Sistering Floor Joists For Attic Renovation - Lumber Size/engineered/LVL Etc. - Building & Construction - DIY Chatroom Home Improvement Forum

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Old 05-26-2012, 07:47 PM   #1
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Sistering floor joists for attic renovation - lumber size/engineered/LVL etc.

Hi folks,

We're in discussions with a builder and an architect about reinforcing the existing floor joists in our attic in prep for an attic renovation. The house was built in the early '30s. The existing joists are "true" 2x6s and I think are roughly 16" OC (I say "roughly" because there seem to be a lot of exceptions to the rule, leaning toward the "more crowded" side). The span is roughly 11 or 12 feet or so on each side.

Our conundrum is that, even though we are considering adding a dormer, we want to retain as much head room as possible. We have a few of those "Attics and Basements" type books from Home Depot etc. They typically say to sister with the same lumber as the original.

However, the architect consulted with an engineer (who has not seen the house) and apparently the recommendation was to sister with 2x10s! Adding a new finished floor to this means we would be losing another 5 inches or so of head room. The existing roof is a typical "triangle" shape (not an expert here!) and I think it is something like 8'2" in the middle, at the top of the center ridge line... so one doesn't have to wander far from the middle before one would hit one's head.

Note that apart from a possible shed dormer, we are not adding another floor or anything. We just figured that 2x10s seem like overkill when these "DIY" books say we can sister with the same lumber.

Also, if we "must" sister with something beefier, what about using engineered lumber somehow? It seems that an "I-beam" or "LVL" board of less than 10" height would provide the "same" strength without the loss of headroom. However, I have been unable to find anything on the web that explains how one would actually sister a "conventional" 2x6 with an I-beam or LVL, or whether this is even doable and/or recommended in the first place. Isn't LVL a lot heavier than conventional lumber? Another idea that I've heard was that it seems it might be possible to just interleave new I-beams (higher than 6") between the existing joists, effectively neutralizing the old boards, save for their own weight still being in the picture.

Obviously what seemed to start out as a straightforward concern has ballooned into a confusing web of questions! Any suggestions/ideas would be much appreciated.



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Old 05-26-2012, 09:56 PM   #2
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Listen to the engineer, he knows what he is talking about.


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Old 05-27-2012, 06:48 AM   #3
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Originally Posted by BeanAcres View Post

Any suggestions/ideas would be much appreciated.

Don't read DIY'er books when it comes to structural work. Very dangerous!!! Listen to your engineer. Ask him if he thinks a 2x8 joist will work on a 11' or 12' span.
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Old 05-27-2012, 07:00 AM   #4
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That idea of sistering 2x10s will only add 2 inches---cheapest way to go---

If you are willing to spend more for doubled 2x8s or engineered joists--tell the architect that the budget will be available--
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Old 05-27-2012, 08:50 AM   #5
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a 12" ( 11 7/8" exactly) lvl ripped in half to 6" would be really strong and maybe cover the 12' span. an engineer would need to approve that of course. the box store diy book does not take into consideration that the way they built long ago is not up to standard code so just nailing on (emulating)what they did does not work now.

another issue, some of the balloon framed houses have the joists nailed to the side of the stud that forms the exterior walls. there would maybe be nothing for you to set the new joists onto at the exterior wall.

also, the 2x6's there now were built for holding up the ceiling in the room below and not built for a floor system for a room above.

Last edited by hand drive; 05-27-2012 at 09:03 AM.
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Old 05-27-2012, 08:58 AM   #6
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Not sure what the end goal for this reno is but you might want to understand local regulations on head room for occupancy.
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Old 05-28-2012, 12:05 AM   #7
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I'd consult another S.E.unless he was told the full dormer and 1/2 roof load is on the floor. With an existing room above and a front dormer addition with a Building permit, it comes down to bearing and design. One choice is bearing the dormer side-walls and 1/2 the roof on doubled or more side rafters (and new full-front dormer header) with new bearing down to earth. This would entail only floor loads on the doubled floor joists, no more. Second choice is bearing the dormer walls/1/2 roof loads on all the doubled floor joists for the over-spanned existing. My choice would be to gable truss the dormer, bearing on the added side rafters. Add doublers for new floor loads. single 2x6, f.b.1500, DF- 12' span = 52# sq.ft. = 39# @16"o.c. DBL 2x6 1500f.b. = 87# or 65#sq.ft.
2x6 (check for species) 1300f.b. = sng. = 34# sq.ft. Dbl. 1300 fb. = 56#sq.ft. I sistered my 12' and 15' span 2x6's of 1915 house in '89 (16d nails, 8"o.c, staggered with construction adhesive, P&L ceilings -- no problems yet-...... no roof loads either). Or, as Joe said- 2x8's work with a 40# load:



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