Part 1

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
Expansion Bolts
Chemical Anchors
Cripple Walls
Reinforce Cripple Walls
How Many Panels
Made To Measure
Ventilation Holes


Bolting the wood frame of an older house to the concrete foundation can significantly
reduce earthquake damage. Mobile homes, portable classrooms, and modular buildings can slide off their foundations during earthquakes. Their supports need to be braced to resist horizontal forces. If portable classrooms are used at your local school, you should ask school officials whether they are properly braced.

Bolting a structure to the foundation is a fairly easy 4-step task. For this project you'll need these materials: 1/2" or 5/8" diameter foundation bolts that are at least 7" long with nuts and square plate washers; rotary hammer drill with an appropriately-sized carbide tip drill bit; (right-angle drill if possible); short-handled sledge hammer for setting bolts; adjustable wrench; measuring tape and chalk line; dust mask and eye/ear protection and a torque wrench.


First, mark the places for each bolt on the mud sill. Make the first mark between nine and twelve inches from the corner, and then measure another four to the next bolt, and so on. Continue this pattern along all of the foundation walls. Place an extra bolt within nine to twelve inches from any joint or step in the mud sill.

Then follow the ABC's.

A. Drill the holesB. Clean the holesC. Install the bolts

A. Drill the holes
Using the rotary hammer drill equipped with an appropriately-sized carbide bit, drill down through the mud sill at least five inches (12.7 cm) into the concrete.

B. Clean the holes
Use flexible tubing to gently blow the concrete dust out of the hole. (This is especially important if you are using chemical anchors.)
C. Install the bolts
Expansion bolts are designed to be hammered into place. This can be done without damaging the bolt's threads by turning the washer and nut past the end of the bolt and tapping on the end of the bolt shaft to hammer the assembly into place. Once

the bolt is in place, tighten the nut down firmly using an adjustable wrench.


If your house is built on a hill or even a slight grade, chances are you have some step-like off-sets in your foundation to compensate. Every step must be bolted down even if it is adjacent to another bolted step.


If you don't have working room above the mud sill to drill straight down and can't find a right angle drill, you can secure the mud sill to the foundation with an anchor plate. This is a metal plate that is nailed or screwed to the top of the mud sill and bolted to the side of the foundation.


When you tighten the nut on an installed expansion bolt, the bolt's other end expands to grip the concrete. When the bolt is inserted properly, you will actually feel it "grab" the foundation as you tighten the nut. Test at least one out of every four new bolts for tightness with a torque wrench applying 40 foot-pounds of pressure.

CHEMICAL ANCHORS (epoxy bolts)

If you have an older foundation and worry about cracking it with the pressure of expansion bolts, consider using chemical anchors (also called epoxy bolts). Always follow the manufacturer's installation instructions. Measure, drill and clean the holes per the manufacturerÕs instructions. Be careful not to drill deeper than the bolt's length. Before you place the bolt in the hole, inject the epoxy mixture into the hole. Press the bolt into place and wait for the epoxy to harden (usually 24 hours). Once the epoxy has hardened, tighten the nut with an adjustable wrench until the washer just begins to indent the wood mud sill. Chemical anchors can be a bit more time-consuming to install. However, they are very effective, and are the preferred method.


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