House Movement in EQs
Before You Buy or Build
Getting The Dirt
Poor Soil
Good Soil
What To Do
% Uninhabitable Homes
Mobile Homes
Wood Frame Homes
Bolt The Foundation
Four Easy Steps
Stepped Foundation
Foundation Anchor Plate
Anchor Bolts
Chemical Anchors

Cripple Walls
Reinforce Cripple Walls
How Many Panels
Made To Measure
Ventilation Holes


Even though most modern homes are bolted down, they can fail because of another weak link called the "cripple wall." This is a short wall that connects the foundation to the floor of the house and encloses the home's "crawl space." The cripple wall is often not strong enough to survive the force of an earthquake and must be braced and strengthened. If not, an earthquake may damage the cripple wall and knock a home off its foundation, even if the house is properly bolted at the foundation.

This home was jerked three feet (1 m) off its foundation during the 1989 Loma Prieta quake due to inadequate cripple wall bracing. © City of San Leandro Development Services
Building Regulations Division


For this project you'll need these materials: 8d and 10d common nails, Simpson HD2A holddown or equivalent, Simpson A35 framing clips with N8 nails or equivalent and anchor bolts. Tools need to complete the work are circular saw, jigsaw, 1-1/2" hole saw, framing square, hammer, plywood blade, tape measure chalk line and a pencil. This will give you an overview of what should be done and where.


Oftentimes bolts alone aren't sufficient to prevent damage from sideways shaking during an earthquake. Bracing cripple walls with plywood helps tremendously.


The number and length of panels needed depends on the height and length of each section of cripple wall and how many stories the cripple wall supports. For all houses, panels should be placed at both ends of each cripple wall section. For a single-story house, additional panels should be spaced evenly so no less than 50% of the total length of each cripple wall section is braced.

Two story houses, should have panels spaced to cover no less than 80% of each cripple wall section. For optimum strength, use the longest piece of plywood possible; instead of multiple pieces of plywood to make up the 4' - 8' panels. Distribution of the plywood panels should be "balanced'. Keep the panels equal in length and as evenly spaced conditions allow. For example, a cripple wall which is 52' long and 12" in height in a single-story house requires a minimum of 26' of braced panels. A typical solution would be a 4' plywood panel at each end and three 6' panels evenly spaced between the end panels.


To provide adequate strength, each plywood sheet must be nailed along all edges, and along the interior studs. In most cases, the cripple wall studs are flush with the mud sill and with the "top plates" (located at the top of the cripple wall). This provides an even nailing surface for each plywood edge. However, if the cripple wall is set back from the edge of the mud sill, you will have to add blocking between the wall studs to create a nailing surface for the plywood.

Measure the height from the top of the double top plate to the bottom of the mud sill. If your condition requires blocking above the mud sill, then measure to the bottom of the cripple studs. Cut the plywood so that it covers this area and reaches from the center of one stud to the center of another. Mark the center of each stud on the foundation and above the top plates. These marks will provide a nailing guide. Remember, you must nail the plywood securely to all studs at the specified nail spacing. Also, note the location of any pipes so you can cut rounded notches in the plywood to fit around them.

Often the mud sill is wider than the stud wall or embedded into the concrete foundation too deeply to allow nailing along its edge. If so, you will need to add a piece of wood 2x4 or 2x6 blocking on top of the mud sill, as shown above, to provide a nailing surface. Install blocking to fit over the anchor bolts per the city's plan set, and nail it to the mud sill using four 10-penny common nails. Blunt the tips of the nails and stagger them across the wood to prevent splitting. If the blocks still split, you may have to pre-drill the nail holes. To prevent dry rot or termite damage, it is a good idea to use foundation grade redwood or a pressure-treated wood for the blocking.


When a job requires a lot of nailing, your arm will thank you for using a nail gun. Not only will the work go much faster, but it cuts down on the wood splitting.

Make sure you get a gun that uses the right size and type nails for the task at hand.

Each sheet of plywood must be nailed every 4" around the edges and every 12" along all interior studs and cross bracing in the "field" area. The edge nails provide most of the strength and the field nails prevent the center of the sheet from bowing outward during an earthquake.

With the plywood in place, drill 2-1/2" to 3" diameter ventilation holes in eachsheet. These holes should be centered between each set of studs and2-1/2" above the mud sill and 2-1/2" below the bottom of the top plates.

The holes provide ventilation and allow inspection of the cripple wall and mudsill bolts. Drill only one hole if the plywood sheet is less than 18" tall. If the wall has an exterior ventilation screen, cut a hole in the plywood opposite the screen and similar in size. Add blocking around this vent hole and nail the plywood edges at 4" on center. With the first sheet of plywood nailed into place, repeat the process to brace the wall of plywood in sheets no shorter than 4' in length. Long continuous sheets provide maximum strength. When installing adjacent pieces of plywood, make sure they join at the center of a stud or that an

additional stud has been added to provide for proper nailing. Check the cripple walls for termite and dry rot damage, and replace any damaged materials before installing the plywood shear panels.

With affection,
Holly and Stan
Seismo and Taco

"Hey Seismo, did you see that solar flare?"
"Nope Taco, that's our plane! Boy, will I be glad to see our sling beds again! This cardboard was OK till it rained. Now look at it! It's falling apart! Buggah!!"
"Quit gripin' Seis, it only has to last one more day. At least we're with The Parents!"
Taco and Seismo weren't the only ones doing time with cardboard! The four of us slept on the floor the past two weeks using our emergency prep gear. Stan and I "camped out" at Camelot Acres after our household things shipped to America. This was our "cardboard office".
"Seis, what in the world did we do?? Are we in jail or what?"
(Taco moaned.)
"Dunno Tacita, but I don't like the looks of this. . .
(Doggies waiting to board Qantas)
Our last minutes at Camelot Acres before friends drove us to the Melbourne airport. This was just part of 11 pieces of luggage we dragged with us!
Stan and Holly Deyo
In Colorful Colorado!

Text and Graphics, 2001 Stan and Holly Deyo, except where otherwise credited

Graphics courtesy