Floor to Floor Connectors

Preventing Damage to Home Additions

The building code does have any guidelines about how best to attach an addition to the main house.  How this connection is made is left up to the contractor. Contractors will put in enough nails to make sure the  addition does not fall down from gravity, but will rarely check  to see if the addition will separate from the main house in an earthquake.  Because they have no code to follow, so the building inspector will pass anything that looks reasonably effective in keeping gravity from separating the two.


Earthquake pushing home addition away from main house

                                        EARTHQUAKE PUSHING HOME ADDITION AWAY FROM MAIN HOUSE

Floor to Floor Connectors

The purpose of Floor to Floor Connectors is to make sure these two floors do not separate.  Floor to Floor can be made of steel bolts, large screws, special hardware known as Hold Downs, or custom made wood connectors designed on site as circumstances require.

Hardware used to prevent earthquakes from pushing addition off of main house


Tension Tie Floor Connectors

Tension Ties or Horizontal Hold Downs are used in many seismic retrofits, especially in houses with stepped foundation, where  installing a plywood shear wall is not a good option.  This is usually due to  limited access or because the foundation steps on a very dangerous hillside home are so steep that plywood shear walls simply won’t work.  Here you can see the earthquake is trying to pull the house away from the foundation and down the hill.   This pulling action is transferred to the Horizontal Hold Down which in turn pulls on the foundation where the force is resisted.

Horizontal Hold Down Hillside


More of the same.

Horizontal Hold Down Tension Tie



In the image below the green arrow points to the Horizontal Hold Down and to the Bolts that attach it to a floor supporting beam.  The red arrow is the foundation.  The blue arrow point to a bolt deeply embedded into the foundation with epoxy and the white arrow points to a nut the attaches this bolt to the horizontal hold down. This assembly prevents the beam from disconnecting from the foundation.

HZDThe other place they are used is when the house has an addition and we don’t want the addition to come apart at the corners.  In the image below the red arrow represents the earthquake pushing on the back of the addition.

Arrow Showing Earthquake Force Pushing on Back of Addition

If the corners of the addition are not securely attached to the main house the addition can be separated sometimes causing catastrophic damage.  This damage is prevented by connecting the floor joists of the addition to floor joists of the main house at each end with a special type of steel called a Tension Tie.

Tension Tie Holding Two Floors together






















Below is a construction detail found in the 2007 International Residential Code pointing to the fact that scientists have come to understand how important this connection between an addition and the original floor is. 



Areas Served

San Jose,Sunnyvale, Fremont,Oakland,Berkeley
And Surrounding Areas


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This website is intended to help clients of Bay Area Retrofit to understand their retrofit proposals.

IT IS NOT INTENDED AS A GUIDE FOR DIY RETROFITS. Retrofit design is based on many factors. Each house is unique. Bay Area Retrofit does not recommend performing a retrofit without the guidance and supervision of experienced and knowledgeable retrofit professionals, and is not responsible for costly errors, injuries, or earthquake damage that result from such attempts.

Bay Area Retrofit assumes no responsibility or liability for use by homeowners, contractors, engineers, or anyone else of the information provided on this site.

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