The One and Only Tested Retrofit Shear Wall
The One and Only Tested Retrofit Shear Wall

HOWARD COOK: In this video,
we’re going to be talking about what your contractor needs
to know about attaching plywood to the bolted
mud sill. There’s only one
tested method. It’s called the flush-cut
method. And it’s the one that your
contractor should be using. There are three other methods
which we’re also going to discuss, which your contractor
may want to use. But you should be aware of all
of these so that when you buy your retrofit, again, you’re
getting what you paid for. And you’re getting something
that you know will work. Here, we will discuss the
different ways of attaching plywood to the mud sill. Normally, the mud sills are
wider than the two by four uprights on the cripple wall. This means that if you were
simply to nail the plywood to the cripple wall, the
plywood would not be attached to the mud sill. As you can see by the photograph
on the right, the plywood is sitting on top of the
mud sill rather than being attached to the mud sill,
where the bolts are. This means as the earthquake
force causes the bottom of the plywood to slide back and forth,
it would slide on top of the mud sill and not be
restrained by the bolts. One way of achieving this is by
nailing two by four blocks to the bolted mud sill in
between the two by four upright studs of the
cripple wall. Then the piece of plywood is
nailed to the blocks and to the two by four studs and the
top plate of the cripple wall. Then when the earthquake force
tries to push the piece of plywood on top of the mud sill,
that earthquake force is transferred into the blocks,
then into the bolts and into the foundation. Here’s a photograph of
someone installing cripple wall blocking. This is normally done
with a nail gun. As you can see, the mud sill is
much wider than the upright two by four studs. Now, in between the studs, the
carpenter is installing the two by four blocks so that
the plywood can be nailed to the blocks. One of the problems with nailed
blocks is that the blocks can split. This happens because for a
retrofit to be effective from an engineering point of view,
large nails are required. Oftentimes, carpenters
substitute these larger nails with smaller nails because the
nails do not split the blocks. However, from an engineering
point of view, nailed blocks that are nailed with small nails
will not be effective. As you can see, there’s plywood
nailed to the block from the other side onto the
face of the two by four. Nailing of the plywood
can also cause this surface to split. Here’s another case
of a split block. As you can see, the carpenter
needed to notch the block in order to fit around the bolt. This further shortened the block
and created a higher potential for it to split. This method is identical to
the nail blocking method, except staples are used. Staples are used because they
will not split the blocks. It is important, then, when
attaching the plywood to these blocks, that staples are used
through the plywood. Otherwise, if nails are used,
the blocks will split. Stapled shear walls are fully
approved by the American Plywood Association and the
International Building Code. This photograph shows why
staples should be used whenever blocks are necessary. The block to mud sill connection
here has the same capacity as 40 nails. Another method of attaching the
plywood to the mud sill is called the reverse
block method. As you can see by illustration
number one, the two components are a piece of plywood
and a two by four. Illustration number two shows
how the plywood and the two by four are connected. You can’t see the nails on the
back, but the plywood has been nailed to the two by four. Then, this entire assembly is
placed on top of the mud sill. And then it’s nailed into the
mud still downward and then also into the cripple
wall itself. Here’s a photograph of a
carpenter using the reverse blocking method. He’s using his nail gun to nail
the reverse block into the mud sill. Once he’s done here, he will
nail the plywood to the cripple wall. The last method we
will discuss is the flush cut method. In this method, the mud sill is
cut flush with the two by four studs so that now the mud
sill is also a two by four. Once this is done, the plywood
is nailed directly into the mud sill and into the upright
studs and into the top plate. In shear wall tests, tests are
always done with two by fours on the bottom, two by fours
on the top, and two by fours on the sides. This is actually the only
method that’s ever been tested, with thousands of tests
done by the American Plywood Association. This special saw is being used
to cut the mud sill flush with the two by four studs. Here’s a photograph of a mud
sill that’s been cut flush with the two by four studs. Now the plywood will be nailed
onto the mud sill and onto the two by four uprights plus
up at the top plate, where the floor is. It needs to be emphasized that
this is the only tested method, based on thousands of
tests done by the American Plywood Association. In these tests, the shear walls
were made of two by fours, two by fours on the
bottom, two by fours on the sides, and two by fours
on the top. Besides the flush cut method,
none of them have been tested before, and I doubt you want
your house to be the experiment to see if
they actually work. The piece of wood on
the left is a flush cut redwood mud sill. The piece of wood on the right
is from a tree farm. Notice the very, very tight
growth rings in the redwood mud sill, which is perhaps
2,000 years old. The reason you want to use the
flush cut method is because you want your shear wall made
of such high quality wood. If you look on the website,
we have a section called Contractor Screening. And on the Contractor Screening
page, you’ll see different questions that you
should ask your contractor before you hire them. It’s important for you to
understand what the answers to those questions are so you can
speak intelligently to your contractor about how to
retrofit your home. Now I’ve seen many, many houses
that were retrofitted where people spent between
$10,000 and $15,000, sometimes more, on retrofits that
simply will not work. And that’s because the
contractor did not understand what he was doing. Even though they may have done
lots and lots of work, and even though there might have
been lots of shiny hardware, it still was not going
to be effective. So that’s why it’s so important
for you as a homeowner to protect your own
home and to realize in general, as far as earthquake
preparedness goes, the government is not going
to help you. You have to help yourself.

5 thoughts on “The One and Only Tested Retrofit Shear Wall”

  1. Tyler M says:

    Incredibly useful and helpful information, thank you!

  2. Albert E. says:

    Hire an architect, not a handyman…

  3. James Gillett says:

    Testing blocks with a pry bar is the only way to know if those super small diameter staples will hold well. If the blocks pop right up from the sill plate (( mudsills are from the 1880s in California where a huge 16" or wider and 8" tall board was laid directly on the soil or like a ice fishing house or a port a john skid plate)) as there are no mudsill buildings in California to speak of. A simple pry bar test will show most stapled sill blocks to be very very very weak. WORSE the staples are coated in the manner of sinker or vinyl nails. They slide in and slide back out. //// With enough nails most any block will stay in place when tested to pry out with a pry bar. In fact one should break out a few blocks to see if they broke up or slid out for the ultimate test. /// Letting some college puke make a video yacking about untested blocking is "out there".

  4. Reynolds Cameron says:

    What about drilling pilot holes into studs and mudsills in order to use 10d nails with 100 year old old-growth lumber (San Francisco Victorians)?

  5. ablooddiamond says:

    What kind of staples do you use for the blocks?

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