Expert: Richard Taylor, AIA - 3/15/2010

Question
I have a 1960 era ranch house with a wood frame and poured concrete foundation. My master bath is very small and I would like to enlarge it and perhaps the bedroom as well.  Is it possible to use a cantilever to expand the exterior wall of those rooms to avoid pouring an addition to the foundation? If it's possible, is it cost-effective and safe?  Thanks for your help

Answer
Athene - thanks for your question!

A cantilevered addition is often an easy and effective way to enlarge a room or two - good idea!  Properly designed, it's as structurally safe as any other part of the house.

But cantilevers don't work in every situation; sometimes it's easier to pour a foundation.  Here's what to look for.

Start with the direction of the floor joists - if the cantilever is going to be extended parallel to the floor joists you're off to a good start.  The cantilevered floor system needs to have at least 3 times the length of the cantilever "buried" in the existing structure; in other words, if the new cantilever is going to add 3 feet, you need at least 9 feet of floor joist length into the house - that means you're going to be buying 12-foot long joists and sliding 9 feet of them into the house, leaving 3 feet sticking out...make sense?

Of course you'll need to move any plumbing, heating ducts, electrical wiring, and blocking that is in the way and reinstall them when you're done.

If the cantilever is going to be extended perpendicular to the floor joist direction, you have a big framing job ahead of you - might be easier to put in a foundation instead.  Every situation is different, so check with a design professional in your area.

You also need to pay attention to the distance the bottom of the cantilever will be above the ground.  After all, you need to crawl under there and put insulation in the floor joist spaces and attach some kind of soffit board to the bottom.  You don't want a small, dark space under the cantilever that will stay wet and attract critters, either!

Finally, you need to consider the roof over the cantilever and the load it will place on the new floor system.  Again, check with a local design pro to figure out if this makes sense for you.

Bottom line - yes it's possible, yes it's safe, and yes it's feasible - sometimes!

Hope this helps,

Richard Taylor, AIA
www.rtastudio.com
www.rtastudio.blogspot.com
www.rtaplans.com