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Wayne Burress 12-09-2007 06:40 AM

how to raise 6" joist to 10"
Over our garage the contractor used 2"x6" joist supported by a hidden beam running across the center. I want to make this into a "frog" or extra room for kids. Is there a way to just add 2"x4" on top saving expense of tying in actual 2"x10" new boards?


JazMan 12-09-2007 10:32 AM

I don't understand what the 2x4's would do? I think you're asking if you can 'sister' new joists to the old ones to increase the floor's strength? If there aren't too many obstructions I think you can do that.

Before you do that though, I wonder if you really have to do that? I believe min. L360 deflection is NOT required in 'sleeping rooms' and maybe other types of rooms? If the max span of your joists is around 10'6", you should be close to L270 and might be OK? Check your local code and with your building department.


Cole 12-09-2007 11:14 AM

I agree, code will be different wherever you leave.

If it was me, I would laminate or sister new joists to the existing. Over-building is not a bad thing.

What is the span for the joists?

You say there is a beam which I assume is halfway in the middle of the garage to split the span so he could use 2x6's instead of 2x12's (etc) for the entire span from wall to wall.

How big is the garage?
How big are you wanting to make the room?

Edit: You will have to probably bring in a structural engineer for the load, the existing beam will only support so much weight. It might be or might not be rated for additional pounds.

Wayne Burress 12-09-2007 12:46 PM

how big is room and garage?
The garage is 20'x20' The 2x6 joist are 12' overlapping over the beam. The new room will be about 20'x12'.

For strength I thought about using the 2x4s across the top of the 2x6s (like a tic-tac-toe grid). I don't know what they are called but they are metal pieces shaped like a cross that attaches to both joist below and studs above.

My contractor said the cross beam is more than large enough to support the additional 2x10s. He won't advise me any "off the record" and I understand.


Cole 12-09-2007 12:53 PM

What will the room hold? Furniture?

Are you planning on using plywood for the sheathing?

IMHO (In My Honest Opinion) I would go for the 2x10's over the grid pattern of 2x4s'.

Kingfisher 12-09-2007 03:48 PM

2x4 on top don't make a 2x10:no: Running them across the 2x6 is even worst adding nothing to the floors load bearing, add the 2x10 to the side of each 2x6 other wise your just looking for problems.

Wayne Burress 12-09-2007 03:53 PM


Thanks for your response.

Mostly the room is to be used to display a model train layout to be built in the shape of a "U" around 3 outside walls. It will not be used as a "romper room".

I am planning to use plywood sheathing as a sub floor and hardwood floors on top of that.


Cole 12-09-2007 05:32 PM

If I was your contractor, the charge from 2x10's to 2x4's would not be a big difference. Take in mind, you will have a stronger floor and it will let you have freedom to do other things with the room in the future. Basically, the labor will be real close, it's just the material cost that will be higher.

My 2 cents, go for the stronger floor!

Wayne Burress 12-09-2007 08:24 PM

Good Advice...
Thanks to all. I will add 2x10 (sister) to the 2x6. I know that will be the safest way to go.

By the way, the house was already completed when I bought it and moved in. I appreciate again all the help!


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