there's a reason that rafters are 24 inch on center vice 30. 4x8 ft
plywood sheets can be easily made to butt halfway into the thickness of a
rafter rather than hanging between them.

These are questions for your building inspector or at least hitting the
library and looking at the code in your area.

On 6-May-2005, wrote:

You'll need to add 3/4 in to the length of
the roofing nail you'd usually use as well.

didn't mean roofing nail.. meant the nails you're using to attach the
sheathing with. my bad

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. That's why I wanted to go with
the rafters on 30" to keep the spacing even
because it will be visible from inside. But I
guess I should go with 24-26" rafter spacing
and just frame in around the wide skylights
with extra rafters to the headers as is
usually done. I am not using plywood
anywhere. Thanks,

--
Jeff

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. That's why I wanted to go with
the rafters on 30" to keep the spacing even
because it will be visible from inside. But I
guess I should go with 24-26" rafter spacing
and just frame in around the wide skylights
with extra rafters to the headers as is
usually done. I am not using plywood
anywhere. Thanks,


If you're using real 3xN rafters, you shouldn't
need to double up on the rafters flanking the
skylights. The problem isn't the ability of the
rafters to hold up the weight of the roof, it's
the ability of the roof-decking to span 30"
with the weight of one man plus tools without
flexing so much that it damages things.

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

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.