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powchutu 10-10-2011 10:42 PM

2x4 rafters/roofing question...
i helped a friend build his garage's roof, kinda of uncomfortable.
he says lowes told him he could use 2x4 for rafters with 1/2 inches plywood for asphalt shingles.
the rafters are 18 foot long without beams or trusses (garage 28x40).
he's already got the 2x4 up for rafters and it's ready for laying plywood tomorrow.

i told him plywood and asphalt shingles would be too heavy.
he could get his money back for the plywood and do tin roof setup instead.
he wanted the 2x4 anyway and is going to think overnight about tin roof or not.
he also mentioned that he'd like to renovate the garage to a house in the future and felt tin roof would not do.

help me out here guys.

tinner666 10-11-2011 05:26 AM

Irregardless of the roof material, maximum allowable span for 2x4's is 10'.
Max span of 2x6's is 20'.
And the less slope there is, the sooner it will collapse.
The plywood alone is too heavy a load, by itself, by far.

kwikfishron 10-11-2011 06:17 AM


Originally Posted by powchutu (Post 746308)
i helped a friend build his garage's roof, kinda of uncomfortable.
he says lowes told him he could use 2x4 for rafters with 1/2 inches plywood for asphalt shingles the rafters are 18 foot long

WOW….. Lowes and/or your friend are wrong and by a long shot. For an 18’ rafter span you’re looking at 2x10 not 2x4 and even then I can’t say because we have no idea what the load requirements are for your area.

This is a recipe for disaster you need to STOP NOW. One of the reasons we have a permit process is to keep stupid **** like this from happening.

I’ve heard of some real bad advice coming from box stores before but this takes the cake.

Daniel Holzman 10-11-2011 07:04 AM

We don't know where the OPS lives, could be in New Guinea for all I know. We don't know what Lowes actually said, all we have is a vague third hand statement attributed to his friend. And we don't know the building code, if there is one, in the area. So in short, we don't know what proper practice, code practice, or even reasonable practice would be in this specific case. If the OPS is uncomfortable helping out, he can point out the potential issues to the owner, and let the owner decide on the next step. Personally, I would be nervous standing on a 2x4 framed roof spanning 18 feet, I have seen roofs fail, very dangerous.

Also note that the OPS did not mention the spacing of the rafters, the pitch, the species and grade, or anything else that might be useful in determining allowable span.

As an aside, substituting tin roof for plywood is not going to improve the situation, as the plywood adds considerable strength to the rafters, whereas the tin does not.

1910NE 10-11-2011 07:48 AM

The best you (the OP) can do is to tell your friend that you have concerns, and suggest that they seek a professional opinion. If that doesn't work, then I would decline to continue helping with the project.

PoleCat 10-11-2011 08:25 AM

Why such a step pitch? The wind may blow it over before the sheathing even goes on.:eek:

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