Room Additions: A Hidden Danger

Earthquake pushing home addition away from main house

You might retrofit your house by precisely following the various retrofit guidelines, including the most recent one FEMA P-1100 Vulnerability Based Seismic Assessment and Retrofit of One and Two-Story Dwellings and your house will be just as exposed to damage as it was before you did anything at all.  Many people do not understand that retrofitting room additions is as important as the house itself. This webpage will help you make sure this does not happen to you.  Many older homes have additions that can seriously compromise a retrofit if not dealt with properly.

It is not uncommon for an older home to have originally been 900- 1200 square feet.  Over time demand for larger homes increased considerably.  Many homeowners built additions on the back of their houses (such as the 1955 house I live in).

The building code does not tell a contractor how to connect an addition to the main house and contractors are not thinking of earthquakes when they build an addition.

Existing Cripple Wall Bracing Is Often Missing

If the houses have wood siding, contractors often completely remove the wood siding from the rear main house cripple wall.  I imagine they do this because redwood siding is expensive.  At the very least they remove the top 3 or 4 courses of siding so they can access the floor framing of the main house they need to attach the addition to.

The studs stay in place.  Because stucco is so much harder to remove, contractors usually break out the top 12 inches of stucco to access the main house floor framing.  In both cases contractors reduce the existing lateral support of this cripple wall considerably.

This severely weakened cripple wall will need to lateral resist forces from the main house AND the addition.  This cripple wall therefore needs additional bracing compared to the other walls.

You could easily do a retrofit on the perimeter (front, sides, and back of the addition) as prescribed by FEMA P-1100, Appendix Chapter A3, or Standard Plan A on a house like this and accomplish nothing because the most serious weakness would have been overlooked.  If you have an addition, make sure your proposal mentions this and find out how the contractor is going to deal with it.

What To Do About It

 The best way to address this is by looking at the addition and the main house as if they are separate houses and each one needs its own cripple wall retrofit.  Another way is to connect the addition to the main house, so they don’t separate. This will also work but that should be a second choice.

Sometimes the addition is attached to the floor in such a way that you can’t attach a shear wall to the addition.  Contractors usually leave the stucco on the cripple wall.  There is no one size fits all solution to this.  Frankly, you simply need to know what you are doing.  These situations usually involve a lot of problem solving.

That being said, lag screws, machine screws, and nails are the most common types of connectors.  Assuming the two floors are butted up to each other, Simpson SDS Heavy Duty Connectors often do the trick.  Be sure the screws go all the way though both floor joists and blocking when you screw them together.  They come in 4 1/2″ to 8″ lengths. Sometimes custom-made floor to floor connectors must be designed on site using all-thread. As far as the quantities go, just put in a whole bunch.  That is a lot cheaper than paying someone to figure out how many you need.


          Simpson Heavy Duty Connector


Earthquake pushing home addition away from main house

                                        EARTHQUAKE SEPARATING AN ADDITION AWAY FROM MAIN HOUSE.

Floor to Floor Connectors When Retrofitting Room Additions

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




Tension Ties in Retrofitting Room Additions

Separation can also occur when the main addition rotates away from the addition.  To the right is one solution using horizontal hold downs.



Tension Tie Holding Two Floors together