# Roof slope less than 3/12



## Rick18071 (Nov 30, 2020)

The IRC does not have a prescriptive way for roofs with less than a 3/12 slope roof. Looking at a car port right now but I may need this info for other projects in the future. As a plan reviewer is there a way to approve a roof with less than a 3/12 roof without requiring an architect?


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## TheCommish (Nov 30, 2020)

using the 2015 IRC

Section R802.2 roof slope greater the 3/12  are coved by the  span tables in sections  802. Slopes less than 3/12 are outside of the scope of the IRC, and therefore would have to be  designed. What the AHJ will accept  is up to them, the  most common answer is a registered design professional,  op possibly a know standard.


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## classicT (Nov 30, 2020)

Please go check out #post-226747. The framing details do not apply, but the span tables still work. 

Cannot be framed with rafter ties/ceiling joists due to low pitch, but can be framed similar to a floor with rafters, beams, and posts.



steveray said:


> Not entirely true....
> 
> 
> 
> ...





classicT said:


> Correct.
> 
> As Steveray pointed out, it is only the framing details in Section 802 that do not apply to roofs with a pitch less than 3:12. Where less than 3:12, roof framing must be done similar to floor framing with traditional ridge beam and rafter framing methods (i.e. no collar ties, ceiling joists, etc.).


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## TheCommish (Nov 30, 2020)

Hmm, I thought I knew what I read, and what I wanted to see is how I read it, now that I have had it questioned and highlighted, what I said is wrong. The IRC tables do allow for the construction of roof ceilings less than 3/12 pitch without an RDP or any fancy calculations.


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## ADAguy (Dec 1, 2020)

An issue of concern would still be slope to drain it.


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## classicT (Dec 1, 2020)

ADAguy said:


> An issue of concern would still be slope to drain it.


Not really....

Minimum slope for rolled asphalt is 1:12 or 0.25:12 (2%) for built up and standing seam metal roofs.

Hell, you can get to 0.125:12 (1% slope) if you use a coal-tar built-up roofs.

See IRC Ch. 12.


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## Pcinspector1 (Dec 1, 2020)

Might check with a wood truss designer, I'd think they can get it designed to work under a 3/12p.


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## Rick18071 (Dec 1, 2020)

Cannot be framed with rafter ties/ceiling joists due to low pitch, but can be framed similar to a floor with rafters, beams, and posts.

What section allows this?


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## Mark K (Dec 1, 2020)

When the slope is low the deflection of the joists may be enough to cause ponding.  Thus the concern with a low slope  roof is not the ability to carry the normal dead and live loads but rather the impact of the added ponding loads.  

If there is ponding certain types of  roofs may be inappropriate.


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## Rick18071 (Dec 2, 2020)

Cannot be framed with rafter ties/ceiling joists due to low pitch, but can be framed similar to a floor with rafters, beams, and posts.

Floor joist spans in the tables are one to two feet shorter than rafter spans in tables. Will using the shorter span floor joist tables for flat or low sloped roofs work for the more snow that could pile up on these types of roofs?


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## ADAguy (Dec 2, 2020)

that w0uld be a nope. Flat roofs unless designed for snow are not the best idea. Snow blocks drains until it melts.


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## Tim Mailloux (Dec 2, 2020)

The contractor should easily be able to get his lumber yard to provide a signed seal design sketch and calculations relatively quickly. Most lumber yards have structural engineers in retainer for just this sort of thing.


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## Paul Sweet (Dec 2, 2020)

Tables R802.4.1(1)-(8) give rafter spans for various snow loads, measured along the horizontal projection of the rafter.  These will work for any slope.

You just have to use a structural ridge when the slope is under 3:12.  Use Tables R602.7(1)-(3) to size the girder (structural ridge).


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## classicT (Dec 2, 2020)

Paul Sweet said:


> Tables R802.4.1(1)-(8) give rafter spans for various snow loads, measured along the horizontal projection of the rafter.  These will work for any slope.
> 
> You just have to use a structural ridge when the slope is under 3:12.  Use Tables R602.7(1)-(3) to size the girder (structural ridge).


They even work for completely flat. Rafters are not required to have pitch at all.

Roof finishes must have slope, and that slope can be made via a built up system.


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## Mark K (Dec 2, 2020)

Tim Mailloux said:


> The contractor should easily be able to get his lumber yard to provide a signed seal design sketch and calculations relatively quickly. Most lumber yards have structural engineers in retainer for just this sort of thing.


Not in this part of  the country.  Manufacturers of engineered lumber may provide that service.


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## ADAguy (Dec 3, 2020)

Tim Mailloux said:


> The contractor should easily be able to get his lumber yard to provide a signed seal design sketch and calculations relatively quickly. Most lumber yards have structural engineers in retainer for just this sort of thing.


Home Depot "approved"!???


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