Roof Rafters

[engineertipsman]

Not sure on this, been a while for wood design. Does code require that roof rafters have blocking between them? I saw a building with looked like 2x12 rafters, no blocking at all. Did have hurricane hold downs, so this was the roof.

 

[woodman88]

As they are 2x12's and if they fall under the conventional light framing (IBC Section 2308) than no. See section 2308.8.5 and 2308.10.6.

Garth Dreger PE
AZ Phoenix area

 

[msquared48]

Only if the roof diaphragm requires blocking for shear strength would you need blocking.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[rscassar]

I would only provide blocking if I needed additional capacity to transfer the diaphragm force into the collector.

The roof truss installer may install blocking if it helps keep the structure stable during erection.

I wouldn't be bothered going to site and not seeing blocking between the rafters unless it was specifically detailed.

 

[csd72]

I would recommend you install at least one blocking at mid span. Rafter can and do warp when they dry out.

 

[msquared48]

...But not full depth blocking unless you provide vent holes above the insulation line.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[engineertipsman]

Thanks. Like I said, this was not my design, just looked fishy not having blocking or bracing. Some longer spans too, looked to be 20+ feet.

 

[a2mfk]

I would like to elaborate on Mike's point. I don't know the details of the structure and diaphgragm, but if the rafters bear on top of a CMU/wood wall/whatever, how does the shear force get from the diaphragm into the shear wall? How is the overturning force induced by the shear force at the top of the members resolved?

If it is a very shallow roof framing member, there are mechanical connectors capable of resisting this in-wall-plane shear force, but none that I know of for 2x12s. Or diagonal straps/braces depending on the force. But you have to check the numbers.

My answer is blocking, almost always does the job, cheap, hard to mess up. Yes you have to coordinate ventilation. There was a multi-page article a few months ago in Structural Design or something similar on this very subject, the only one I have ever come across... Yes I am in high wind country but even in low wind country how do these forces get transferred without blocking or diagonal straps?

Andrew Kester, PE
Florida

 

[southard2]

I agree with Kester. Lack of blocking at the bearing points is one of the big oversites in the load path that architects, engineers, and contractors neglect. I don't do a lot of wood design but I would also check the uplift case on the rafters with full span being used as the unbraced length. If it works fine. If it doesn't I would add blocking or straping as required.

I think above people were refering to a prescriptive method that is allowed. In Florida I think if you go above a certain wind speed prescriptive methods can not be applied. I really could be wrong about this. The way I look at it though is if someone is hiring me to engineer the structure that is what I'm going to do prescriptive method or not.

John Southard, M.S., P.E.

http://www.pdhlibrary.com

 

[a2mfk]

John- agreed, I don't know much about prescriptive design, that kinda takes away the fun of being a structural engineer