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Unread 12-29-2020, 07:52 AM   #1
dbernd
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Sistering joists to increase live load

Hello,
I am remodeling a townhome in PA built in 1901. I would like to put a bathroom on the second floor. The flooring in the new bathroom will be tile.

Currently the joists are a true 10 inches by 2 inches (since they are old) spaced 16 inches O.C. and the span is 18 feet 3 inches. The current flooring is tongue and groove pine at 7/8 inches thick.

I would like to sister new joists along the old to increase the live load to the needed 40 lbs according to code to allow this construction.

The question I have is in two parts.
1. What is the minimum size of joists needed to be sistered in order to achieve this goal?
2. Do I have to use joists the same size as the existing joists in order to follow code guidelines? Ex. If adding 2 x 8 joists would be an adequate load increase is there any reason I would need to use 2 x 10 joists instead.

Thank you
Damon
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Unread 12-29-2020, 08:02 AM   #2
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No you don't need a 2x10 just because that's what is existing, you need whatever will provide sufficient stiffness. Logic suggests that 2 2x10s are better than 1. An lvl or tgi would be even stronger, but are more expensive than lumber.

But the best thing for you to do, is to make a quick call to a local architect who could give you the best advice for your local code in about 30 seconds, and probably wouldn't think of charging you. Have your measurements handy.

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Unread 12-29-2020, 09:04 AM   #3
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Hi Damon,

I know I'm treading on shakey ground here, but I would give it the Jump Test.

How large a bathroom do you intend to construct? Are there no supporting walls under the floor we're talking about? It's hard to imagine an upper floor with an eighteen foot unsupported span.
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Unread 12-29-2020, 09:17 AM   #4
cx
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Welcome, Damon.

For clarity, what you need to correct is the excessive deflection of your joist structure at the mostly standard code requirement of 40psf live load. Your current joist structure is quite capable of supporting that load, and quite a bit more, but it will deflect more than the allowable amount while doing so.

The L/360 deflection requirement for most residential building codes is also the same as the L/360 deflection requirement of the ceramic tile industry for a ceramic tile installation.

In the dark blue bar at the top of the page you'll see our Deflectometer listed. If you fill in your known values it will give you a good go/no-go indication of your current structure. You'll see there that your current joist structure does not meet the L/360 requirement, regardless the species and grade. With a little experimentation you'll also see that you'll need to sister your joists with good quality nominal 2x10 joists to meet the standard you're shooting for.

A second option would be to shorten the unsupported span of your joists by adding a support wall or beam to shorten the unsupported span by a few feet (depending upon the grade and species) if that's at all feasible in your situation.

My opinion; worth price charged.
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Unread 12-29-2020, 09:26 AM   #5
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john,

As I stated this is a very old house. It was originally a corner store on the lower level with rooms above.
There are walls under that section of the 2nd floor, however, none of them were there when originally constructed.
As far as the size of the bathroom it will reach approximately 10 feet along the joists from one of the walls and span 7 to 8 joists.
Due to the current renovated layout of the lower floor, sistering joists with no additional support from underneath would be the only option for me to feasibly do this project.
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Unread 12-29-2020, 09:45 AM   #6
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CX,

Thank you for the information. I am a bit confused using the deflecto-calculator to find the results you did.

Putting in my current info I used 10 inches by 2 inches for the size and 18.5 for the span (this is 3 inches longer than the current span) which came out to L/229. Using 18 feet I get L / 246. Would 18 feet 3 inches come out to L/ 238?

To check "L" value with a sistered joist do I just add the new dimensions to the old - EX. adding 1.5 inches (total 3.5 inches) to width and still using 10 inches for the height (added joist only being 9.5 inches) I get L / 401 (L /431 using 18 foot span)
How can I calculate what adding an 2 x 8 (1.5 inches by 7.5 inches) joist would do?
How do I calculate with different quality of wood - new and old together vs all one type?
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Last edited by dbernd; 12-29-2020 at 09:49 AM. Reason: more information needed
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Unread 12-29-2020, 09:46 AM   #7
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Quote:
Originally Posted by Damon
There are walls under that section of the 2nd floor, however, none of them were there when originally constructed.
We'd need to see a drawing your your floor plan, but if there are existing walls under the joist structure in question I would think they could be made to be load bearing, reducing the unsupported span of the joists above. What am I missing there?

[Edit] I see you slipped in more questions while I was typing. The calculation you're looking for is straight-line math, Damon. You can enter the dimensions of your existing joists as you've done and get a deflection reading. You can then enter a separate set of dimensions for the sister joists you have in mind and get a second reading. You then simply add the two numbers for your resulting deflection of the new structure. I think you'll find that a nominal 2x8 is not gonna get you where you wanna go.

My opinion; worth price charged.
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Unread 12-29-2020, 10:08 AM   #8
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Additional information:

The main reason I am trying to see if a 2 x 8 sistered joist would be adequate is because the ceiling on the lower floor is original plaster over lathe and by using a joist with less height I would be more able to place them without disturbing any lump of plaster that may be in the way of using a 2 x 10 joist (even with a 1/2 inch to spare)
For [cost] feasibility, joist sistering will need to be done for the top by removing existing floor so as to disturb the least amount of ceiling on the lower floor.
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Unread 12-29-2020, 10:26 AM   #9
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CX,

Ok using that method I can see that the 2 x 8 joist would still come out below the needed L / 360 deflection.
The reason the I don't think the walls on the first floor would help much is that they don't span all of the necessary joists needed for the project and changing them really is not an option.

as I stated this is a town house (row home) and all rooms span the entire width of the house.

Thank you for all your help. I see that I will have to plan to use the 2 x 10 joists for the sistering operation and will submit that to the township in the new plan.
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Unread 12-30-2020, 07:02 AM   #10
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Damon,

Keep in mind that to be effective your sisters need only be 2/3rds the length of the existing joists, and roughly centered on the existing joists, so 16' 2X10's, or even 12's, will do the trick. Also know that a nominal 2X10 is only about 9.25" high so you have about 3/4" of wiggle room. Your glue and screw schedule will be important.
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Unread 12-30-2020, 10:27 AM   #11
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What do you recommend? I have seen suggestions of fasteners every 1 foot or every 16 inches
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Unread 12-30-2020, 11:58 AM   #12
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When you install sistering lumber, determine if you need a straight line, if you are installing tight to top or bottom.

Determine if floor is sagging and needs to be supported temporarily to correct position before sistering.

I'd use liquid nails type adhesive in a z pattern. I like installing structural screws in pairs. First pair about 4-6"in from ends, second pair 4-6"in from there, then a pair every 12 or 16, which measurement depends on what is necessary. If you don't actually know what you are doing or why, I'd go stronger than weaker.

Again a30 second conversation with your local architect/ engineer would give you all the knowledge you need.

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Unread 12-30-2020, 03:40 PM   #13
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While normally I’d concur with the standard 2/3 span rule for sistering, I tend to go with the longest I can work into position with old ungraded wood. A foot or so short is fine. Look for clean straight pieces and adjust up or down as required to ensure your floor is nice and flat.

Recently described the how to and tips for doing exactly this type of thing.

https://www.johnbridge.com/vbulletin...87#post1540087
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Unread 12-30-2020, 03:59 PM   #14
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Just remember deck screws are not structural, and are not intended for creating beams. They are intended to hold down a deck board and resist corrosion.

The shearing force of a beam flexing is an entirely different force, and requires the correct fastener. Not that you can't get away with deck screws, just not the correct use.

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Unread 12-30-2020, 05:08 PM   #15
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Quote:
Originally Posted by mykcuz
Just remember deck screws are not structural, and are not intended for creating beams. They are intended to hold down a deck board and resist corrosion.
I agree that using the proper fastener is critical part of any structural work, but your statement that deck screws are not structural is not a accurate. Deck screws such as Deckmate (Home Depot) and Grip-Rite (Lowes) are specifically designed to fasten wood to wood. Per the National Design Specification for Wood Construction, 3"/#8 screws described above have 4X the pullout resistance of the 16d nails (168# vs. 33#) while providing virtually identical shear strength as a 16d nail gun fastener (90# vs. 95#).

The increased pullout force is particularly significant in the application described by the OP and the details provided in my post since the dominant action of the screws is to provide permanent clamping of the two pieces to improve the performance of the adhesive and the resulting beam.

My one complaint with the use of deck screws versus a screw such as Strong-Tie SDWH is the self countersinking design which reduces the screws ability to close the gaps. On the other hand the deck screws are inexpensive ($30 for 400 versus $32 for 100 SDWH) and the additional of a washer make the deck screws highly effective for the purpose described.
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