Guys, I’m afraid I am just feeling lazy today and concerned this is affecting my thinking aboout this job.
I finished installing the edge blocking up in the roof rafters (to fasten the roof sheating). The edge blocking is 2X4’s laid flat between the rafters and “toe nailed”. The entire stack of rafters are now very much stiffer than they were. I was intending to install some 2X6 blocking (installed with 6″ dimension vertical) but I am thinking it really doesn’t need it, as it is a roof and not a floor.
Am I wrong? Does the roof need the vertical blocking? If it doesn’t need the blocking, would the vertical blocking hurt anything?
For reference, the roof is a low slope “flat” roof. The rafters are 2X8’s. The sheathing is 3/4″ T&G plywood.
Any advice or help would be welcome. Thanks in advance.
Replies
Mike ,
I think you're overthinking this job . It will be just fine the way it is .
Walter
Why would you put any blocking at all, especially to nail sheathing too?
Did an inspector make you do this for some reason?
No, it was part of my design. As I build it, and see it in 3D and can "test" it (as opposed to analyze it), I am thinking I have over-designed it. Still, I am not sure if I will be giving up something structural if I leave it out.
We do get earthquakes with their shaking in all three axes. What happens when the rafters start shaking? I was going to add the blocking to reduce the torsional (twisting) dynamic mode.
Edited 12/5/2009 4:19 pm ET by Mike_Mills
Mike, you're over thinking it. Because you're using T&G and breaking your sheathing on rafters, you really don't have any need for any blocking at all unless you're building this thing on top of a moving vehicle or something.
As usual, you guys are correct. There isn't a single stick of blocking in the roof of my house, so why put any in the roof of the sunroom?
Thanks for the sanity check.
I dunno, those sun rays get pretty heavy some days. Mebbee you wanna double up that blocking. It has to hold the weight off all that 3/4" plywood too.But if you have us all in for drinks, I'm sure we can lighten things up
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lolol
All the hot air coming out of the owner ought to keep it inflated, even if the roof is too heavy.
Edited 12/6/2009 1:27 am ET by Mike_Mills
In CA. generally there is blocking between the rafters full height over the exterior wall. Blocking attached to plate with clips, not toe nailed. Sheathing nailed to blocking per nailing schedule. Purpose is to transfer roof shear to wall shear.Assume this is true of all earthquake areas.John
That makes sense.
If I was building a deck, rather than a roof, what would the proper spacing be for the blocking? You'd put in a row of blocking every,...???
Midspan.
Mid-span on anything longer than 8'.
Thank you, gentlemen.
I know I'm jumping in late, but - The only reason I've ever seen blocking on roofs is if there's some reason to tie the whole roof together so it's all one big diaphragm. That would typically only be for wind or seismic loads.
every eight feet
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Mike,
Example of a typical carpenter's approach to what you're doing, just to ease your mind: I added a ~20'X20' 1/12 pitch patio roof to my parents' SoCal home about fifteen years ago. The 2X12 rafters are on 24" centers. The sheathing is 5/8" 5ply Doug Fir exterior grade, not T&G. I added clips. No blocking or bridge bracing between the rafters.
The roof has served very well and will continue to do so for the life of the house.
Edit: regarding earthquakes; the house is about five miles from the epi-center of the North Ridge quake, the last big one (7.2 right?) in SoCal. Nothing happened to the patio roof. Didn't even crack its sheet rock ceiling.
Worth considering: all modern types of rolled roofing materials have sufficient elasticity to be uneffected by movement, either earthquake or deflection of the sheathing due to heavy people walking on the roof.
Edited 12/7/2009 4:20 am by Hudson Valley Carpenter
Thank you for that input. It is specific in design and the stress environment.
If you were close to the epicenter, the Northridge quake was a huge (7+) vertical acceleration - out of plane to the roof. To me, so far way, it had a huge sideways component. It is the lateral accelerations that have me thinking about blocking in the roof.
Guys, you are all a terrific resource. Thanks to all of you for the input.
It's raining "cats and dogs" here, probably all week long. So, I have time to do aome analysis and to think about this.