rafter size - InspectionNews

Robert A. Kuzmick · #1 (**permalink**) 06-29-2007, 12:40 PM

In a cabin that is 20' front to back by 25'wide, I plan on using 2x8 on 12" ctr for rafters on a vaulted ceiling. Underneath will be an exposed beam. Snow load is pretty heavy as it is in upstate NY. Should I switch to 2x10 rafters on 12' ctr? Currently, there are trusses that have been damaged by previous owner(ie: cut/sistered/etc) Any input is very appreciated.

Bruce Breedlove · #2 (**permalink**) 06-29-2007, 02:49 PM

You should pay an engineer (or an architect with access to an engineer) to design this for you.

Bruce King · #3 (**permalink**) 06-29-2007, 09:05 PM

The charts I have require the following info. in order to figure.

Horz. span (not rafter length) of rafter
On center spacing used
light or medium or heavy roof covering
live load required by code in lb per SF
dead load required by code in lb per SF
drywall ceiling or not
wood species used
lumber grade used

Robert A. Kuzmick · #4 (**permalink**) 06-30-2007, 04:31 AM

This is what I know Bruce:
1. 20' span
2. 12 oc
3. medium-I'm putting on a metal roof over existing shingles.
4. live load-minimal,
5. dead load = 60
6. no drywall. t&g pine
7. spf
8. #2

thanks,

Bruce King · #5 (**permalink**) 06-30-2007, 06:52 AM

Are you sure about the 60 psf dead load? Is that due to the T&G ceiling or code?

The best match I can find in the tables are:

50 psf live
15 psf dead

SPF is not even in the table, too weak probably even if 2x12.

Southern Yellow pine on 12" centers:
#2 2x10 = 17' 5" span
#1 2x10 = 19' 5" span
#2 2x12 = 20' 5" span

Hem-Fir will not work unless #1 grade = 20' 5" span

D.Fir-L
#2 2x12 = 19' 8" span
#1 2x12 = 20' 9" span
Select Structural grade (special order) 2x10 = 20' 3"

I would consider eng. I-joists for this application.
i-level.com or search for engineered I-joists
These products come with free eng advice.

Jerry Peck · #6 (**permalink**) 06-30-2007, 11:16 AM

Quote:

Originally Posted by Robert A. Kuzmick

This is what I know Bruce:
1. 20' span

No, as you described it, it's a 10 foot span for the rafters, unless you are planning on a 'shed' style roof.

Bruce Breedlove · #7 (**permalink**) 06-30-2007, 11:06 PM

Yep. 10' span. You should think about hiring an engineer or an architect with an engineer to design the roof for you. In my neck of the woods Regional Building Dept. would require the roof design to have a PE stamp before they would issue a building permit.

Philip Desmarais · #8 (**permalink**) 07-02-2007, 02:35 PM

The higher the pitch the more impact on the roof by wind. I would think pitch would need to be considered.