Progress on Floor Prep for Slate Tile

14 years ago

Hello, Here are some pictures of my DH getting the joist in place for my bathroom remodel. Background info can be had at the link at the end of this post, below. It was a post I started in the Remodeling Forum last fall. It answered most of my questions and helped me proceed.

One note that sticks in my head, from the Remodeling Forum link below, is where Renovator8 states:

"- The existing floor that you describe would have a maximum deflection of .135" (L/844). This is so high because the Moment of Inertial of the old joist lumber is 42% greater than that of the modern lumber. (I didn't bother to add the probable greater Modulus of Elasticity)"

The wood is old, and we used even older salvaged wood to put in the new joist. All in great condition. The "new" salvaged wood was taken from an old wooden grainery that my DH had taken down that was build around 1900. It was beautiful to see when we cut a length and saw the end grain.

DH getting the joist in place and getting ready for the next cut:

All but one up there. There is one more to go up on the very left of the picture. They are 10" on center. The existing subfloor is 1-1/2" thick which we will screw through to the new joist from above. The underlayment of plywood will be 3/8" of ac exterior grade:

And here's the happy worker :)

So using JB "Deflectolator" my original floor was:

"For joists that are SYP or Douglas Fir, in good condition, 7.25 inches tall, 1.5 inches wide, 20 inches on center, and 9.5 feet long between supports, the deflection calculated is 0.262 inches.

This translates to a deflection of L / 435.

Since the maximum deflection for tile is L / 360, and for natural stone is L / 720, your floor is rated for Ceramic tile, Congratulations!"

To this:

"or joists that are SYP or Douglas Fir, in good condition, 7.25 inches tall, 1.5 inches wide, 10 inches on center, and 9.5 feet long between supports, the deflection calculated is 0.131 inches.

This translates to a deflection of L / 871.

Since the maximum deflection for tile is L / 360, and for natural stone is L / 720, your floor is rated for Ceramic tile or Natural stone, Congratulations!"

Now if I consider what Renovator8 stated in his post on the other forum, I could have some outrageously high L/? value, taking into consideration that this is most certainly old growth timber we've used.

Hope that this info is of help to someone.

Here is a link that might be useful: Adding Joist to Existing & Intact Floor

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Comments (4)

MongoCT
14 years ago
Th boys on the other thread were indeed having a grand old time.

I like what you did. There are two areas of deflection you need to be concerned with:

1) Deflection of the joist itself, in terms of how much the joist will "sag" along it's length when under load. That's the basic "L/XXX" number.

2) Deflection of the floor itself between the joists. For your floor, this "span" used to be about 20", by adding additional joists you halved it to about 10".

That was the prudent choice versus sistering the new joists to the old.

Did you calculate the deflection of the fence rail your husband is sitting on? And shame, shame on you. How come he doesn't have a cold beverage in hand after working in that crawl space?? ; )
enduring
Original Author
14 years ago
Mongo, thanks for your input. Yes, I agree that was some discussion, and I hardly understood the stuff. It was worth a re-read (several). I get more and more out of it with each read. I really like looking up into that crawl space now with the way we've re-enforced it. It looks rock solid. Thanks for your vote of confidence.

That fence rail is one of the joist that DH put in, stretched across the saw horses. He was goofing off. I took this picture and a short video of him testing the deflection, bouncing up and down. It didn't move much. Is that criteria for laying tile?

And I might add, he didn't have a beer in his hand because he "weren't" done yet :) If I would have given him a beer before we were finished, he would have been dead in his tracks. Later we went into town and he got a nice steak sandwich and a beer.
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MongoCT
14 years ago
Actually, the old criteria for testing floor deflection was to put a glass of water on the floor and to jump up and down.

If the water shimmered a bit, that's okay. If it formed white caps, oh, that's bad.

Your crawl space looks great. Congrats to the two of you!
enduring
Original Author
14 years ago
Mongoct, I tried the jumping to check the shimmer! Very low shimmer :)

Getting ready to install the floor warming cables so need to get the floor prep finished. Here is the 3/8" ply screwed down 4"oc as Bill V recommended.
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Now I have just realized something :( ... I have the underlayment installed and I still don't have the last joist on the far left in place yet. When we screwed the new joists to the floor we didn't have the underlayment in place. Now we need to get that last joist in there and the underlayment is in place.

I added a link to the old floor discussion below. That links to the "Remodeling" forum. I have posted more detailed information on there. Its about my progress getting the floor to this point and was posted today.

Here is the new joist positions, but the far left is missing the new addition:
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1) So is it ok to screw through the underlayment into the joist on that last one?

2) Or should we put bridging in between the joist and the plate and toenail these short pieces into the joist and plate?

I thought the principle of the layers was to help isolate them from one another. If I screw that last joist to the floor it will go through all of the layers and I will lose the layering effect, right?

The reason for the underlayment going in was I had the help and I forgot about that last joist. The plumber isn't done, but he said he didn't care if the joist was there or not.

Here is a link that might be useful: Adding joist to existing floor...