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Old 12-09-2017, 09:09 AM   #1
ben5243
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Floor joists for tile and tub question

Hi, first time poster. I've spent the last couple days looking at different ways to stiffen up the floor for tile and that seems to be pretty straight forward but I'm wondering how the dead loading and live loading is determined with a bathtub.

The house was built in 1975 and the master bathroom has not been updated. First house, just purchased a year ago knowing a full gut was in order.

I'm still in the early stages of planning a full bathroom gut and remodel. The current bathroom is tiny so my plan is to remove a wall and expand into the master closet and recreate the master closet elsewhere.

I've come up with 5 or 6 different layouts but the one I'm leaning towards is probably the worst case for loading and puts the free standing tub smack in the middle of the joist span. We don't have an exact tub selected but I'm using this one as a rough idea of what we want:
(Apparently I can't post links but it's an Empava 67" freestanding tub EMPV-FT1518 on Amazon)

Okay now for the fun stuff, here is the floor plan I'm working on with the joist locations. Joists may be off by a tiny bit but should be pretty close to these locations. 48x48 corner shower at the top left (not to be confused with the wood stove visible through the transparent floor). There is a wood stove chimney pipe in the upper right that will get a chase around it.


The joist span is 12'10" as measured in this side view. You can see the tub is right in the middle.


Joists are 2x8, 16 o.c. and I don't currently know the species of wood. It is stamped and I will hopefully be able to find a stamp tomorrow. The subfloor is plywood but I don't know exactly how thick. I plan to remove the entire subfloor and replace with 3/4 plywood anyway. The joists are in good condition and there's no notches or bored holes in any. The only obstructions are recessed can lights at the approximate location of the circles in the floor plan.

For Dug Fir/SYP the deflectolator gives me a L / 244

So I have two main questions:
1. If I remove all the subfloor and sister the joists with full height 2x8 (I don't think I'll be able to maneuver 100% of the span into place, but maybe 90%). How do I calculate a new deflection? Do I just input the same numbers but use 3" as the width? What about for sistering both sides? 5.5"? I assume doing the calculation for only sistering 2/3 of the span gets more complicated? I also would add perpendicular bracing to help.

2. How do I know the assumed dead load and live loads are enough for the tub? For the sample tub, it's 89 pounds and can hold 58 gallons. Most tubs I've found have leveling feet so that weight is concentrated on those, this tub has 6 feet shown by one of the review photos.

If I roughly estimate the footprint of the bathtub feet to be a 4.5 sqft rectangle, with 89lbs of tub, 484lbs of water, and worst case 400lbs of humans over 4.5sqft is 216 lbs/sqft? Would it even be possible to use a tub like this even with double sistering and bracing and be able to tile the floor or is my math entirely based on wrong assumptions?

Thanks!
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Old 12-09-2017, 10:16 AM   #2
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Hi Ben,

Welcome to the forum. This topic came up recently for someone else. I responded with perhaps an overload of math, but you might find the thread of interest: http://www.johnbridge.com/vbulletin/...d.php?t=124140

If you have adequate access to the joist bays and they are unobstructed, then certainly it is possible to add enough structure there to support your tub. It's just a matter of figuring out how much is required.

On your question #1, the answer is Yes, 3" or 4.5". If I have a chance I'll respond further about your specific situation later today.

Cheers, Wayne
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Old 12-09-2017, 10:40 AM   #3
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Welcome, Ben.

1. Yes, it's all pure addition at that point. I wouldn't bother with 2x6 sisters unless I could put them at the bottom of the existing joists and if you can do that you can sister with 2x8 material.

You must sister a minimum of the center two-thirds of the joists to be in any way effective in reducing joist deflection and the calculation is the same as for full length sisters.

Adding perpendicular bracing will not improve your deflection calculations.

2. You find out who did the assuming. The manufacturer should be able to give you an accurate weight for the tub and the water capacity. The tub is dead load, the water is live load as are you and your friend. If your tub sits on individual feet, that will be a subflooring problem rather than a joist problem. If it will be a problem, you can deal with that after you have done whatever joist stiffening you decide upon.

But the first thing you need to do is determine exactly what you currently have for joists.

My opinion; worth price charged.
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Old 12-09-2017, 04:54 PM   #4
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Thanks, I will certainly see what information I can get off a lumber stamp. Hopefully I can find one near the current hole in the ceiling without cutting a new one.

Wayne, I had read through that thread but got lost in some of the confusion about the remodel plans. I reread it and understand better how you are taking the loads on a couple of joists and calculating in sections. One thing I was confused about is you mention his span was strength limited rather than deflection limited. I interpreted that to mean the shear loading near the supporting walls is too much. Meaning sistering anything short of a full span between supports wouldn't help, correct?

I guess my main confusion is in converting point loads to distributed loads. It seems to get complicated quickly once you start loading multiple joists.

CX, I should be able to get 12ft sister joists in there when the wall is opened up to move the door frame but lowering them into place without dropping them onto the drywall and popping nails below will be the real test.

Could you explain what you mean by bracing won't help the deflection calculations? Do you mean the calculator won't take that into account or that bracing won't improve the overall deflection from a point load? I get that it won't improve your overall deflection from uniformly distributed dead load like tile, but if you have a person standing on one joist, the deflection on that joist while the adjacent joists remain stationary would cause the most stress on the tile, right? But if you brace them together that person's load is distributed to 3 or more joists. Or is the intention of the plywood sheathing so that the joist to joist deflection is less than the span?

Is there any way to determine how much 1/2 vs 3/4 vs 1inch plywood would help to distribute load from a bathtub with feet? Or is it better to just assume the load is only on the joists it's on or between?

Sorry for all the questions. I'm really excited to tackle the bathroom and learn. I've been watching shower pan and tile installation videos for about a year but just found this site a few days ago and already learned a lot.
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Old 12-09-2017, 06:54 PM   #5
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Ben, your joist design deflection is based upon a uniform load being applied to the floor. Generally 40psf live load and 10psf dead load for residential applications. The point loads between joists is handled by the subflooring, which does, of course, also help to distribute the floor loads.

The primary purpose of between-joist blocking is to prevent the joists from twisting under load and therefore allow them to display their full potential in resisting deflection. They do not reduce the design deflection, but they can help achieve it, especially in longer spans. Cross bracing is the most effective type of such blocking.

I know of no calculation for subfloor deflection under point load. The common test for ceramic tile installation is ASTM C627, which uses loaded wheels to determine when subfloor/substrate/tile installations fail. From such testing we get general subflooring requirements for various tiling substrates.

In your case, if you'll have very heavy point loads between your joists that might be a problem, I'd want to reinforce the subflooring in those areas after creating an adequate joist system. You could accomplish that by using cleats on the sides of the joists and installing a first layer of 3/4" plywood between the joists in those areas flush with the joist tops before installing your actual subfloor.

My opinion; worth price charged.
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Old 12-09-2017, 07:10 PM   #6
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I was just thinking, you probably don't need to add the weight of the people and the water both. If there are people in there, they are taking up the space of some of the water. Considering how people float, not far from 1:1.
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Old 12-09-2017, 07:16 PM   #7
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True so long as the people get in first and then add water, Jeff. But if the people fill the tub with water and stand in it, there would be a whole lot more weight there.

Only until they sat down, of course, at which time they'd have a completely different sort of problem with an even layer of water on the bathroom floor. Briefly.
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Old 12-09-2017, 08:03 PM   #8
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I'm also thinking that with 6 feet sitting on at least 3/4" (hopefully more) plywood, the load on the joists is getting spread out significantly from actual point loads. I fear that sometimes we overthink these things. As one who is an expert overthinker, I know it when I see it!
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Old 12-09-2017, 08:38 PM   #9
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That tub has a projected area of about 32" by 66", or 14.7 square feet. A standard 40 psf live load for that area would be 587 lbs. So if the tub full of water and people weighs more than that, the excess load should be addressed in the joist design.

If you go with your original numbers, the excess load is 400 lbs, or with 2 joists 200 lbs per joist. The tributary area of one joist is 16" x 154" or 17 square feet. So if the excess 200 lb load were spread out over the length of the joist, it would be a load of 23 psf. Since the load is concentrated in the middle third of the joist, the deflection is greater than that of a uniform load; the amplification factor is about 1.4. That is the deflection of the central 200 lb load per joist is about the same as an extra 33 psf live load over the full length of the joist.

The deflectolator is based on a 50 psf total load, so if we want to add an extra 30 psf to it, we need to multiply the results by 8/5. If your current joists give you L/244, then you will need to double them everywhere, which will give you L/488. But 8/5 * L/488 = L/305 > L/360. So you would need to triple the two joists under the tub.

Or if you can find a modern species and grade stamp, you could investigate what the AWC calculator at http://www.awc.org/codes-standards/c...tware/spancalc says. For example, if your joists were #1 Douglas Fir-Larch, then the maximum span for 15 psf dead load and 40 psf live load is 12' 10". In which case you could leave the field joists unsistered (just passes) and just double the two joists under the tub.

If you want to say that the tub can only hold one person when filled with water (or that two people displace 200 lbs of water), then the excess load is half as much. So you'd want deflectolator results for 65 psf, or 13/10 of what it prints out. 13/10 * L/488 = L/375, so you'd be Deflectolator-OK with just doubling the joists under the tub instead of tripling them.

Cheers, Wayne
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Old 12-10-2017, 11:04 PM   #10
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Thanks Wayne that analysis really helps! I will play around with those numbers as I approach a more final layout.

Jeff, I definitely considered the displacement of water but wanted to be ultra conservative if for whatever reason two large people were to stand in the tub with it full. Our combined weight is only under 300 I was just thinking build it extra strong.

CX okay I understand the bracing more now. The joists currently have 2 locations where they are criss cross braced with 1x2 (best guess from memory). Is that would you mean by cross bracing is best to reduce twist or is solid perpendicular braces that fit snug better?

I just measured and the subfloor is 3/4 plywood. I've read suggestions people use a 1/2" layer and 3/4" layer going perpendicular to also help distribute load. Is there a good way to go from a carpeted bedroom to a tile floor that's got an extra 1/2" of subfloor that doesn't cause a toe stub or trip hazard or just look funny?

I looked really hard in 3 different spots and cannot locate a lumber stamp on the joists anywhere. Is it possible they stamped the 2x edge? I saw a 2x10 in the ceiling which creates a soffit but isn't structural. It had markings on the edge but I couldn't read the entire length.
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Old 12-11-2017, 06:09 PM   #11
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Yes, the 1x2 is the style of cross bracing I had in mind, Ben, but I'd want it to be at least 1x4 material.

You want all layers of structural subflooring to be oriented with the strength axis perpendicular to the joist structure. No exceptions.

Easy to ramp your carpet up a half-inch to meet your tile surface. There are materials made specifically for the task or some folks use cedar shingles.

Lumber grade stamp won't be on the edge of the joist, it'll be on the face.

My opinion; worth price charged.
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Old 01-05-2018, 09:24 PM   #12
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I'm in the process of ripping up the subfloor now. I just measured and my joists are 1.5x7" exact measurement. Not 7.25 like common 2x8

No lumber stamps anywhere but I'm fairly certain it's Douglas fir.

Where can I find 1.5x7 lumber? Or should I just sister them with 2x6s?
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Old 01-06-2018, 07:07 AM   #13
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Not likely to find exact 1.5X7's at any of the common locations, and given your situation I'd hesitate to use 2X6's - which will net out to something closer to 1.5X5.5 - ish. You could run the numbers again with 2X6 sisters - maybe they'll give you what you need.

You might have to rip down some 2X8's. If you have to resort to that be certain to cull through the stack for the straightest ones possible, or find some place that sells engineered lumber.
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Old 01-06-2018, 10:02 AM   #14
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It looks like you are remodeling the full area above the joists, so you can drop in full length joists. You don't need to slide anything under existing subfloor?

In which case you could just get kiln dried 2x8s (which I recommend anyway), those may be only 7-1/8" or even 7" wide. If you can accept an extra 1/8" or 1/4" of floor height, that would be fine; if not, you could use a power planer to lower and flatten the joist tops in situ.

Since you have structural concerns, you might as well get Select Struct joists, which is the highest grade generally available, and will be kiln dried.

Cheers, Wayne
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Old 01-06-2018, 01:31 PM   #15
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Yes I currently have the entire span minus what's under the walls open. I'm hoping I can get full 13ft pieces into position so the sisters are supported on both ends but I haven't tried yet.

If I used anything larger I would have to shave them down to 7" at the ends where they go under the walls.

I'm just really confused I can't find any information about 1.5x7" joists or anyone who has had a similar situation online. The house was built in 1975 and every other piece of lumber I've seen is a standard size.
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