What Is Holding My House on Its Foundation?

Securing Houses

• In newer construction, houses are secured to their foundations by L-bolts set four to six feet apart in the wet concrete of the foundation when it's made. The L-bolts pass through holes drilled through the sill plate to which the floor joists are fastened. The sill plate is anchored to the foundation by washers and nuts threaded onto the bolts and tighened down. Houses built on concrete slabs also should be anchored to the slab with L-bolts. Some builders use flanged steel anchor plates set into the wet foundation concrete and bolted to the floor joists.

Why Fasten

• Foundations normally support the vertical gravity load of the house, contents and residents. However, windstorms and earthquakes can impart sideways motion to a house. In an earthquake, an unsecured house can be shaken off its foundation by sideways ground motion. In windstorms, the force of the wind can push an unsecured house sideways off its foundation. If a disaster shifts your home off its foundation, resetting the house can cost $25,000 or more.

Retrofitting

• If your house isn’t bolted to its foundation, two main retrofitting methods are available for securing the house. If you have room for a drill, you drill vertical holes about every 4 to 6 feet apart through the sill plate and down about 8 inches into the foundation. Insert an expansion bolt into the hole and tighten the top nut to secure the fastener. For added strength, set the hold-down bolts into epoxy. If you don’t have the room to drill vertically, secure your house to the foundation with angle irons bolted into horizontal anchor holes that are drilled into the foundation and into the floor joists. Set the irons four joists apart.

Mobile Homes

• Mobile homes represent a special case. Because they must be moved, they generally are not bolted to a foundation. They must be securely anchored to prevent them being blown over in windstorms. You need to select the anchor type for your soil and wind conditions. Many mobile homes are secured by anchors screwed or driven into the ground, but the most secure anchors are embedded in concrete set deep into the ground. The anchors are secured to the mobile home with over-the-top vertical tie downs or diagonal tie downs fastened to the home’s under frame. Consult with your mobile home seller or installer on how to anchor your home.