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Rafter tables and slope

rktect 1

rktect 1

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Joined
Oct 20, 2009
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1,117
Location
Illinois
I was just wondering if anybody knows if the rafter tables provided 802.5.1(1) through 802.5.1(8) are good for all slopes. I'm really just curious as it seems like it would be getting pretty flattened at 3:12 or 2:12 slope. 802.3 seems to imply that 3:12 slope or less requires a ridge beam to be designed.

Maybe that is the answer.
 
Re: Rafter tables and slope

Go way back to your 2nd year statics class. You'll remember that for practical purposes, the effective span is really what matters with rafters. The F sub b is the typically the controlling factor and that will come from trigonometric load component acting perpendicular to the grain.

So yes, within limits, it is safe to use the tables regardless of slope.
 
Re: Rafter tables and slope

or assume that we've managed to dumb it down for the worst cases..

use the tables..
 
Re: Rafter tables and slope

rktect 1 said:
I was just wondering if anybody knows if the rafter tables provided 802.5.1(1) through 802.5.1(8) are good for all slopes. I'm really just curious as it seems like it would be getting pretty flattened at 3:12 or 2:12 slope. 802.3 seems to imply that 3:12 slope or less requires a ridge beam to be designed.
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The prescriptive code is always safe. The tables are part of the prescription.

The situation is that for some cases an actual computed answer will give you a few more inches of span and allow you to get by with smaller rafters. If that is important, you do the math.
 
Re: Rafter tables and slope

Use the rafter tables for all slopes. At slopes less than 3:12 you essentially have a floor, so it is the supporting construction that has to be beefed up.

R802.2 Design and construction.

The framing details required in Section R802 apply to roofs having a minimum slope of three units vertical in 12 units horizontal (25-percent slope) or greater. Roof-ceilings shall be designed and constructed in accordance with the provisions of this chapter and Figures R606.11(1), R606.11(2) and R606.11(3) or in accordance with AFPA/NDS. Components of roof-ceilings shall be fastened in accordance with Table R602.3(1).

R802.3 Framing details.

Rafters shall be framed to ridge board or to each other with a gusset plate as a tie. Ridge board shall be at least 1-inch (25 mm) nominal thickness and not less in depth than the cut end of the rafter. At all valleys and hips there shall be a valley or hip rafter not less than 2-inch (51 mm) nominal thickness and not less in depth than the cut end of the rafter. Hip and valley rafters shall be supported at the ridge by a brace to a bearing partition or be designed to carry and distribute the specific load at that point. Where the roof pitch is less than three units vertical in 12 units horizontal (25-percent slope), structural members that support rafters and ceiling joists, such as ridge beams, hips and valleys, shall be designed as beams.

There are beam span tables in the WFCM...hip and valley beams in Table 3.28 and ridge beams in Table 3.29B.
 
Re: Rafter tables and slope

A roof at less than 3:12 doesn't become a floor.

The uplift does change however.

As does the way in which roofing must be installed etc.

At that point, it requires different prescriptive methods or calculated design.
 
Re: Rafter tables and slope

I was just studying these sections this morning because I am designing a roof with flat rafters and was pondering and studing this!

Jake points out the only requirement is to design structural members as beams--there is no requirement that the spans for the rafters/joists be calculated from anything other than the prescriptive tables.

Brudgers comments refer to covering and uplift. I have parapets (no overhang for uplift) and EPDM over a tapered foam deck turning up the parapets.

The ends of the rafters are supported by loadbearing parapet walls.

Does anyone see any basis for having to engineer this?
 
Re: Rafter tables and slope

Robert Ellenberg said:
I was just studying these sections this morning because I am designing a roof with flat rafters and was pondering and studing this!Jake points out the only requirement is to design structural members as beams--there is no requirement that the spans for the rafters/joists be calculated from anything other than the prescriptive tables.

Brudgers comments refer to covering and uplift. I have parapets (no overhang for uplift) and EPDM over a tapered foam deck turning up the parapets.

The ends of the rafters are supported by loadbearing parapet walls.

Does anyone see any basis for having to engineer this?
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The rafter span tables in the IRC do not cover your situation. Posts that were made alluding to different prescriptive methods are outside of the provisions of the code (2006 IRC, Section 802.2). They are however easily calculated, and whether or not engineering is required is up to the jurisdiction in which you are working.
 
Re: Rafter tables and slope

texasbo said:
Robert Ellenberg said:
I was just studying these sections this morning because I am designing a roof with flat rafters and was pondering and studing this!Jake points out the only requirement is to design structural members as beams--there is no requirement that the spans for the rafters/joists be calculated from anything other than the prescriptive tables.

Brudgers comments refer to covering and uplift. I have parapets (no overhang for uplift) and EPDM over a tapered foam deck turning up the parapets.

The ends of the rafters are supported by loadbearing parapet walls.

Does anyone see any basis for having to engineer this?
Click to expand...
Click to expand...
The rafter span tables in the IRC do not cover your situation. Posts that were made alluding to different prescriptive methods are outside of the provisions of the code (2006 IRC, Section 802.2). They are however easily calculated, and whether or not engineering is required is up to the jurisdiction in which you are working.

I beg to differ:

R301.1 Application.

Buildings and structures, and all parts thereof, shall be constructed to safely support all loads, including dead loads, live loads, roof loads, flood loads, snow loads, wind loads and seismic loads as prescribed by this code. The construction of buildings and structures in accordance with the provisions of this code shall result in a system that provides a complete load path that meets all requirements for the transfer of all loads from their point of origin through the load-resisting elements to the foundation. Buildings and structures constructed as prescribed by this code are deemed to comply with the requirements of this section.

R301.1.1 Alternative provisions.

As an alternative to the requirements in Section R301.1 the following standards are permitted subject to the limitations of this code and the limitations therein. Where engineered design is used in conjunction with these standards the design shall comply with the International Building Code.

1. American Forest and Paper Association (AF&PA) Wood Frame Construction Manual (WFCM).

2. American Iron and Steel Institute (AISI) Standard for Cold-Formed Steel Framing—Prescriptive Method for One- and Two-Family Dwellings (COFS/PM) with Supplement to Standard for Cold-Formed Steel Framing-Prescriptive Method for One- and Two-Family Dwellings.

R301.1.2 Construction systems.

The requirements of this code are based on platform and balloon-frame construction for light-frame buildings. The requirements for concrete and masonry buildings are based on a balloon framing system. Other framing systems must have equivalent detailing to ensure force transfer, continuity and compatible deformations.

R301.1.3 Engineered design.

When a building of otherwise conventional construction contains structural elements exceeding the limits of Section R301 or otherwise not conforming to this code, these elements shall be designed in accordance with accepted engineering practice. The extent of such design need only demonstrate compliance of nonconventional elements with other applicable provisions and shall be compatible with the performance of the conventional framed system. Engineered design in accordance with the International Building Code is permitted for all buildings and structures, and parts thereof, included in the scope of this code.

The WFCM should be in every building department library. You are allowed to use it prescriptively. Your engineer is also allowed to use it if you like.
 
Re: Rafter tables and slope

TJ: You are correct; I simply meant the design requirements were not actually contained in the code itself. And of course engineering should not be required if designed to the standards referenced in the code, provided AF&PA has design criteria for the rafter designs we are discussing (I'm not in the office right now). Thanks for clarifying, and sorry if my post was misleading.
 
Re: Rafter tables and slope

You can continue to use the rafter tables for slopes under 3 to 12. My only concern is when you get below 1 to 12 the deflection might be enough to create ponding problems.

I personally prefer to use the Western Wood Product Association's Span Computer (Slide Rule) for sizing beams, joists & rafters.

http://www.wwpa.org/store/product1.asp? ... duct_ID=13
 
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