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Re: eave blocking in high wind and seismic zones

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Here are answers to some of your questions, which were:

1. Do you require eave blocking? =20
2. If the forces at the roof diaphragm were greater or less than about =
200 plf would your answer to #1 change?
4. Would flat or vertical blocking at the truss/rafter tail eliminate =
cross grain bending?
5.   Do you see any problems with a discontinuous roof diaphragm at =
the roof peak because it was cut for ridge vent placement?

1.    As luck would have it, I was looking into this on Tuesday.  The Code
sections you cite do  require blocking at eaves, whether you have rafters or
trusses.  UBC Section 2320.12.8 says "solid" blocking.  In my situation this
would have meant 23-inch deep blocks, since that was the truss depth at the
support point.

So, yes--I require blocking at eaves.   I also require sheathing nailing
into this blocking.  Guess what?  The UBC does NOT require such nailing  in
the Conventional Light-Frame section, so you'd better specify it on your
plans if you want the framers to do it.  Funny, the code requires
toe-nailing to the top plate from the blocking, but not nailing to the
blocking from the sheathing.  But you know this if you've gotten to that
part in my book ;-).  Most  lower-48 builders DO install eave blocking.

For venting through the blocks you could bore some pretty big holes before
compromising their shear capacity.  Unless of course you have wood that
tends to split in line with the holes.  Not sure what quality of lumber is
typical up there.  Another (uglier) option is to cut V-shaped notches in the
tops of the blocks.   1/8-inch mesh over such holes or notches would do much
better than window screen, which will clog up with paint.

2.  No.

4.    Vertical blocking could probably bring cross-grain bending under
control.  This would depend on the length of the eave overhang.   It would
seem reasonable to leave a NARROW  ventilation gap between the eave block
and the plate--although this would compromise the insulation's
effectiveness.  Framing clips typically used for connecting rafters/trusses
to the plate have a significant capacity to transfer shear in line with the
walls, and could substitute for the toe-nailing from blocks to top plate.
Although 200plf could overstress  such connectors.   For not-so-high wind
and seismic regions, forces parallel to the ridge can often be resisted by
the three toe-nails from each rafter or joist to the wall top plate.

5.    In theory, the diaphragm shear is zero at the center of the roof.
Someone else on the forum pointed out that for trussed roofs any force that
did have to transfer across the peak would tend to shift the truss top
chords at the connector plate.  This would be true unless you called for
blocking at the ridge (i.e, the diaphragm is already discontinuous at the
plywood joint at the ridge).   Ridge blocking is something that I hardly
ever see installed.  For ridge venting you could install a 2x6 or wider
block oriented horizontally, with the edges beveled to match the roof slope
for solid nailing from the sheathing.  Boring holes through these blocks
would allow air passage to the ridge vent.

Given the probabilities, moisture and water problems are likely to occur
every winter if you do not ventilate the attic, whereas earthquakes and
hurricanes are less frequent.  I would tend to favor some level of venting.
In Seismic Zone 3 (where wind almost always governs wood-frame designs, even
at only 70mph)  I have no problem leaving out every fourth eave block to
install a screened vent.  With A35's from blocks to top plate you could
leave out every other block and still get your 200plf shear into the wall.

Before I returned to college to finally finish up my degree, I helped my
father build a house for himself.  He used 2x12 rafters with 2x10 eave
blocks, and we ran a narrow strip of mesh to cover the 2-inch gap above the
blocks.  I thought this was an exceedingly clever idea and wondered why I'd
never seen it done before--until I got into my timber design classes.

I thought that ice-dams had been well-understood for decades.  I'm surprised
that Alaskans would have problems with them in recent construction;  it
seems that engineers who ignore moisture/ventilation problems in favor of
seismic or wind design could be setting themselves up for trouble.  It's not
hard to accommodate both needs.

Best wishes,

Thor Matteson, SE

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