dcsimg
Self Help Forums

Go Back   Self Help Forums > Building > Building Codes - Residential
Register FAQ Members List Calendar Today's Posts

Building Codes - Residential International Residential Code 2000 or 2003

Reply
 
Thread Tools Display Modes
  #1   IP: 24.63.236.56
Old February 10th, 2011, 02:11 AM
bobm bobm is offline
Senior Member
 
Join Date: Oct 2004
Posts: 213
Default rafter span tables/No pitch

Hello All,

I am curious why rafter span tables do not include a spec for roof pitch? Load (live and dead) and clear span are considered but not pitch. It seems to me that the greater the pitch the less stress on rafter but perhaps I am missing something. Thanks Bob
Reply With Quote
  #2   IP: 75.110.94.120
Old February 10th, 2011, 04:50 AM
pushkins's Avatar
pushkins pushkins is offline
Senior Member 'Self Help Master'
 
Join Date: Oct 2003
Location: Living in, not from North Carolina
Posts: 1,424
Send a message via Skype™ to pushkins
Default

From an engineers perspective there are, structural engineers often work with pitch calcs. when designing. I think why in residential building your don't see them in span tables is that it's much easier to have a "standard" table covering all pitches. A 3:12 pitch yes will have more live load in regards to snow sitting on it as opposed to a 12:12 with snow sliding off, but the wind loads on the 12:12 is much greater than the loads wind would put on a 3:12.
__________________
Reply With Quote
  #3   IP: 68.95.197.193
Old February 10th, 2011, 06:40 AM
bobm bobm is offline
Senior Member
 
Join Date: Oct 2004
Posts: 213
Default

Good point, In addition, the length of the rafter will be a little longer for the steeper pitches as well, so there may be somewhat of a balance as you stated.

Thanks Bob
Reply With Quote
  #4   IP: 69.97.17.152
Old February 11th, 2011, 10:52 AM
Wgoodrich's Avatar
Wgoodrich Wgoodrich is offline
Super Moderator

 
Join Date: Sep 2002
Posts: 7,451
Default

A rafter span is not figured from the length of the rafter. Instead the distance from the outside wall horizontally to the center of the roof span is the legal rafter span.

The IRC now has in it calculations to address wind load for upsizing rafters required by this calculation. Also the location of your home geographically changes concerning snow load that also affects size of rafter.

Also the ceiling is considered the normal collar tie allowing the rafter size matching the span charts. However if you have say a cathedral ceiling and a collar tie is higher then the rafter is required to be adjusted larger than the chart given in percentage comparing height of ridge versus height of collar tie. Ceiling as collar tie at wall height is 100 % span matching chart. Collar tie installed 2/3 way up to the ridge requires a 50% reduction in span capability requiring a rafter double the size said in the charts in the IRC matching the snow load rating of your area.

Hope this helps

Wg
Reply With Quote
Reply






Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump


All times are GMT -4. The time now is 05:23 AM.


Powered by vBulletin® Version 3.6.7
Copyright ©2000 - 2014, Jelsoft Enterprises Ltd.
© 2002 - QuinStreet, Inc.
http://www.selfhelpforums.com