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PC7060 01-20-2013 12:02 PM

PC's Bathroom Renovations
My wife really wants porcelain tile in our 8x5 bath rooms but I am stuck with a home with 2x8 joists with spans of 11.5 feet up to 14' (don't get me start on the whole "self inspection" process that the local building authority implemented to keep up with the building boom in the 1980s.)

I used the deflectometer (very clever) to calculated the deflection for the bathroom to be around L/337 for a 2x8 with SYP and a 11.5 span.

The local tile guy says floor is stiff enough but I'm an engineer and believe numbers generally don't lie.

I removed the ceiling of our dining to to get full access the the underside of the bathroom floors so I can plumb for the new tub. This gives me room to implement additional structure to meet the deflection requirements.

My options seem to be:

1) sister in additional joists but the area is obstructed with water and drain lines which would need to be removed and re-installed (not a happy thought).

2) add blocking between joists to ensure load is distributed. Partial solution; adding another layer of plywood on top would probable achieve the same results.

3) glue and screw 3/4 ply to underside of joists to create a box beam. This would make future maintenance a real joy.

Any ideas?
Is there alternate flooring or grout which give the feel of the tile but can withstand higher deflections? I can post pictures if that helps.

Thanks in advance.

cx 01-20-2013 06:31 PM

Welcome, PC. :)

Our deflectometer is a pretty conservative tool, using a full 50psf loading for it's calculations. We prefer that to the customary 40 live/10 dead load calculation used by most building codes.

If your joists are of good species and grade and your joist spacing (I see no indication) is no more than 16" on center, chances are good they met the code requirement of L/360 on the 11.5-foot spans when installed, even with a dead load of 20psf. Tile industry standards accept that as suitable for a ceramic tile installation.

Adding between joist blocking will do nothing at all to improve your joist deflection. Adding another layer of subflooring is always a good idea in my opinion, but also does nothing at all to improve your joist deflection.

Up you you to decide what you want to do based upon information you might be able to glean from grade stamps on your joists.

My opinion; worth price charged.

technical 01-20-2013 07:13 PM

There is deflection based on joist span/length, joist size and thickness. There is also deflection between the joists width, or firmness of the subfloor. The larger the tile the more problems with joist span deflection. Large tiles may not remain bonded with the curvature of the joist span under load.

Generally bath floors are not subject to varying live loads like kitchen and dinning areas. What your joist are presently seem fine for a bath floor as long as your subfloor is 5/8"+ T&G. I would use an underlayment (over your confirmed T&G subfloor) to adhere your tiles to. If it is stone to be installed L/720, different conversation.

I hope this helped.


PC7060 01-20-2013 08:47 PM

L J. /CX

Thanks for the feed back. I have 2x8 #2 Fir joists which are nominally 16" OC. Down the center of the bath is a double joist. Overall bath is essentially 5x14 two zone dressing (5x6) and bathing (5x8) area with the joists running along the 14' axis.

Re the feedback on the joist deflection, I am gluing and screwing 2x4 to the side of all the joists to create a L beam. This should result in significantly improved joist deflection. Make the drywall guys happy too on big flat surfaces to work with.

Re type and size of tile, We are looking at using 6-8 inch porcelain tile. The subfloor is 3/4 T&G plywood in good shape overall. I did notice a couple areas around the edge of the old tub where some degradation has occured; less than 1 Sq ft. I will install 1/4" Hardie backer board over the subfloor. Is there any reason to use 1/2"?

Thanks again.

cx 01-21-2013 10:10 AM

1. That will be helpful if done correctly and is a whole lot easier than your initial plan to put the 2x4s on the bottom of the joists.

2. There is no advantage at all to using a thicker CBU. Follow the manufacturer's instructions.

There is, however, a significant advantage to adding another layer of subflooring.

My opinion; worth price charged.

Lakee911 01-21-2013 12:02 PM

To make the most out of your 2x4, use continuous lengths. If you can manage a twelve footer, trim it to fit rather than using two pieces.

PC7060 01-21-2013 06:59 PM


Concur on the additional 5/8 subfloor, and I have the bc exterior grade plywood cut and ready to go down in the kids bathroom. Waiting on the master bath for some reason my wife does not want me work on both baths at the same time. Ha!

Jason, you also get a concur, I have a bunch of 12' in the van ready to go up. Time to get to work.

HooKooDoo Ku 01-21-2013 07:32 PM


Originally Posted by PChestnut
Make the drywall guys happy too on big flat surfaces to work with.

Drywall guys? You're not doing the drywall yourself? It's not too difficult to do it yourself (however initially getting the drywall on the ceiling is easier if you have a buddy, as well as make a pair of T-Bars from some 2x4s to hold the drywall in place until the 1st set of screws are in place). If you don't know anything about finishing drywall, checkout www.drywallinfo.com. The guy who build that site shows DIYer how to finish drywall. He takes more steps than a pro, but his focus is to show a DIYer how to get professional results.

cx 01-21-2013 07:41 PM

Nonsense! He don't need no steenkin' "T-Bars" nor helpful friends. Onliest tool a reasonable homeowner needs to repair a drywall ceiling is a telephone. Leave him alone, already. :shades:

Same tool works wonderfully well for any but the smallest insulation jobs, too. And concrete. And some other stuff. :D

PC7060 01-21-2013 08:21 PM


Funny! I've put up a bunch of drywall in my day but now I tend to focus on things that I need done a certain way; like this kind of stuff, cabinets, trim work, etc. I have couple drywall guys who are reliable and surprisingly inexpensive. I still do the small dw jobs around the house but if it more than a sheet or two I follow CX example and and pick up the phone (well send a email anyway.). Well back to work.

Jaz 01-21-2013 08:52 PM


Here's another way to stiffen the joists. It's the same theory as the normal X-bracing, but much better. http://www.luxorcorp.com/data/IBS/IB...Connectors.pdf


cx 01-21-2013 09:16 PM

Mark me down as really, really skeptical, Jaz.

Cross bracing does nothing to increase the design deflection of any joist system, they simply allow the joists to remain vertical so they can display their full capacity. And those "connectors" look to me like nothing more than a blocking system dressed up to look like cross bracing.

And suggesting that they will improve the deflection of an engineered joist system, which will already include any necessary blocking or bracing, is a bit far fetched, seems to me.

Not much information on that pdf except the claim and some phone numbers. I think I'd want a lot more information on their "system."

My opinion; worth price charged.

Jaz 01-21-2013 09:49 PM

I don't know how much bracing improves deflection, but I'm sure that it does. I haven't read the entire pdf or searched further.

As you know, bracing works by 'sharing' loads with each adjacent joists. So some of the load on a joist is transferred to 2 others, how much? Bracing is not only just to keep the joists vertical. So if regular bracing works, I think this design should work better because it's built sturdy.

I look at my own bracing and can see they do some good, but not that much since they are made from 1x3 and they tend to split when toe-nailed.

I received the info about this system the other day, I don't make any claims. Maybe one of our engineer types can take a look see.


cx 01-21-2013 10:14 PM


Originally Posted by Jaz
I don't know how much bracing improves deflection, but I'm sure that it does.

And I'm equally sure it doesn't, Jaz. :)

Design deflection is based upon a maximum, uniform loading over the entire floor over the joist system and it presumes that the joists remain upright without any lateral flexing to provide that capacity. When the joists are long enough that some such flexing is anticipated, cross bracing or a similar bracing is specified to maintain the shape of the structure to allow the joists to continue to perform at the design level.

While the sharing of which you speak may help alleviate the effects of some substantial point loading, that does not change the design performance under the uniform loading used to calculate deflection.

Cross bracing can also serve to dampen some vibration and otherwise make a floor feel better, but the design deflection is based entirely upon the structural capacity of the joists.

My opinion; worth price charged.

PC7060 01-22-2013 08:18 PM

5 Attachment(s)
Jaz / CX,

Things are moving along smoothly, I've attached a PDF with a few photos. I included the type of blocking I prefer to use. Also included a picture to illustrate what happens when you leave HVAC installers unsupervised.

Blocking with 2x4 attached to side of 2x8 and the installed blocking. I cut the blocking down by 1 1/2 inches to 5 3/4" to accommodate the 2x4 portion of the L beam. Everything is glued and screwed. I'll be adding the 2x4 to all single joists after I complete the plumbing for the new tub.
Attachment 136868

Here's another shot showing the blocking style I use, very strong and easy to build to required size. I understand the blocking doesn't increase the overall rigidity but does stabilize any lateral deflection in the joists and provide solid mounting and alignment points for the 2x4 portion of the "L" beam. The one shown here are full size (7 1/4). Joist grade stamp also shown.
Attachment 136867 Attachment 136866

Found this gem after I stripped the drywall from the dining room. HVAC guys way back in the day cut through ½ of of a double joist and left the cut joist hanging by a couple 10p nails.

Reworked the problem by glueing, clamping and screwing the two joists together and then sistering (glue and screw) a short 2x8 inside the plumbing box shown above to reinforce the cut out area.
Attachment 136865
Attachment 136864

Take care,

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