Geeky engineer question: Floor deflection? - Calgarypuck Forums - The Unofficial Calgary Flames Fan Community

teamchachi · 01-16-2007, 07:38 PM

So I'm finally getting around to upgrading the flooring in my 1999 house. I'm trying to determine what I need to do to get the subfloor on the main level ready for porcelain tile or slate.

Joists are TJI Pro 120TS - 24" OC and the longest span is 14'.

Subfloor is Weyerhauser 23/32" Sturdifloor OSB.

Anybody out there know how to calculate the floor deflection? I want to make sure that I'm sturdy enough to do a tile installation!

Flames_Gimp · 01-16-2007, 07:45 PM

The deflection rule for ceramics is L/360, where L = the length of the floor in inches. So for example, if your floor is 12' wide, then 12' x 12" = 144", divide that by 360 gives you 0.4". This rule tells you your floor is allowed to deflect (sag) .4" under normal loading. The rule for stone tiles is L/720, essentially twice as stiff. With the same example as above, the floor would only be allowed to deflect .2" (less than a 1/4" over it's entire span).

http://www.mikeholmesfanforum.com/ a good forum to ask these questions!

MacDougalbry · 01-16-2007, 07:51 PM

With engineered joists, you would probably need to consult the manufacturer's specs to calculate the deflection of your floor, but odds are that the builder has given you building code minimum, which would not be sufficient for natural stone or slate for sure.

Couldn't find the specs for your 120 joists, but as an example, see the fancy formula on page 3 of the following document:
http://www.ilevel.com/literature/TJ-4001.pdf

I was checking the installation guidelines for Ditra, and with 24" joist spacing, they recommend adding a minimum of 3/8" of plywood plus the Ditra.

You might want to post this question for Tileman on the Mike Holmes Fan Forum.

MacDougalbry · 01-16-2007, 07:53 PM

Oh yah, here is the Ditra info link I was going to provide:

http://www.schluter.com/english/prod...32)_052005.pdf

teamchachi · 01-16-2007, 09:29 PM

Not sure how to read the tables? I think that this might be the right PDF that mentions my 120TS TJI joists? http://www.trusjoist.com/PDFFile/2040.pdf

I should note that they are the 11 7/8" joists and its a great room concept. The only thing loading the floor is the osb and flooring.

MacDougalbry · 01-16-2007, 09:39 PM

Link doesn't seem to work for me...

teamchachi · 01-16-2007, 09:55 PM

Oops..

http://www.trusjoist.com/PDFFiles/2040.pdf

Flames_Gimp · 01-16-2007, 10:04 PM

Quote:

Originally Posted by teamchachi

Not sure how to read the tables? I think that this might be the right PDF that mentions my 120TS TJI joists? http://www.trusjoist.com/PDFFile/2040.pdf

I should note that they are the 11 7/8" joists and its a great room concept. The only thing loading the floor is the osb and flooring.

i know that minimum for stone tiles are 2x12 joists, so you should be ok.. but post and ask tileman on mike holmes forum...

teamchachi · 01-16-2007, 10:57 PM

Looks like its 3/8" plywood over top of the OSB and then a layer of Ditra. Anybody know the best place to pick up Ditra in Calgary? What is 3/8" plywood going for these days?

Eddie Bronze · 01-16-2007, 11:11 PM

Quote:

Originally Posted by teamchachi

What is 3/8" plywood going for these days?

Nothing if you go to a random construction site in Calgary and steal a couple sheets.

Probably save yourself around $20 a sheet, at least.

PM me, you have a truck? I can get you a couple sheets, no problem.

I can't believe your floor joists are on 2' centers. Wow. I've been framing for over 3 years and never seen that once. Wall studs yes, floor joists never.

teamchachi · 01-16-2007, 11:26 PM

Thanks, but I'm going to need more than a couple of sheets. I'm looking at 300+ ft2 of tile.

I'm not terribly surprised that my joists are on 24" centres. Its a Jayman house and I get the impression pretty much everything in the house is built to the absolute lowest standard while still meeting code.

They are 12" engineered TJI joists if that makes a difference. And the longest span is only 14'.

Flames_Gimp · 01-22-2007, 06:35 PM

Quote:

Originally Posted by teamchachi

Looks like its 3/8" plywood over top of the OSB and then a layer of Ditra. Anybody know the best place to pick up Ditra in Calgary? What is 3/8" plywood going for these days?

3/8 ply is about 13$ for 4x8 sheet at home depot, they also sell ditra but is expensive there.

I would order from www.tile-experts-canada.com i need ditra as well, we should combine an order and splt the shipping. pm me if interested.