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Old 10-22-2007, 01:12 PM   #1
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1918 Craftsman Attic Floor


Just bought a 1918 Craftsman style house in Minneapolis MN that has balloon framing. We want to finish off the upstairs with a master suite that is 500sf and has a full bath. The Problem is the current floor jousts are only 2x6 on 16" OC with a 11ft Span from exterior walls to interior bearing wall. My Home inspector suggested using double thick subfloor so 2 x 3/4" T&G OSB to add strength to the floor but I wonder if sistering would be a better way of doing it? Please see pic if it helps. any input is appreciated I'm looking to submit plans for a permit soon. [IMG]file:///I:/DOCUME%7E1/THESCO%7E1/LOCALS%7E1/Temp/moz-screenshot.jpg[/IMG]



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Last edited by CwrenchSeth; 10-22-2007 at 01:17 PM.
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Old 10-22-2007, 03:20 PM   #2
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1918 Craftsman Attic Floor


I think you should call a local engineer to help you out.

We may able to give you some ideas, but they are ideas and no one can physically take a look at your current situation.

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Old 10-22-2007, 06:15 PM   #3
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CW: I agree with Cole to be 100% sure. That said if it was my home( which it is close to being :}:}:}:} ) I would sister every joist, that will get you to 4 x 6's on 14 which should be fine, just make sure they are full length and run from plate to plate NOT just attached to the existing ones. Glue and screw your sub flooring down, 3/4" T&G minimum. Also glue and screw the sisters.
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Old 10-23-2007, 09:14 AM   #4
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1918 Craftsman Attic Floor


My attic too has 2x6 joists, are we talking about sistering every 2x6 across the entire floor? I agree with the idea but in my own home my thoughts were to do this only under the bathroom.
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Old 10-23-2007, 10:17 AM   #5
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Money; If you are going to make it a living space and plan to walk to and from the bathroom then yes every joist. the joists were used only as nailers for the ceiling underneath NOT as floor members. To take a live load from scratch they would be usually 2 x 10's on avg, therefore by doubling up you create 4 x 6's which are very close strength wise to 2 x 10's.
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Old 10-23-2007, 10:52 AM   #6
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Originally Posted by skymaster View Post
just make sure they are full length and run from plate to plate NOT just attached to the existing ones.
But in this balloon framed structure there is no plate for the joists to rest on. I don't know much about balloon framing, but it seems sketchy to just rely on those nails holding the joists to the side of the studs!
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Old 10-23-2007, 10:58 AM   #7
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Nate; normally what you dont see is instead of a plate as in todays platform framing there is a 1 x 8 let into ( like a big dado ) the wall framing and the joists rest on that and are nailed into the studs. Acts similar to a plate. My house is balloon also.
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Old 10-25-2007, 06:33 PM   #8
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1918 Craftsman Attic Floor


I agree with the others. Usually when you size beams, you are sizing them based on the required moment of intertia. In this sense you can "tack on" another 2X6 to the other side and meet your intertia requirement. However the neutral axis is key when designing for flexure (bending). This determines what portion of the beam in under compression and what portion is under tension. The only thing you are doing by adding another 2x on the other side is reducing the distributed load. There are two ways you can increase the strength of your floor:

1. increase the depth of the floor joist (this would involve scabbing another board onto the joist)

2. decrease the distributed load applied to the floor (space your extra 2x6 joist evenly across the floor. So now you have your joist 8" o.c. and an equivelant depth:distributed load of a 2X12)

Either way youll probably be ok. If anything youll feel the deflection the most, which is where your 3/4" plywood will come in handy. If it were me I would space them 8" o.c. If you still feel like you want to tack one on the other side, dont do anything dumb like put a filing cabinet or water bed up there.

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Old 11-14-2007, 10:24 AM   #9
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1918 Craftsman Attic Floor


I am remodeling a recently bought home, removing load bearing walls etc - only to find that a previous owner knew just enough to be dangerous. He added a second story, but did not reinforce the 2x6 ceiling joists. He simply layed 2x4's perdendicular to the "ceiling" joists 16" OC and used a single layer of tounge and grooved 3/4 " floor boards. upon inspection there are many places where the crossing 2x4 do not even tough the underlying joists. I have two sections of the house that have 2x6 southern pine #2 14.5' floor joists 16" oc. There are no calculators that I can find online that will allow me to calculate appropriate spans as a function of sistered 4x6 joists or at a spacing of 8" oc. Where might I find the equation?? One of the upstairs rooms is a game room and I plan on putting a pool table in it. Should I sister 2x6 joists or space then 8" OC. Should I mount an additional 2x4 along the top of the joist so that the joist contacts all the 2x4's of the 2nd floor support lattice?

thanks in advance

mike

thanks

Last edited by mike.MGI; 11-14-2007 at 11:19 AM.
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Old 11-16-2007, 08:06 AM   #10
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Originally Posted by mike.MGI View Post
I am remodeling a recently bought home, removing load bearing walls etc - only to find that a previous owner knew just enough to be dangerous. He added a second story, but did not reinforce the 2x6 ceiling joists. He simply layed 2x4's perdendicular to the "ceiling" joists 16" OC and used a single layer of tounge and grooved 3/4 " floor boards. upon inspection there are many places where the crossing 2x4 do not even tough the underlying joists. I have two sections of the house that have 2x6 southern pine #2 14.5' floor joists 16" oc. There are no calculators that I can find online that will allow me to calculate appropriate spans as a function of sistered 4x6 joists or at a spacing of 8" oc. Where might I find the equation?? One of the upstairs rooms is a game room and I plan on putting a pool table in it. Should I sister 2x6 joists or space then 8" OC. Should I mount an additional 2x4 along the top of the joist so that the joist contacts all the 2x4's of the 2nd floor support lattice?

thanks in advance

mike

thanks
Theres not just one equation, there are at least three that a typical structural engineer would check; flexure, shear, and deflection. A lot of these have subparts or charts or variables which need to be quantified. These online calculators you find sometimes give you a small snapshot of whats going on but you really need someone who knows what they are doing to look at your scenario. As far as the 4x6 vs. an additional 2x6 of equal spacing...think of it like this. Take a piece of paper....hold it with one hand on each side. Its flexible in the middle right? Now lets say its 8.5X11. Does it help to increase the area? would a 11x17 sheet make any difference? No, but if you were to get a stack of 100 sheets of paper and hold it in the same manner it will flex much less. You need to increase the load/depth ratio. This can only be accomplished by increaasing the depth of the beam or decreasing the amount of load applied to the beam. Since I suppose your trying to maintian a ceiling height, your only other option will be to decrease the distributed load to each beam and thats done by decreasing the spacing of the beams not by making wider, same depth beams. Hope this helps some.

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