Wood Diaphragm Connection v.s. Architect's Ventilation 1

[bigmig]

I have always been taught that the correct way to transfer lateral loads from a sloping roof diaphgram is to provide full height blocking at the rafter/stud plate interface. The wood diaphragm sheathing is nailed to the full height blocking which as the term implies, extends all the way to the underside of sheathing.

More and more I am finding that carpenters and architects do not like this detail and pretty much ignore it. My research of products on the market that provide ventilation show detail after detail of improperly installed blocking (partial height, held down to allow a "rafter baffle").

Well known wood product manufacturers on the other hand, show the correct details with "v" notches, alternating bays blocked etc.

When I point this out I am greeted with the following arguments:

1. We have never ever done it that way
2. Our propietary underwriter approved "rafter baffle" won't fit with your detail so we ignored it
3. Our roof can't breathe with your detail so we ignored it

It seems pretty elementary to me that you build a roof with full height blocking, but I am amazed that there are products on the market that not only ignore this, but make it impossible to install full height blocking.

Does anyone else callout full height blocking on their designs? Does anyone else notice architects trying to install partial height blocking and call them on it?

 

[msquared48]

Sorry guys, but it seems to me that by not installing the blocking you are adding liablilty for yourself to save the client money. Why would you do that? Why would you want to be Mr. Nice Guy in a court room? I have made this mistake before and will NEVER do it again!

Blocking serves more than one purpose here. It is not only structural, but also serves to deter rodents, squirrels and birds, and bees, from entering the attic space when the vent holes are properly screened.

Professionally speaking, I do not care what the IBC or the IRC "allows" here. Engineering judgement trumps that line of reasoning. I even have a problem with partial depth blocking in that the diaphragm edge nailing is not provided (6" minimum). That alone is a structural code conflict, let alone the roll over and cross grain bending issues. I always, ALWAYS provide full depth blocking with vent holes at the top of the blocking with clearance for any insulation. No exceptions here for me, ever.

I'll get down off my soapbox now.

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

 

[DaveAtkins]

Mike,

Omitting diaphragm edge nailing is not an issue if you use the diaphragm capacities for unblocked sheathing.

DaveAtkins

 

[msquared48]

I understand what you are saying Dave, and for Gable end structures you are correct, but for hip configured roofs, you will run into problems with the "continuous edge" scenario that does necessitate edge blocking.

I just prefer to use the blocking all the time to avoid not having the blocking where it is needed.

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