Expert: Daniel Humphrey - 1/25/2008

Question
QUESTION: We are having an addittion built between our house and garage. It is 15 feet wide, attached the house on on side and the garage on the other. Underneath is open no foundation and about six feet off the ground.

The builder has run 2 x 12 joist between the two building,
the first one is doubled. He says no other support needs to be added to the floor.  He did say that there is a 15 ft header that supports the roof and that the only weight on the joist is the wall and that is not much.
I thought a center beam with footings and columns would be used under the joist.  The room will be used as a family room.
Should I be concerned about the span?
Would or should the building inspector pick up on this when he inspect the job?

Any advice you can share would be greatly appreceated.

ANSWER: Rich,

I'm assuming the house and garage are 15 feet apart. A 2x12 joist system on 16" centers is pushing it for the that distance, not so much for strength, but for stiffness.  To keep the bounce down, you could put in a saddle beam beneath the midspan that is hung by metal straps from the walls; this will join all the joists and dampen the bounce. Even if the span is shorter, this will help keep deflection down, especially if the room is 15 feet wide as well.

You have plenty of space for a good size saddle beam. A nice one would be made of two 2 x 12 sandwiching a 11-1/2" strip of plywood, all glued together and nailed very well.

I know a doubled 2x12 doesn't seem like much, but you'll realize how strong that wall system is when you consider those outer walls will act as deep trusses if built correctly. Your builder is right, but Make sure you sheath the walls with plywood and maybe even put in diagonal blocking--Old School. This will ensure that the outer walls act as trusses.

You do not want a mid-span support going to the ground. This could create a see-saw effect if the garage and house move at different rates. Lots of buildings with mid-span piers have trouble with the floor bowing up, doors sticking, and sheetrock cracking. A saddle beam hung securely from the outer walls will do.

I know these suggestions are a bit unusual for most residential builders. But they are sound and inexpensive, even if they are mainly for your peace of mind and to keep the vibrations down when the family, or family dog, trots through!

Good luck,

Daniel

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

QUESTION: I'm already feeling better thanks.
Not sure what a saddle beam is?
I just tried to google it and not much luck.

Answer
Rich,

A saddle beam is one that hangs by metal strapping underneath the joists, rather than being supported by pillars. It provides stiffness by tying all the joists together on the lower plane. It functions in the same way as a summer beam, except that there is no direct load path to ground.

It is used in bridges most commonly, to support the road decking by hanging from the two large outer trusses. It's called a "saddle" beam because the joists sit on it, and because the beam itself often sits in a saddle bracket hung beneath the wall trusses. It's also called a "drop beam", perhaps more in the Eastern U.S. where you don't have as many cowboys bending nails.

BTW, I added some info just as you were reading the last response. Refresh and see if you can get the new info there in the first post.

Dan