Is my subfloor stiff enough?

jwatson15 · Mar 9, 2013

Greetings! Well im getting ready to start my first ceramic tile flooring endeavor however i'm concerned that my subfloor may not be stiff enough and I am looking for some expert advice. My home has 14'' deep i-joists and the two locations that I am considering flooring have different spacing between the joists. the joists under my foyer which is approximately 20 sq/ft are 16'' OC and my bathroom which is 50 sq/ft is 21'' OC. I dont know a whole lot about engineered i joists but they are stamped APA if that means anything. The subfloor is 3/4 inch plywood. If you need anymore information just post and I'll do my best to supply it.

Ernesto · Mar 9, 2013

I'd go buy this book. http://www.tcnatile.com/faqs/47-installing-tile.html

Also check through the FAQ section for quick tips.

FloorMaven · Mar 9, 2013

According to this -

http://www.openjoist.com/tech/360_25.htm

your structure should be fine for ceramic tile.

Ken · Mar 9, 2013

I say go buy this book: "The Lorax" by Dr. Suess!

Ernesto · Mar 10, 2013

FloorMaven said:
According to this -

http://www.openjoist.com/tech/360_25.htm

your structure should be fine for ceramic tile.

I don't trust that. I'd say one layer is not sufficient with the 21" on center. Just to be safe another layer of ply and brace the floor underneath if possible.

What is the acceptable deflection for a floor that will be tiled?

Traditionally, the accepted minimum requirement for floor rigidity is L/360 - before the tile underlayment is installed. The L/360 standard means that the floor should not deflect more than the "span" divided by 360. If the span of the joists is 10 feet (between supports), then the deflection should not be more than 1/3" between the center and the end. Frequently, there is misunderstanding regarding deflection between joists. For example, while joist manufacturers regularly meet the standard L/360 criteria for code construction with 24" o.c. (on center) systems, these floors often have deflection between the joists exceeding L/360.

Recent research has shown tile to fail, under some conditions, when the floor is more rigid than L/360. In fact, failures at L/600 have been observed. It is for this reason that recommendations for floor rigidity are not based on deflection measurements but on empirically established methods found to work over normal code construction.

http://www.tcnatile.com/faqs/30-deflection.html