First, I will not use any internet advice as a sole source for determining structural capabilities. I just would like any insight before I have a conversation with the inspector.
To build an ADA shower in my own renovation, I would like to use the technique shown in an issue of FHB about a year ago where he trimmed a section of the joists to ‘sink’ the shower. This prevents the need to raise the floor by a significant amount negating the accessibility side.
The floor is currently 2x8s spanning 12 feet. My thought is to trim 4 joists by 1.5 or 2 inches in the shower area and then sister 2x6s along those joists.
So far, no problems assuming that a doubled 2×6 is greater or equal to a 2×8 (I need to confirm).
My issue is that for 2 of those joists, it will be logistically impossible to get the sistered joist in place so that it bears on both support ends. It’s a matter that I cannot get a joist of the right length in place. So the sistered joist will rest on one support end but will be 6 inches short on the other.
My question is whether there is any code/precedent for gluing/nailing/screwing/bolting a sistered joist to provide the support along the length of the joist but where only the primary joist is actually resting on the support. In this case, let’s say the sistered joist is 6 inches short.
If this is not clear, I will draw a picture to better explain.
Thank you in advance.
-Jonathan
Replies
On the side that falls 6" short. Could you put a third joist along side it that rests on a bearing surface? Just a shorter joist to bear on one side and splice that area?
Certainly. Can you help me understand if this changes the structural properties of the support given by the sistered joist? There would be bearing support but does the fact that there is a spliced piece make up for not having a continuous member?
I'm no expert but from my experience there are a couple of things to keep in mind.
One is that that this is a one off design that is being built once. So if you use more materials than is warranted it doesn't matter. People get caught up in the use of materials and optimizing them. With multiple units it does matter but with one just put more wood in it.
The other is that I grew up when there was a lot more cheap wood. Typically wood structures were over built. There was way more wood in a structure that necessary. That has changed with trusses and manufactured beams. But in your situation you can just overbuild it and be done with it. The balance between labor and material costs is tilted way over onto the labor costs at this point.
If you splice wood joists just keep them long enough and use plenty of 16D nails. It will most likely be overbuilt. Do you get what I'm saying? One thing I've done with something like this is to build it and then check it by standing on it and bounce on it. Your weight on it without sheathing will let you know if it is strong enough. I know I might get razzed for what I just said. Keep in mind these are seat of the pants building practices. You do have to have some sense of what will work and what won't.
You did say that the building inspector will look at it. Ask him what he thinks. Ask him if it is right at the margin or overbuilt. He's seen enough of this to know. It helps to have feedback from someone else that is right there.
I just read Riversong's post. I think he said the same thing but from a very different angle ;^ )
Edited 3/3/2008 1:27 pm ET by popawheelie
Thanks Popa. After reading your response and doing more research, I guess my question is better asked as rules for joist splicing or partial length splices. I see references in other threads to this practice and nailing/bolting schedules but nothing comprehensive. It appears that this will be workable.
I'll keep looking but if anyone has anything definitive, it would be appreciated.
Thanks!
To determine equivalency of joist options (at least in terms of fiber strength), simply compare the section modulus (bd²/6).
2x8 (1.5" x 7.25") has a section modulus of (1.5 x 7.25²)/6 = 13.14 cubic inches
(2) 2 x 6 (3" x 5.5") has a section modulus of (3 x 5.5²)/6 = 15.125 cubic inches
The sistered 2x6's will be stronger than a single 2x8, but they will have greater deflection because that is based on moment of intertia which increases with the cube of the depth rather than the square (I = bd^3/12).
You don't need any additional bearing surface, so it doesn't matter that the sistered joist doesn't reach both plates as long as it is well fastened to the original joist (3 10d nails every 16" - glue-nailed even better).
Solar & Super-Insulated Healthy Homes
Edited 3/3/2008 1:23 pm ET by Riversong
Thank you Riversong. This makes a lot of sense.
If headroom is not a problem you could sister on 2"x8"s two inches down, or glue and screw 2"x4"s to the bottom of the existing joists.
Not to be a stickler <g> but this would be considered an excessive notch under section 502.8.1 of the IRC 2006 (exceeding 1/6 of the depth of the member) - max. notch is 1 3/16" +/-
If the shower is in the middle of the span, it's not permitted at all.
Note - section modulus units are inches cubed, not cubic inches.
Jeff
Edited 3/4/2008 10:19 am ET by Jeff_Clarke
Thank you Jeff for the reference. The shower is 4' along the edge of a 12' span.
Note - section modulus units are inches cubed, not cubic inches.
Same thing. The first is the algebraic expression and the second is the geometric expression. They have the same meaning. One is convertible to the other.
Riversong HouseWright
Design * * Build * * Renovate * * ConsultSolar & Super-Insulated Healthy Homes
I would put it this way - it is a number that has the same value but describes something totally different.
Cubic inches describes the volume of a solid.
Section modulus in units of inches cubed describes the property of the area shape and depth of a section, specifically the relative stiffness of the cross section.
At least, that's the way I was taught to think ;o)Jeff
At least, that's the way I was taught to think
That's why we need to learn to think outside the (cubic inch) box. ;)
Riversong HouseWright
Design * * Build * * Renovate * * ConsultSolar & Super-Insulated Healthy Homes
It's a convention; the same way you measure torque in pound-feet, but you measure work in foot-pounds. The units are technically the same but one's right and one's wrong for a given application.
It's a convention; the same way you measure torque in pound-feet, but you measure work in foot-pounds. The units are technically the same but one's right and one's wrong for a given application.
Is that so? As a certified Master Auto Mechanic, I can assure you that all torque specifications in the US are in foot-pounds, not pound-feet.
Sure they're not killogram-meters?
If your view never changes you're following the wrong leader
Ok, so I picked a bad example of a convention. Motor torque is always reported in pound-feet. Work is always reported in foot-pounds, as are fastener torque specs.
My point still stands-- each application has its convention, and the convention for section modulus is "inches cubed".
Motor torque is always reported in pound-feet.
Hmmm... Engine torque is always specified in foot-pounds.
From Wikipedia, the free encyclopedia
"The SI unit for torque is newton meters (N m). In U.S. customary units, it is measured in foot pounds (ft·lbf) (also known as 'pounds feet')."
Now you've got me questioning your Master Mechanic certification... I can't seem to find a single auto manufacturer's website that lists engine torque in foot-pounds.
Anyway I'm done arguing about it.
Instead of 2x6 sisters I'd look at using 2x8s that drop below the existing ones the required amount (assuming that the loss of headroom isn't an issue).
On the end closest to the "surgery" notch the 2x8 sisters' ends to rest on the sill, and on the other end make sure they're securely fastened to the original joists with through-bolts or plenty of nails.
Also would be wise to use a joist hanger with the flanges bent over (or some other suitable strap) on the notched end to reenforce the end and prevent splitting at the notch.
the sheer (shear?)strength of a 2x8 trimmed down for your shower (essentially a 2x6) as in where the wood rests on the plates, etc.. is incredible. so by placing a sistered 2x6 on the 2x8 and having it fall 6" short is not as great as having it as close as possible to solid bearing. the idea of a no-barrier bathroom is amazing. something i would like to replicate as well. screws(10" o.c) and glue are essential in the sistering process. good luck.
Thank you. I will post pictures when the framing is complete.
Mojo, question for you off the subject of your knocking joists.
I assume you are trying to build a shower with NO step up or step over at the threshhold of the shower. If so how much drop are you planning to build into the shower between teh bathroom floor and your drain?
Good question that I do not have a definititive decision on. The stall will be 4'x6' and I thought 1" slope to the center would be appropriate. There is probably a guideline on this that may be more or less. Do you have any insight?
Mojo, no I don't have much insight on the issue. Part of my interest is that I would like to do the same thing someday. You might look at the code and or ask the question here. I think I heard once that a 2 inch drop was required but that included the step-up or step-over treshhold. If true, and you have no curb then you need a 2 inch drop. That seems like a lot of drop, but I don't write code.
I hope I have not fabricated a non-issue for you. But I would like to know as well.
Mojo, one more thing. If you are building a mud base shower pan you need to account for the fact that your dropped joists must account for the thickness of the mud base which has some minimum thickness (is that 1 1/2 to 2 inches) and then add your planned drop from the threshhold.
???????
Yes, I would need to account for all the levels of substrates.
I know that 1/4" per foot is generally accepted for drainage slope for showers. I found the article from FHB last February/March which covers the topic and there is a thread over on JLC which covers the topic as well.
http://forums.jlconline.com/forums/showthread.php?p=364044
I guess what I am saying is that by the time you knotch the joist to account for a 1 inch pitch to the drain and 2 inches of mud under the drain to set it in, you will need to remove 3 inches from those 2x8s. Thus, giving you only 4 1/4 inches of wood to work with. Make sense? Mine mistaken?
Why dont you just drop the subfloor below the level of the top of the joists with plywood. We typically will run 2x2's 3/4" below the top of the joist and cut plywood to fill in the space to creat a flush floor that is lower than the flooring outside the shower. This is our common practice as we love flush transitions between differing floors.
sotabuilder, your solution will give you 3/4 inch differential (the thickness of the subfloor in the bath. The issue is that you need two or three inches difference (I / we think).
correct, you will have 3/4" difference in the subfloors, you will need a thicker underlayment for your tile in the bathroom and a thinner underlayment in the shower, if this is a small shower you do not need that much pitch, trust me this works we just finished doing a 6x8 shower in ADA home in St. Paul with no transitions from tile in the shower to the bath floor to the carpet in the bedroom.
sotabuilder, "you said you don't need that much pitch". Doesn't code require a definite drop? I keep thinking I have read two inches. I think it was even written in one of the attached links in this thread. 2 inches seems like over kill, but . . . . code?
What is your experience with inspectins in this regard.
Go with an 1/8" per foot by the door and could increase to a 1/4" as you move in.
but sotabuilder, does not code require a certain total drop in inches to the drain from the threshhold? And secondly, don't you also need a minimum thickness of mud built up ontop of the subfloor and under the drain flange?
All i can say is check with your local building inspector, give em a quick phone call and they should be able to answer those questions over the phone.
I'm not a structural engineer but doesn't this negate the value of plywood spanning multiple joists? So now if you stand on a joist, the entire load is on that joist?
Either way, I need more than the height of plywood though.
Here is a photo of one such installation, i should note that we only do real mudset tile beds which can be sloped and screeded to whatever depth is desired.
KISS solution: You only need 1-1/2" of bearing to be fully supported so if you can slide the new joist in at least 3" on one side and nothing on the other and pull it back to center between the bearing points you're okay by code.
Often this means the new joist is wedged in very tightly under the existing floor, a seemingly impossible thing to slide back the 1-1/2". The trick is to use both a liberal amount of parafin on the friction points and leverage to your advantage. Screw a block onto the side of the new joist so an 8' 2x4 can be used to lever it over. In extremely difficult situations a portable hydrolic ram or small hydrolic jack suitable for horizontal use makes short work of it.
Second best: Add solid blocking between the joists on one end, slide your new joist(s) in, on the unsupported end (the end that's blocked) add a simpson hanger that encompasses both the new and old joist, nail hanger into place, and you're done. It would be rare to have an inspector question this fix. Be careful in your selection of hangers or the guy at Billy-Bob's Lumber will sell you a $50 hanger when all you need is the light duty $15 version.
If those two solutions won't work (they almost always will) I'd invite the inspector over for a consultation. The majic words for this are, "Hi inspector Bob, I have an oddball framing dilemma and would like your guidance and direction on what you'd like to see as the correct fix," even if I know good and well what should be done, but need his blessing for something outside the letter of the law.
"I would like your guidance and direction..." is a magic phrase that opens people up and makes them more agreeable in any situation.
Good building!
Beer was created so carpenters wouldn't rule the world.
What's all this dithering about joist bearing?
If the original joists have sufficient bearing, and the load on each joist is not being significantly increased, then the sistered joist pairs will have sufficient bearing.
The purpose of sistering is to return the notched joist to at least it's original strength and deflection. The sister strengthens the span between bearing surfaces. No additional bearing is necessary.
Riversong HouseWright
Design * * Build * * Renovate * * ConsultSolar & Super-Insulated Healthy Homes
No additional bearing is necessary.
You are 100% correct--if and only if the inspector will allow the assumption that the sister'ed joist will fully transfer the loads to the original joist. I have yet for one of the anall inspectors, especially one without construction experience, to go for that argument without either a manufacturer's spec sheet on some sort of connector specifically for the installation (none I've seen), a page out of the code book (nadda), commonly accepted construction practice (nope), or an engineer's stamp ($$$).
You and I both know that something as simple as a 1" wide 18g simpson strap wrapped around the end of the joist and up over the existing joist and nailed off would accomplish what we're after, but there is no chart with a rated number to associate with the fix.
What I do know is that every inspector I've ever dealt with would go for a solution with either 1-1/2" of bearing on each end or a hanger rated for the load and used in a manner for which it was intended.
Good building
Beer was created so carpenters wouldn't rule the world.
If the original joists have sufficient bearing, and the load on each joist is not being significantly increased, then the sistered joist pairs will have sufficient bearing.
Bingo!
So the sistered joist will rest on one support end but will be 6 inches short on the other.
Whoa. I just read the 6" short. It can't even be fitted in to at least equal the space between supports? If it's just running into blocking I'd cut it out and reinstall after the new joists are in. If it's some other obstruction that can be cut and patched or replaced that's often the path of least resistance.
If it's just not humanly possible to get closer than 6" from one side, I'd see if the inspector would go for a doubled blocking (w/hangers) at the end of the short joist and hanger the old and new, nailing the hanger into the cross blocking.
12' is a long way if you have considerable weight, so perhaps it's going to be better to double the 2x6's or even use LVL's.
Beer was created so carpenters wouldn't rule the world.