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Gray 06-23-2010 11:41 PM

floor joist spans
We have a two story home with an attic that's not used. We would like to remove a wall to make our living room (which is on the first floor) bigger. The wall supports the second floor (which has a bedroom and closet) and the wall between the bedroom and closet is directly above the wall we want to remove. Above that wall on the second floor is the ceiling joists (which are also the floor joists of the attic, except there is no floor up there, just insulation)

We would like to remove the first floor wall and have joists span the 21 foot distance from outside wall to outside wall. The joists we have are 2x7 and 16" on center. We know that we need to do something else so that we can span the length we want. We were thinking that we could add 2x10s sistered to the 2x7s. From charts we could find online, we could use 2x10s but they would have to be 12" on center. But none of the charts take into consideration that we are sistering them to 2x7s. We really want to have the room completely open with no beam where the wall was and we can't go any bigger than 10" for the floor joist depth. We would be willing to put 2x10s on either side of the 2x7s or space them even closer than 12" on center, but we can't find any charts to say that this would even work.

What do you think?

Ron6519 06-24-2010 01:09 PM

Bring in an onsite professional to spec the beam. Guess work is folly.

Gray 06-24-2010 01:35 PM

Well, how does a pro do the math to figure things? I would love to find a table or joist calculator that would allow me to find the maximum joist span for a 2x10 spaced 6 or 8 inches on center with 40 live load and 20 dead load.

drtbk4ever 06-24-2010 03:23 PM

I think a beam woud be a better choice as I doubt a 2X10 will span 21 feet.

But hey, that is why I personally would hire an engineer.

Daniel Holzman 06-24-2010 05:22 PM

It is possible to determine how much a sistered joist combination could handle. The method is to compute the moment of inertia of the composite section, factoring in differences in modulus of elasticity of the different pieces (every species of wood has a different ME). The fasteners also need to be designed appropriately. This is typically done by an engineer, as there are not going to be standard tables for composite sections such as the one you are proposing.

Determination of allowable span starts with precise geometrical information about the members. I wonder if you have a 2x7, it is certainly not a commercially available structural element. Maybe you have a dressed 2x8, which should be approximately 1-5/8" x 7-1/4". You also need to know the load you will be supporting, which is typically taken from code requirements.

A structural engineer could take all the information and determine the most cost effective solution to your problem. This may be a good way to go, especially if you plan to do this work legally and pull a permit, in which case the code enforcement official will probably require a stamped drawing showing the size and installation method for the joists.

jklingel 06-24-2010 06:21 PM

Daniel hit it
No way in hell are 2x10s going to work for that span, just based on experience and what the engineer called for in my house and others. To get a very, very rough idea of what a board will carry, there are plenty of tables available at lumber yards, etc. I have used one at the back of my Stack Steel handbook, but that is just for rough numbers, not something serious like a house. Hire an good engineer. P, EOS.

jogr 06-24-2010 08:37 PM

A 2x10 won't span 21 ft even sistered to 2x8s even at very close spacing. You'll need to put a 9" steel beam across at mid span within the floor. And hang the 2x10s from the beam. An engineer can spec the beam for you. It will need proper support posts at each end. That support must be solid all the way down to footing. The engineer can help you with that too.

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