Expert: Ted Barnhill (Principal, Design45 LLC) - 3/5/2009

Question
QUESTION: I want to turn an existing bay window in Michigan into usable floor space with
the same footprint.  Is there a maximum amount of distance I can cantilever
out BEFORE I need to get a structural engineer.  I was told it is 3 feet.

Also, what is the best way to insulate and heat this new floor area.  Should I
build a complete enclosure from the ground up to the new floor area or just
close it up somehow and get some heat in there by extending a heat duct
from the room the bay is in?

Thanks in advance

ANSWER: Rick,

There is a table in the International Residential Code which shows allowable cantilever spans based on a number of factors: size and spacing of floor joists, roof width, and ground snow load in your area (because the code assumes that the cantilever supports a wall and roof).  In a nutshell, 36" is on the optimistic side, though in extreme cases it can be up to 48".  If you have an older home, it is more likely to be less than 24".

Your local building official will be able to tell you the ground snow load (ranges from 20 to 100psf in Michigan) and if you measure your joists, he/she will probably help you read the IRC.

As for insulation, you would not want to enclose to the ground as you'd defeat the purpose of cantilevering.  Typically, I specify spray-foam insulation for cantilever.  You probably won't need to modify your heating layout for just a small cantilever, though if you can manage in-floor radiant heat it takes the chill off nicely.  I hope this helps.  Good luck, Ted.

---------- FOLLOW-UP ----------

QUESTION: The reason I asked about building an enclosure to the ground was so I could
enclose the new floor area as it will now technically be "outside".  The
enclosure would not be "structural", rather just a way to make an enclosed
area that I could heat or at least insulate a little better.

The floor heat grille is centered right in front of the existing bay window so I
have to move it anyway or it will get stepped on.  Since I plan to add a French
door to the new bay area and 2 windows (one on each side of the bay), I was
concerned that this area would get cold.

I understand building the enclosure is sort of overkill, yet wouldn't this help
with keeping this new area warmer seeing is will be mostly glass (doors and
windows).

Lastly, there is a glass block window centered right under the window bay
right now.  Should I remove that and fill in that area with cement block so as
the new cantilevered joist do not sit on top of the block window?  Can I leave
the glass block if their in a metal piece over it for support (lintel ??)

Thanks!  

Answer
Rick,

There is plenty of room within the depth of your floor joists for adequate insulation.  It would be a good idea to have a heat register in your new space near the windows and door.  If for some reason you can't run it within your joists, then extending the structure down to accommodate the ductwork is an option, though it may be odd looking from outside.  However, you wouldn't want to extend to the ground, as frost heaving in winter could damage your structure, and wood should not be in contact with soul.  Maintain at least a 12" space.

The lintel at your glass block is currently load bearing.  While your cantilevered joists will increase the load, in many cases the existing lintel will be able to carry it.  However, it is an aesthetic and cleanability issue; depending how far you cantilever, I would consider removing the window.  If you want to keep it, have a professional look at the lintel.

By the way, sorry for the delayed response; I injured my back and have been limited for the past week.  Good luck with your remodel.  -Ted