DIY Chatroom Home Improvement Forum

DIY Chatroom Home Improvement Forum (
-   Building & Construction (
-   -   joist sistering ( 09-23-2008 08:26 PM

joist sistering
if i sister ajoist do i have to go the whole lenght of the old joist , i can only go about six inches from the end of the old joist because there is no way i can go the whole lenght idont have enought room

Termite 09-23-2008 08:40 PM

It depends on why the joist is being sistered. Can you elaborate on the reason they're being sistered? 09-23-2008 08:47 PM

they are being sistering because of a sagging floor

Termite 09-23-2008 09:57 PM

Adding material to the sides of the joists will certainly stiffen them. That will reduce their deflection under load, which is the bounce that you probably feel when walking across the floor.

Ideally, you'd run the sisters from bearing point to bearing point.

I asked why you were sistering to make sure that the joists weren't spliced, cut, notched severely, or otherwise damaged. If they were, the sisters' installation would be more critical.

In order to be effective, the sisters must be securely attached to the existing joists. I'd recommend construction adhesive and nails. Lots of nails.

Nestor_Kelebay 09-23-2008 10:46 PM

Normally, the stresses on a joist are highest at mid-span and become smaller as you approach the supports. My understanding is that you can get nearly all the benefit of sistering the whole length of the joist by sistering only the middle 2/3 of the span.

You can also increase the strength of your joists more effectively by adding wood to the bottoms of the joists than by sistering them.

Take a look at this:

Termite 09-23-2008 11:13 PM


Originally Posted by Nestor_Kelebay (Post 160737)
You can also increase the strength of your joists more effectively by adding wood to the bottoms of the joists than by sistering them.

Take a look at this:

Sorry Nestor, but that is not a factual statement unless you've devised a way to grow 2x12's from 2x10's. Tensile stress will occur at the bottom of the loaded beam, and lumber added to the bottoms of joists would have to be fastened in a way that would resist the tensile stress to lower the joists' tensile chord...Not easily done. I've seen and inspected hundreds upon hundreds of engineers' designs and fixes for damaged joists and excessive deflection, and not one of them included the addition of wood to the underside of a joist. Nearly all incorporated addition of wood to the sides.

The linked thread was muddied by engineering gobbledy-gook that doesn't necessarily hold water in the real world, and generally isn't promulgated on a DIY site because it really isn't of benefit to the average poster. We need to give out advice that is practical and usable. I spent 5 years in college dealing with theory of structures, and assure you that trying to loosely educate most DIYers (and a lot of carpenters for that matter) is not going to accomplish anything but to frustrate them.

I do agree with the statement that the most reaction is occurring in the middle third of the span, and that sisters 2/3 of the length of the span will provide some benefit. The closer to the full length of the span the sisters are, the more effective they will be.

Nestor_Kelebay 09-24-2008 08:19 PM


Originally Posted by thekctermite (Post 160744)
Sorry Nestor, but that is not a factual statement unless you've devised a way to grow 2x12's from 2x10's.

The fact is that it's done all the time. Consider an engineered joist consisting of a 2X3 on top and bottom with a piece of 1/2 inch chip board holding them together. Where is all it's strength coming from? The chip board? No, it's coming from the wood at the top and bottom that has been GLUED to that chip board. You could make a stronger engineered joist if you glued more wood to the top and bottom of it.

As long as the wood you added was as strong or stronger as the wood they used, and the glue you used was as strong as the wood they used, then you could have complete confidence that you had made the joist stronger regardless of whether or not you had the formal training to have confidence in what you'd done.

Similarily, the skating rink in my home town has huge laminated joists that hold up the roof. Those joists aren't made from a single piece of wood, but from 1X8's that have been glued together to make a 24 inch tall by 8 inch wide rafter that, along with the other such laminated beams, holds up the roof.

Imagine that we have a 2X12 in one corner, and we have a 2X10 with a 2 inch thick piece of wood glued to the bottom of it in the opposite corner...

And I stipulated only the two following conditions:

1. The wood we glued on has to be as strong or stronger than the wood the joist is made of, and

2. The glue bond between the new and old wood has to be as strong or stronger than the original wood.

Then we have a situation where what's been added is as strong or stronger than it would have been had we originally made a 2X12 instead.

How can you claim that what we've made is weaker than a 2X12?

It couldn't be. If the new wood is as strong or stronger than the original wood, and the glue holding it on is as strong or stronger than the original wood, what we've made is as strong or stronger than a 2X12 of that same original wood.

You may have seen lots of plans, but I graduated from mechanical engineering, and I can assure you that there is NO reason why you can't add 2 inches of wood to a 2X10 to make a composite beam that's equal or greater in strength to a 2X12.

Similarily, you can sandwich a 2X10 between steel plates and grow that 2X10 joist so that'll be much stronger than a 2X12, or even a 2X14 for that matter. Or, we could add steel to the bottom of the joist to make it stronger too. There is nothing significant about adding anything to the SIDE of anything. What's important is the coefficient of stiffness, which is the product EI, where E (the modulus of elasticity of the material) and I is it's Moment of Inertia, which is a function of it's shape. The stiffer the material, the greater the value of E, so steel has a much higher value for E than spruce of fir or oak or chipboard or walnut. I for a rectangular beam is given by the formula I = b*h^3/12 where b is the width of the beam and h is the height of the beam.

You can see for yourself that what you were saying about adding stuff to the side of a beam simply can't be true if you consider a 2X12. It can be equally well used as a springy diving board or a rigid floor joists depending on it's orientation. That's because by turning it 90 degrees, it's that 11 1/2 inch dimension that gets cubed, not the 1 1/2 inch dimension, and that results in a much higher value of I, and that results in a much stronger beam and less deflection.

All of this is the kind of stuff you learn in 2nd and 3rd year engineering, and the original poster would lose little time or money by simply phoning up any engineering or architectural consulting firm and asking about the feasibility of glueing wood to the bottom of a 2X10 to make a 2X12.

The fact that other people don't do it doesn't make the idea wrong, it only means people are reluctant to embrace new ideas. And that, my fellow termite, is nothing new.

Termite 09-24-2008 08:43 PM


Originally Posted by Nestor_Kelebay (Post 160976)
If I were to eight spruce 2X2's and glue them together with a glue that formed a bond that had the same shear strength as the original wood, I would have a beam that is equal in strength to a 2X12 (neglecting the fact that nominal sizes are different than measured sizes). As long as the glue between the 2X2's was as strong as spruce is, that laminated beam would be as strong or stronger than a 2X12.

You give me one reason why it wouldn't be.

And, of course, would it matter if I started with 7 2X2's glued together and then added an eighth one many years after the house was built? No it wouldn't.

This is great in theory, but doesn't hold water out in the real world of a jobsite.

What you're describing is the same principal that beam manufacturers use to make glued-laminated timber (glulams). 2x dimension lumber stacked and glued on top of each other. Yes, it works very well and they're very strong.

Glulam manufacturers use resorcinol glues that aren't available at the corner hardware store. They use heat. They use jointed faces that are perfectly flat. They use dried lumber. They use tens of thousands of pounds of pressure to make the laminations' connections. After much R&D, testing, 3rd party certifications, and investment, they can produce glulam beams that work.

When they need a deeper glulam than the one they have, they don't go out in the yard and add another 2x4 with some more glue and a bunch of clamps, and you can't do it on the jobsite either for the same reasons.

Termite 09-24-2008 08:51 PM


Originally Posted by Nestor_Kelebay (Post 160976)
You may have seen lots of plans, but I graduated from mechanical engineering...

I don't think we need to compare resumes in an effort to one-up each others' credibility, but I hold a degree in construction engineering and have a solid grasp on how structures work. I've never seen a structural repair with a mechanical engineer's stamp on it.

We'll have to agree to disagree on this one. But, there's nothing wrong with getting everyones' ideas out there for the poster to make his/her decision.

Nestor_Kelebay 09-24-2008 09:18 PM

I agree that the manufacturers of glulam beams may have glues and machinery and techniques available to them that a homeowner wouldn't. However, it's not important that the wood be added in a high tech way.

All that is important is that the glue be as strong as the wood of the original joist, and with the new moisture cure polyurethane construction adhesives, like LePage's PL Premium, that is certainly do-able by a homeowner or contractor.

I agree that a comparison of resumes is about as useful in here as an insult hurling contest.

And, that the free exchange of ideas is the life blood of these forums. Knowledgable people arguing a technical matter is the ONLY way for newbies to learn enough about every aspect of the issue to walk away with a good enough grasp of the problem and solutions to form their own informed opinions.

I expect the only reason why strengthening beams by adding wood to the bottom of them is because VERY strong construction adhesives have only recently become available to the general public. I have no doubt that PL Premium would form a bond at least as any softwood, including pine and fir. I don't think PL Premium has been available for more than about 5 years now.

All times are GMT -5. The time now is 04:25 PM.

vBulletin Security provided by vBSecurity v2.2.2 (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
User Alert System provided by Advanced User Tagging (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.

Search Engine Friendly URLs by vBSEO 3.6.1