Mudsill Connection in Retrofit Shear Walls VIDEO


Please open and read the “This letter” link below.  The four methods mentioned of making a critical connection are evaluated in this letter.   The four methods are carefully  describe on this webpage given its importance, is worth understanding.

This letter contains the authoritative opinion of the leading shear wall research laboratory in the world. It was given to the author of this article when helping develop the Bay Area’s Regional Retrofit Guideline known as Standard Plan A.

Before looking at the rest of the page please take note of their opinion of the Nailed Blocking Method used by most contractors and engineers and the Flush Cut Method used by Bay Area Retrofit. This connection can make your retrofit one that is reliable and based on science and testing or when the shaking starts makes you say “I hope this works!”.

The Problem

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.

This method cuts the mudsill flush with the 2 by 4’s using a special saw. The contractor then nails the plywood directly to the mudsill. A retrofit shear wall built in this way exactly matches those tested by the American Plywood Association. Research has shown removing part of the mudsill does not weaken it.  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.

When the blocks split, it is quite tempting for the installer to leave those split blocks in place because of the time and labor involved in removing them.  The plywood covers the blocks when nailed to the wall framing.  Private home inspectors cannot tell if the blocks  split when installed.  This can leave lingering doubts in the mind of the home buyer.  Seismic retrofit contractors often believe “the more and bigger the nails, the stronger the shear wall” which further exacerbates the problem.

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.

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.


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

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