Attic joist strength for attic conversion

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Old 08-10-14, 06:28 AM
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Attic joist strength for attic conversion

I am having trouble interpreting these online calculators for load to make sure my Attic has proper strength. As such, I am curious if someone knows how I could ensure my attic truss system can support what I want to do.

I have knee high wall truss style for my attic, and I want to put in a stair case and convert the attic to ~600 additional usable sqft. Currently it is unfinished.

Although the house is 32' by 40', there is a load bearing wall on one side at 8', and on the other side at 8 ' is a 4 board think HUGE (2x12/2x14 or so) support beam that the trusses are hanging on with joist hangers.

This gives 16' wide room, where the joists are 2x6 that are spaced between 14 inches and 16 inches apart. The wood kind I am uncertain of, but it felt a bit rougher than normal fir or pine. I would assume for calculations a weaker wood.

Thoughts on how to calculate if I can make this work?

Also, thoughts on how to make it stronger? Cross beams that distribute weight? Some kind of baseboard/plywood that makes the floor stronger?
 
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Old 08-10-14, 06:42 AM
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The minute you say "truss" you're out of the realm of online span calculators. If they are engineered trusses they are probably not designed to handle the load of finished space up there unless they were specifically designed to do so. A 2x6 bottom cord also suggests that it was designed to carry more than the weight of the sheet rock ceiling below. Best would be if you can locate the records for your house and track down the original engineering for your trusses, you might get lucky and find that they can carry the load. Baring that the next option would be hiring a structural engineer.
 
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Old 08-10-14, 07:13 AM
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Truss may be the wrong word since it's 2x6's on joist hangers. Not an engineered truss as the bottom beams tie to the large support beams mentioned previously. I am not sure the difference between what is up there and what I would call a truss. House built in 1950s. There are windows already in the attic looking out on both sides already. Was odd to see the joist hangers for me, since it's a 1950s house. Hiring anyone around here is... hard. Even running after people with a check ($), it seems like there is a lot of work and people are booked up. It's more a curiosity right now but trying to get someone out here is still a bit of work. The area is rural just outside a large metropolitan area so most people get centralized work here closer to them.
 
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Old 08-10-14, 08:18 AM
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Years ago my mother wanted to do the same thing with her attic. She was told that the 2x6 joists needed to be 2x8s. That was the end, of the conversion idea.
 
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Old 08-10-14, 11:08 AM
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Any way you can post a drawing or sketch of the joist layout and their spans? I'm thinking 2x6" is the end of the story but you might be able to sister somehow with larger material.
 
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Old 08-10-14, 11:13 AM
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If all you want it to be able to store some lightweight objects in the attic the 2x6 joists will probably be okay. If you actually want to make it living space then no way will the 2x6s be adequate. There are also the matters of insulation, heating and cooling to consider.
 
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Old 08-12-14, 06:52 PM
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Thanks all. As for heating, cooling and insulation, it's not structural and I can hire out for that part. I will see if I can get some pictures the next time I am out at that house (project house).

I was thinking about if I could hang stronger beams somehow, if on the one side they use joist hangers to the huge "header" like support beam, and on the other side they somehow rest on the load bearing walls. Since the 2x6s use joist hangers, I would not know if removing them to replace with something else would be much of an option I'd like to go with. If I could build the floor up and lose some height, I would be fine with that.

I am planning to make this into livable space, like a study or hobby area.
 
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Old 08-12-14, 07:15 PM
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Also, one of the later options, I had been looking this over: Beefing Up Attic Joists for Living Space - JLC Online

But I still don't know what my psf is now and how to get it to a suitable number in the most optimal way.
 
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Old 08-13-14, 04:18 PM
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First off, we can't give structural suggestions sight unseen and your description leaves a lot to the imagination. You can't use 2x6's that span 16' as living space. If the "beams" you speak of would happen to be large enough to do what you suggest, and their supports are correctly transferring the load path to the foundation (big if's) then perhaps removing the 2x6's and their hangers, then replacing them with 2x12's in hangers would provide the support you need for a floor.

But I would suggest you hire a structural engineer to actually look at it and formulate a plan. This isn't something you can design over the internet.
 
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