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.

The width differential between the 2 by 4 studs and the 2 by 6 redwood mudsill (piece of wood on the concrete) on most cripple walls must be addressed .  Below is an illustration showing why this is a problem.

Screenshot at Sep 07 18-40-55

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 Nailed Blocking Method

Here is a construction detail from a set of engineered plans that tells a contractor how to do this.   As shown by the red arrow 8 nails are specified through the mudsill blocking which has a very high potential of splitting the block. Unfortunately, this is the method recommended by all extant retrofit building codes  These guidelines were written by building officials and engineers who never checked to see if it worked in a practical way

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

On the left, 2 by 4 blocks have been nailed onto the mudsill.  On the right, the plywood has been nailed to the blocks.


Shear Wall Blocks being Installed

2 by 4 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

Block Split during an installation


Retrofit Shear Wall built with a Split Block

Split Block will not work perform as intended


Another Split Block on a Cripple Wall Retrofit

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.

What is I have Nailed Blocking?

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

 The largest shear wall research laboratory in the world evaluated these methods in this letter and determined the the Nailed Blocking Method is the least desirable of all the   Their opinion can be seen if you go here


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