Mudsill Connection in Retrofit Shear Walls

By Howard Cook, Founder and General Manager of Bay Area Retrofit

The Vital Retrofit Mudsill  Connection in Cripple Wall Shear Walls.

Cripple Wall Shear walls are complex in nature and retrofit shear walls are even more complex. When building a retrofit shear wall, the studs are  2 by 4 Douglas Fir and the mudsills (piece of wood on the croncrete) are full sized 2 by 6 redwood.

The size differential between the 2 by 4 studs and the 2 by 6 redwood mudsill presents a problem that requires a modifications to the mudsill that does not produce split blocks.  The American Plywood Association has conducted thousands of tests on various ways of constructing shear walls and their tests always used 2 by 4s on the top, sides, and bottom.  The challenge is to make our retrofit shear wall  the same configuration as those tested by the American Plywood Association.

Screenshot at Sep 07 18-40-55

The four ways of making this connection are explored their fullest in this paper, though the material below and watching the video will tell you most of what you need to know.


The Nailed Blocking Method

Untested Nailed Blocking Method of Connecting the Plywood to the Mudsill.

2 by 4 blocks put between 2 by 4 studs and plywood nailed to the blocks

On the left, blocks have been nailed onto the mudsill.  On the right, the plywood has been nailed to the blocks.


Shear Wall Blocks being Installed

Blocking for plywood nailing


Why are Blocks a Problem?

The problem with this method is that the blocks split. The blocks, usually 14 inches long and often dry, are installed between the studs on the cripple wall as shown in the photo above.   If the blocks split, then the shear wall will fail.

Photo: Block Split in an earthquake retrofit

Photo: Block Split in an earthquake retrofit


Retrofit Shear Wall built with a Split Block

Obviously, this Split Block will not work in a Retrofit


Another Split Block on a Cripple Wall Retrofit

Retrofit Guidelines require that Nails that are too Large be using on the block. This is what caused the split here.



Furthermore, once the plywood is nailed there is no way for a private home inspector to see whether the blocks are split.  He can only tell the buyer he hopes they are OK.  Unfortunately, this is the method recommended by all extant retrofit building codes.  

The Flush Cut Method of Attaching the Plywood to the Mudsill.

In this method the 6-inch-wide mudsill is cut flush with  the 2 by 4s.  The plywood is then nailed directly to the mudsill.

Screenshot at Sep 07 19-07-15


The Flush Cut Method is the only way to build retrofit shear walls that is identical to those tested by the American Plywood Association, a National Laboratory for Testing Shear Walls.  These tests are the basis of the building code provisions for new shear wall construction for the entire country.     The following photographs show the step-by-step process used in this method of retrofit shear wall construction.

Carpenter Cutting Mudsill Flush for an earthquake retrofit

Removing mudsil from foundation bolts

One of the reasons this method is used is because the plywood can now be nailed into old growth redwood that is many times less prone to splitting than wood grown on tree farms.


Pnoto: Flush cut sill is best for earthquake retrofits

The Stapled Blocking Method.

Staples are an excellent way to prevent splitting of the blocks and also have a very high shear capacity.   The staples in this block have the strength of 35 nails.


Wood Blocks in Cripple Wall Retrofits should be Stapled.

This Block has over One Hundred Staples and never Split.

The Reverse Blocking Method

The image on the left shows a 2 by 4 being attached to the plywood.  The center image shows how it is then placed on top of the mudsill.  The 2 by 4 is then nailed to the top of the mudsill.  Because the 2 by 4 is so long it will not split.



The carpenter is nailing into the reverse block with his nail gun

 These 4 different methods were evaluated by the American Plywood Association as consultants to an International Code Council committee developing a retrofit standard that became Standard Plan A.  Their opinion can be seen if you go here


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