Homeowner Forum / Construction / January 2006

The code  (IRC 2000) specified live load for rooms other than sleeping
rooms is 40 psf live + 10psf dead. For that load, 2x10s @ 16" O.C. of
the common lumber species can span 15'-0" and a bit more. Check the
spacing of the 2x10s and the distance between supports. You can
probably add a beam & columns at the garage level if you need to.
TB

I'll give you my opinion as to what all's involved. What you actual end up
doing is your business.

*First you will need a building permit as it adds a habitual room (living
space) to the existing dwelling.

*The separation between the room and the garage will need to be increased to
5/8" Type X gypsum (R309.2)
(This is important for fire safety and insurance purposes).

*Next you need to concern yourself with natural light and ventilation
(R303.1)
(You can get by with art. light & mechanical vent at the rate of 0.35 air
changes per hour, bath vent fan could probably do this.)

*Ceiling height will need to be 7', except 50% may be less that 7' but not
than 5' (This would govern where to place the stubwalls) (R305)

*The existing ceiling joists may need to be increased in size depending on
the grade and material originally used. Table R502.3.1(2) gives a rough
approximation.

For a 30 psf snow load, spanning (approx)18' @16" oc:
   #2 or better Southern Pine 2x10's will span 18' (OK)
   #2 or better S-Y-P (most common in the east and Midwest) will go 17'-2"
(Probably OK)
   #2 or better Him-fir will go 16'-10" (getting a little bouncy).
(actual clear span between exterior walls)
(Assuming a 40 psf live floor load and a 10 psf dead load (no interior
partitions.)
If the existing joists are #3, do not have sufficient span capability for
this situation.
If the existing ceiling joists are spliced, you have no alternative but to
install new joists along side. (But they should be full length).
You could make the install stronger by bracing the joists at mid-span in the
garage and gluing the floor decking down to the floor joists. Keep the
supports in-place until the glue is fully cured (depends on temperature and
what construction adhesive was used.) This would give a T-joist effect by
increasing the Section Modulus and a stronger floor.

*You will need to install electrical outlets (6' from a doorway and 12' oc
thereafter). You will also need adequate wall-switch controlled lighting at
a rate of 6 foot-candles at 30" above the floor (a couple of 100 watt bulbs
will do this).

*You will need to insulate the underside of the roof, walls and 'garage
ceiling/office floor' to comply with your state energy code. The area will
extend your interior envelope (the amount of area that the existing furnace
will need to heat) and may be a consideration with the local building
official (adding a 4' or 6' electric baseboard will be easier than extending
the existing heat ducts.)

*If would ever have plans to convert this to a future bedroom, you would
need to add a 5.7 sq. ft. window (for emergency egress and fire dept.
access.)

This is only an opinion, but I think it probably is doable.
Dennis