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-   -   Joist re-enforcement vs "sistering" (https://www.johnbridge.com/vbulletin/showthread.php?t=63630)

Plainrider 06-12-2008 05:57 AM

Joist re-enforcement vs "sistering"
Taking CX's advice :) , I am posting this message to briefly discuss joist re-enforcement vs joist sistering.
Now, sistering is the addition of extra wood joists to a floor that, for the considerations of floor tiling integrity, are to stiffen the floor. This approach can work well but is often difficult to implement in homes previously built (e.g. due to the interfering equipment, such as wires, plumbing, gas lines etc) that would inhabit the space between the joists. Sistering has an "additive" effect on floor stiffening i.e. doubling the number of joists will double the floor stiffness.
Joist re-enforcement on the other hand, however, involves the direct stiffening of the existing joists themselves - which in turn, will stiffen the floor. Joist re-enforcement can be done in many ways, but the approach in my previous threads has focused on the adhesion of 2x4s along the bottom edges of the joists. This approach has a multiplicative effect on stiffenening the joists i.e. adding a 2x4 to the bottom of a 2x10 joist can about double the joist stiffness (and can be performed easily after the fact by the homeowner, tiler etc as no electician, plumber, or gas technician is required because interfering with existing equipment within the joist spaces is not involved. (Much more detail on this method is contained in my many posts :blah: on this topic!)
It is generally true that joist re-enforcement will substantially increase floor load carrying strength also: however, for the purposes in this forum pertaining to tiled floors, no credit for this effect is being promoted here - only the stiffening effect of the approach is being highlighted. This allows the joist stiffenening approach to be reliably implemented by pretty well "anyone", because no safety issues are then involved (i.e. regarding claims in the increase of the load carrying capacity of the floor).
However, in this respect, it may now be useful to mention the consideration of "poor/damaged" joists. For example: where the bottom of a joist has been "notched" to facillitate the routing of water or gas lines, its strength and stiffness will be substantially reduced (esp. if the notch is within the mid-span area). In this case, a 'localized' joist re-enforcement would repair the damage! So, for joists that are otherwise in acceptable condition (except for the localized damage), joist re-enforcement would be very desireable. (Note that any sag in the joist can be rectified easily at the time of joist re-enforcement). Now, the actual notch can remain in the joist without concern as the load carring stresses in the joist are simply re-routed through the re-enforcement.
Anyway, it would seem that poor joists in this respect are not only good candidates for the joist re-enforcement approach - they are excellent beneficiaries of this approach (e.g. regarding damage repair).

John R 06-12-2008 09:18 AM

For the engineer--

If you have the option of sistering or of adding in extra joists half way between existing joists, which would be better for strength/deflection?


ddmoit 06-12-2008 09:28 AM

Excellent question, John!

Mere speculation on my part, but I would suspect that either method would get you about the same deflection score parallel to the joists. The big difference would be in the deflection between the joists - which I believe CX says is often a greater concern. Seems to me that new joists placed midpoint between existing joists would have an overall advantage.

cx 06-12-2008 02:19 PM

I would agree with Dan on that point. All other things being equal and the same, twice as many joists half as far apart would give a fella the most advantage from a subfloor standpoint.

That would make good sense, however, only if a fella needed to add joists to obtain the necessary joist rigidity. Adding a layer of subflooring to correct the between-joist deflection deficiencies would be far more practical if the joist structure itself was already adequate.

By the way, my information about a tile installation failure being more likely related to between-joist deflection than to joist deflection is from reports gathered at places such as the CTEF by folks such as Dave Gobis who have spent years answering phone calls about real failures in the field. It's good not to hafta guess about everything or to make stuff up. :)


Originally Posted by Plainrider - emphasis mine
Joist re-enforcement can be done in many ways, but the approach in my previous threads has focused on the adhesion of 2x4s along the bottom edges of the joists. This approach has a multiplicative effect on stiffenening the joists i.e. adding a 2x4 to the bottom of a 2x10 joist can about double the joist stiffness (and can be performed easily after the fact by the homeowner, tiler etc as no electician, plumber, or gas technician is required because interfering with existing equipment within the joist spaces is not involved.

And that's where we start to disagree, Michael.

I have no engineering degrees but do have a good deal of real-life, hands-on building and remodeling experience. I've framed and repaired framing under a lot of different conditions, some of which would compare with the worst photos posted on these forums by visitors with serious structural problems.

I do know that it is sometimes much more difficult just to attach a full sister effectively to an existing joist simply because of the conditions of the existing structure and the work area. Been there, done that, have many tee-shirts. Given some of these conditions, I can't imagine how a fella would effectively attach a 2x4 to the bottom of the joists for stiffening. It's a good theory, but it ain't gonna happen in the real world much of the time.

Rough, dirty, knotty, split, waned, twisted, joists frequently don't have a bottom that is suitable for such an application. The very same joists could sometimes be effectively sistered or just have a full joist added beside the existing with good effect and far, far less opportunity for an improper installation or subsequent failure.

What we're dealing with here most of the time is not new construction or test laboratory conditions. Our visitors are generally bringing along their old, time-worn, sometimes poorly built, existing structures. They want to make them suitable for a tile installation and we try to help them do that with methods that they are most likely to be able to execute effectively.

I'm not saying your method can't be effective, I'm saying it's not gonna be anywhere near as simple in a real-world application as it would sound from your descriptions. Would I use it in the proper circumstance? Certainly. Have I used it? Yes, that's why I know it's not a slam-dunk, especially in anything less than near-perfect conditions. Just taking the sag out of existing joists while using that method (which I know you also think unnecessary) makes it real tricky, especially for someone working alone, as is usually my case. Then there's the matter of getting the pieces adequately glued and screwed in place and leaving it all sufficiently braced for the required number of days. Some of this is required for the most effective sistering as well, but the margin for error is far smaller in your method.

Again, I have no quarrel with the theory of your method; the principle is sound. It's the actual application that I think makes it less reliable and useful. If someone has the right circumstances, resources and abilities to use the method, I've got no argument. But for general joist structure deflection improvement under most circumstances, I would generally feel much safer recommending one of the more generally effective methods such as joist addition/sistering or mid-span bracing. All that is, of course:

My opinion; worth price charged.

scuttlebuttrp 06-12-2008 03:12 PM

Maybe someone could explain a few framing things to us folks on slabs so we can follow along and edumacate ourselves?
Sistering a 2x4 to the bottom of a 2x8? To give you a 2x12?
Sistering usually means screwing two joists together to get "one" board 3" thick?
Bracing means adding a wall or some other structure under the joists? Or do you mean attaching new boards perpindicular to the existing joists?
If you have joists that are 12" oc. You sister every other joist. Does this get you halfway to doubling? or is it just halfassed? :)

kevjob 06-12-2008 03:21 PM

1. no sistering is the process of attaching same size joist to the side of the joist.
2. blocking perdicular usually at mid point helps stiffen the floor.

3. We try to sister every joist if possible. this allows you to flatten and level the floor.

robbin 06-12-2008 03:40 PM

I'm not visualizing what is being proposed. Is the addition of the 2x4 to the bottom of the 2x10 in effect making a 2x14? Or is it being sistered on the side aligning the bottom edges? Or possibly having the 2x4 horizontal making the bottom half like an I-beam? How is the attachment being made? Screw and glue? Please describe the process in detail.

ddmoit 06-12-2008 03:44 PM


Or possibly having the 2x4 horizontal making the bottom half like an I-beam?
That's what I envision - screwed and glued.

Plainrider 06-12-2008 03:56 PM

CX: I respect your experiences and your opinions. Of course, your preference for joist 'sistering' has merit. After all, it is fairly intuitive that adding joist sisters will directly add to floor stiffness to the extent of the added sisters e.g. doubling the number of floor joists will double the floor stiffness.

However, we do have a substantial difference in our perspectives and basic understanding of the two issues of "ease" and the "effectiveness" of sistering vs joist re-enforcement.

First of all, to address your above comment, I would mention that although 'engineering' skills and experiences can be invaluable in providing the underlyiing understanding for many physical phenomena, this knowledge is sometimes impractical when considering the individual renovator, often working on his own to solve pressing "real world" problems. So, I would emphasize that my suggested "joist re-enforcement' approach stems from my being raised on a derelect farm - where every solution to a problem was required to be solved quickly, effectively and on our own - and then lived with - because 'professional' help was too far away, took too long, or just cost too much money. So, practicality is the basis of my suggested approach!

In this respect, a main reason that I generally recommend the joist re-enforcement approach (vs. joist sistering) is because it can be done by an 'individual on his own', without the need to hire licensed tradesmen that are often legally required to re-route plumbing, electical wires, A/C & heating ducts, gas lines etc within the joist spaces. Accordingly, the elimination of this expensive and troublesome reqiurement regarding the joist re-enforcement approach is a major benefit for the homeowner, or even for a contractor, who does not have ready access to these tradesman.

Regarding effectiveness, you have a point that the joist re-enforcement approach does have some requirements regarding the joist condition. However, I do not agree at all that the "effectiveness of the joist re-enforcement approach involves significant uncertainties or difficulties for the average homeowner in this respect i.e. with regard to the stiffening increase in the joist.
In a nutshell, if the existing joist is capable of carrying its intended load, then the joist re-enforcement approach can readily tolerate "imperfect" site conditions because no credit is given for increasing the load carry-capacity of the joists by this method. It is that simple. In other words, if the joists are capable of handling the floor loads (but are simply not stiff enough for tile), then the joists are immediately amenable to the joist re-enforcement approach to provide substantial increases in joist stiffness. However, if the existing joists are not strong enough to carry the floor loads ( generally a rare occurrence), then this is an entirely different issue that has nothing to do with tiling directly.
I have performed both approaches myself - adding joist sisters and also re-enforcing existing joists. For nealy every home that I have inspected (e.g. built within the last 35 years), the joists are generally in excellent condition for the joist re-enforcement approach. That is, the joists are typically sound.

But, again, I would agree with you that where the joists are warped, rough, knotty, split, waned, twisted, etc, then it may not be a good idea to re-enforce them (i.e. without consideration of their suitability for repair, for example): but in this case, it would seem that the joists are not suitable for their intended design loads anyway!

Plainrider 06-12-2008 04:00 PM

Robbin; Scuttlebuttrp:
If you check out the posts in my thread "Bedroom to Bathroom", I have provided nearly all the answers to your questions.
In a nutshell, the bottom 2x4 re-enforcement makes the joist a "T-beam" (like an I-beam) which imparts substantial strength and stiffness to the joist much greater than that of the single 2x4 itself.
After you read the posts on that referenced thread, I would be pleased to answer any of your questions.

cx 06-12-2008 04:43 PM


Originally Posted by Kevin
1. no sistering is the process of attaching same size joist to the side of the joist.
2. blocking perdicular usually at mid point helps stiffen the floor.

3. We try to sister every joist if possible. this allows you to flatten and level the floor.

1. Not necessarily the same size member, Kevin. That is the easiest to calculate, though.

Frequently we sister smaller or larger boards to the existing joist/stud/other. All depends upon what a fella's trying to do at the time. Some folks call it scabbing instead of sistering. Maybe they had sisters? I dunno. :shrug:

2. Blocking will help the joists achieve/maintain their design load capability, but it doesn't increase the design deflection. By bracing I meant installing vertical support of some kind to a floor joist.

3. That's a good thing. Sistering half the joists for purpose of reducing deflection still leaves a fella with "soft" areas in his floor.

Like so many construction terms, Royce, Sistering may well be interpreted differently in different parts of the country. And, of course, you know we have some people hereabouts to whom it might mean something quite different all together, eh? :D

My opinion; worth price charged.

Plainrider 06-12-2008 04:53 PM

Your question about adding sister joists mid way between existing joists vs. joist re-enforcement is good - I would genberally agree with Dan that all things being equal, adding the sister joists would be the preferred approach. Especially, if there are no (or relatively few) obstructions between the existing joists (e.g. cross-bracing, wiring, plumbing etc), then there would be little problem in adding sister joists.
If this is not the case..., then the preferred approach would be joist re-enforcement, I would expect for most situations.

robbin 06-12-2008 04:59 PM

I always find it helpful to have all necessary information in one thread. So for anyone else who may have missed the originating thread and don't want to search for it as I had to do, it can be found here


Plainrider 06-12-2008 05:20 PM

3 Attachment(s)
To streamline the understanding of the suggested "bottom wood-flange" joist re-enforcement approach, I summarize some of the information on this method that I have discussed in the referenced thread.

An easy & quick way to substantially increase the stiffness of 2x10 floor joists, for example, is to "screw & glue" 2 x 4s along the lower edges of the joists (see Figure 1 below). This approach eliminates the complications of adding sister 2 x 10s between existing floor joists and it more than doubles joist/floor stifness.

A diagram of this approach is shown in Figure 2. Of course, a drawback of this method is that the actual "headroom" in the room below this re-enforced floor is reduced by 1.5 inches - although the 2x4s can act as strapping for the ceiling in the room below.

Now, if 2 x 6s were added to the bottom of the floor joists (rather than 2 x 4s), floor stiffness would be increased even more, as is shown in Figure 3 (various combinations of floor joists and re-enforcing boards are shown in this Table).

To use this Table, the Deflecto calculations can be increased by the indicated factors in the Table for various re-enforcing situations.

Example: say you had calculated a Deflectio amount of L/400 for a certain 2x10 floor joist floor. But this is not quite stiff enough for an intended installation of natural tile. When you consult Table 3 for the 2x10 joist bottom-flange stiffening factors, you can see that the addition of a 2x4 bottom wood re-enforcement board would increae the joist stiffness by a factor of 2.1. Therefore, the equivalent deflecto reading for a 2x10 floor with added 2x4 re-enforcing joists would be L/840 - which is L/(2.1 x 400).

Because the Deflecto stiffness criterion for a natural tile floor is L/720, you are able to quickly and easily beef up your floor for natural tile by adding bottom-flange 2x4s to the joists - an effort that typically takes about 20 minutes per joist!

koihito 06-12-2008 06:34 PM

I think the practicality is going to depend much on location. In NC I see a bunch of 70's crawl spaces with 2x8 construction that are in the low 300's on the deflecto, but otherwise in fine condition. It's easier to go in and add a 2x4 (or 2x6 for stone) and not touch any plumbing, wiring, or insulation than to try and add a beam (that often cuts off access to parts of the crawl space) or remove everything and sister.

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