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Old 11-30-2011, 10:24 AM   #1
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New Floor Joists for 3rd floor attic remodel


I have a existing 2.5 story with stair access to a 3rd level. This existing attic space has 2x6 @ 16oc floor (2nd floor ceiling). 2nd floor ceiling's are plaster/lathe. I have a large cathedral attic ceiling and installed closed cell spray foam in rafters and want to build an adequate attic floor system to support it. Attic spans are 10'-14'. Two questions:
1.) Can I get by with 2x8's both floated and/or sistered to existing 2x6's or do I need to use 2x10's? With some blocking mid-span my hope is to use 2x8's.
2.) My basement is being updated with steel structure but do i need to be concerned about the 1st/2nd floor wall construction and the additional weight bearing on it?

Thanks!

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Old 11-30-2011, 11:13 AM   #2
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New Floor Joists for 3rd floor attic remodel


2X8s will be fine for a 10 foot span but will be a bit bouncy for 14 foot. If I were installing the joists I would hold the joists up away from the lath and plaster. Fastening the joists to the existing joists will be tricky also as it will be easy to disturb the plaster. You for sure do not want to use a hammer on the joists.

I would be sure to check to see if your walls were able to handle the extra pressure. There is no way we can know that, you may have to call someone in to check that. The added pressure will be on headers, foundations etc.

Being the third floor, some one will sure have their work cut out for them.

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Old 11-30-2011, 11:25 AM   #3
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New Floor Joists for 3rd floor attic remodel


I don't know where you are located but the CBC which is based on the IBC shows that 2 x 8 s 16" O.C. just barely makes the grade if using Douglas fir #2 for a 14' span.
Deflection is an L/360 which should be comfortable enough.
Maybe you should have this drawn up and taken to the local Building Department?

Andy.
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Old 11-30-2011, 12:25 PM   #4
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New Floor Joists for 3rd floor attic remodel


I had planned to lay either 1x or 2x pressure treated pieces on the existing top plates between the existing 2x6's and rest the 2x8's on them to prevent damage to the plaster. If I were to move to 2x10's that's another 20% in weight and I may not have enough clearance to connect the 2x10's to the top plate as the roof line would interfere. I could chamfer them down I suppose but I may already have to chamfer down the 2x8's in a few areas. I have an engineer and architect on board and have drawings, just looking for more encouragement before I attack this myself.. House was jacked up this morning and new steel posts/beams being installed to help carry the attic weight. Thanks again.
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Old 11-30-2011, 12:58 PM   #5
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New Floor Joists for 3rd floor attic remodel


Do you doubt the architect/builder that you have? I don't know why you're in a forum. sounds like you got it figured out pretty good.
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Old 11-30-2011, 02:03 PM   #6
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New Floor Joists for 3rd floor attic remodel


Yes you have some good ideas. The advice you got from JiJu1943 and Andy is spot on. If you go the 10'0'' span use 2''x8'', or 2''x10'' for 14'0''. You can chamfer the absolute minimum amount to fit the 2''x10'' under the roof space (go between the roof fafters and nail floor joists to roof rafters at edges). This will strengthen both the floor and the cathedral roof.

Fill between floor joists with 4'' batts to improve insulation and reduce noise transmission from attic to 2nd floor. Trust your architect to do the right thing (they carry indemnity insurance as a last resort if something goes pear shape). Finally, don't exceed 1'6'' spacing between joists.

As far as using 4''x1'' spacers (laid flat on studs), that's fine, as long as it doesn't increase the amount you have to chamfer from the ends of the joists.

Regards from Joe in Oz
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Old 11-30-2011, 02:43 PM   #7
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New Floor Joists for 3rd floor attic remodel


Did anyone take into account he would SISTER the existing 2x6?

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