Sistering 2×6 Floor Joists

sistering 2×6 floor joists

A lot of the older houses I work on have framing that is overspanned and undersized.  The standards were different back then.  You might want to reconsider that King Sized Waterbed up there.

You won’t find too many floors framed in 2×6 unless the house is pretty small or very old or both. Having a max span of under 10 feet, they’re not real useful on houses wider than 20 feet unless there are multiple beams used to hold them up.

Today’s newly constructed floor systems contain joists that are usually a minimum of 2 inches by 10-inches. Older homes, however, sometimes contain smaller-dimension floor joists. Additionally, attic floor joists often contain smaller dimension lumber, which requires reinforcement before adding an attic loft.

The Problem

When designed correctly, the floor joist system provides a measure of support for the walls and the roof as well as the subfloor. Its structural integrity, however, is only as good as the combined strength of the floor’s members. Joists install on-edge, which creates a stronger overall system, but two-by-six floor joists are relatively small, and when used to span distances greater than 8 feet, can result in structural problems. The floor might feel spongy or bouncy underfoot, or you might notice a sag in the middle. There are a few ways to correct the problem.

Reinforce the Joists

If the floor is bouncy but the joists are not sagging, the simplest fix is to reinforce the joists by installing additional joists adjacent to the original ones. Called “sistering,” this is done from beneath the floor in the crawl space. New two-by-six joists are cut to fit snugly beside the old ones. Because the weight of the house bears on the original joists, it’s a tight fit at the ends of the joists. To get the new joists in place, you might have to beat them in with a sledgehammer. Once in position, attach the new joists to the existing joists with framing nails or screws. You can sister the joists on just one side or on both sides for added strength.

Support Sagging Joists

If the floor is sagging, the joists beneath are also sagging, which makes it impossible to sister them in their current warped configuration. Before you can reinforce the joists, you must first push the sag out. This is usually a job for the pros because an engineer should study the house, or at least the house plans, to determine where to position a beam in the crawlspace for support. The size of the beam and its exact location depends on the weight disbursement of the structure above.

Typically, the beam installs perpendicular to the floor joists and is supported by house jacks that rest on concrete pads. Over a period of a few weeks, or even months, a foundation contractor gradually adjusts the jacks to slowly push the joists back into shape. After the joists are level, you can sister them or simply leave the beam and jacks in place, whichever the engineer suggests.

Beef-up Attic Floor Joists

Beefing-up attic floor joists to add an upper floor is simpler than fixing ground-floor joists because you can reinforce the joist system from above. You can sister the joists with additional two-by-six boards, but if you’re not pressed for headspace in the attic, you can use larger dimension boards for a stronger floor. For example, you can install new two-by-tens right beside the old two-by-six boards. The new joists will raise the level of the floor by 4 inches but you’ll have a stronger floor that will support heavy furniture without sagging.

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Assuming you are going to obtain all of the necessary permits to convert or add the bedroom. The local building inspector may not allow to use of the 2×6’s for floor joists, even if they are sistered. Usually 2×8 is minimum, which would mean building a new platform on top of the original joists, which could be a problem with ceiling height. I would check with the local building dept. before you proceed.

You could sister 2×8’s to the existing joists and extend them to the perch of the originals. I think adding the additional 2 1/2″ shouldn’t be a height problem, minimal at best. I believe that if you are going to the trouble to add a bedroom, then do it right the first time so that you suffer no problems later. If there is wiring thread through the existing joists, I would suggest that you pull it out, redrill the new and then re-wire.

Yes, you can sister to only the middle of the span, but 2×6’s aren’t going to work for 16 ft. span (12 ft span is more reasonable for 2x6s). 2×8’s might work, but if you sister to the middle you’ll have the problem of not having an even surface to nail floor deck to. If you only sister to the middle, you will have to pay special attention to how you attach the two joists together so they act as one and you’ll have to bridge joists any way you do it. There would be some calculations to figure out the minimum required.

You can beef up only a portion of a joist or beam – it’s done in commercial construction all the time. It’s not very common in wood framing. Our code would allow 2×10’s at 16″ o.c. hem-fir #2 for a full length 16 ft. span at 30 psf live load. 

16 feet is just too far for anything less than 2x10s. If you have the headroom available, that’s the way to go. 2x10s, 12 in. o.c. And sistering without running the sisters or partners all the way to the supports is folly. The entire joist needs to be stiffened, not just the middle.

Whether you need to go to 2×8 (or more) will depend on alot of variables, including species of the existing wood joists, condition of the joists, anticipated live loads, etc. Note that unless you actually attatch the new joist to the old in a proper fashion, you are not necessarily gaining the advantage of the combined strength of the two. You will likely need to run full length and alternate spike them together every 8-12 inches. You may also need to laminate them with plywood, gluing as well as spiking them together.

I have 2×6 and don’t want to use larger (2×8 or 2×10) lumber for sistering if that can be avoided since head space would become and issue due to the roof. Also, since there is a ceiling below it would be very difficult to jack up the existing joists if they are sagging some. Currently it does not seem like the floor has much sag to it but I won’t really know until I pull up the subfloor.

 I did stiffen the floor by sistering onto the existing 2×6 joists with more 2×6.  I also added solid blocking and glued down a new plywood subfloor.  Of course you probably would be better served by sistering on 2×8 but in my case I didnt want to lose any headroom either above or below.

Sistering deeper joists would be best – and most direct solution if you are not too concerned with headroom. If you are, and the floor is not outrageously bouncy as it is now, sistering 2×6’s might be enough (even one on each side of each original joist?) to make you feel comfortable. Topping it with a good stiff subfloor material will add a lot of stiffness, too.

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You could sister the existing 2×6’s with new 2×6; this will effectively give you 4×6 nominal for calculation purposes. A 2×6 SPF SS-grade (good luck finding any new ones!) is good for 9’6″ on 24″ centers if my memory serves. A 4×6 would be good for a substantially increased span, but you will have to have it calculated as that is not a standard size you will find in any table.

You need to plug in the joist spacing, too. There is a difference for acceptable scantlings among the three most common spacings of 12″, 16″, and 24″.

Another option to sistering would be to insert additional 2×6 joists between the existing ones, halving your joist spacing. So if you were on 24″ centers, you would wind up with 12″ centers.

Lastly, glue-N-screw for a new subfloor of ¾ ply on top of the joists will definitely give you a stiffer floor deck. Whether it would meet code or not would be something your engineer would have to determine in conjunction with the local inspector.

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Allowable Span For Sistering 2×6 Floor Joists

Regarding the allowable span of a sistered 2 x 6 joist. I have done many attic renovations and I have spanned up to 13′ with out a problem. The allowable span for a reg 2 x 6 by itself is 9′.anyway.

Species and grade of the 2×6 can affect span and therefore have to be considered too.

It can be defined a compliance criteria of 2-2×6 SPF #2 16″ oc. sistered floor joist will only span 11′-8″ with a 40# Live Load & 10 # Dead Load with a Live Load Defl. of L/360 & a Total Load Defl. of L/240. Furthermore, a single 2×6 SPF #2 16″ oc. floor joist will span 9′-3″ with the same criteria above.

Weight Support

how much weight can 2×6 floor joists hold? To give you a general idea though, floor joists are sized to support 40 pounds per square foot of “live” load (moving weight) or 10 psf “dead” (stationary) load.