Ceramic Tile Advice Forums - John Bridge Ceramic Tile

Ceramic Tile Advice Forums - John Bridge Ceramic Tile (https://www.johnbridge.com/vbulletin/index.php)
-   Reference Liberry :-) (https://www.johnbridge.com/vbulletin/forumdisplay.php?f=8)
-   -   Wood framed floors (a collection of posts) (https://www.johnbridge.com/vbulletin/showthread.php?t=114739)

Bud Cline 03-30-2002 11:53 AM

Wood framed floors (a collection of posts)
Where you aware that not all floor structures are suitable for the installaion of tile?

Unfortunately sometimes you have to "Just Say No" To Tile.

Your floors "deflection" under both dead and live load cannot exceed 1/360th of your span, and in the case of natural stones the limit is half of that at 1/720th.

Here is some very boring yet none the less very important information you should have from the:



By Dale Kempster

The TTMAC (Terrazzo, Tile and Marble Association of Canada) released the completely revised 1997 "Specifications Guide 09300" which succeeds the 1989 edition.

In the new "Tile Installation Manual" the reader will find several references to " … deflection under both dead and live loads does not exceed 1/360 of span." This reference can be found in details; 310F-98, 311F-98, 312F-98, 313F-98 and others. For many individuals this criterion of deflection may not be easily understood; both for the calculation and the actual ramifications for the installation in question.

First, a definition of deflection should be made; "A variation in position or shape of a structure or structural element due to effects of loads or volume change, usually measured as a linear deviation from an established plane rather than an angular variation". In other words, when a load is applied to a surface such as a plywood floor, that floor will often bend downwards in relation to the amount of weight applied and the structural capability of the plywood and the support ( i.e. joists) underneath.

It should be noted that the deflection is measurable both as the length of the span, as well as between joists, in the case of a residential application. In actual practice, what this means to the installer is that they have to know if the sub-floor will be able to support the tile and not exceed the deflection criterion of L/360.

In practical terms this means in a residential application with joists 16" o.c. (400 mm) the maximum deflection that can be allowed when using L/360 is 16/360 which equals 1/25"= .04"or (1 mm). Now how does this relate to a single layer of plywood sub-floor. When looking at the span tables published by the American Plywood Association, it will be found that a single layer of plywood 19/32" (15 mm) on joists spaced at 16" o.c.(400 mm) the maximum amount of weight that can be applied is 368 lbs (167 Kg) before the deflection succeeds L/360 (remember 1/25" or 1 mm)

Now, this may seem like a lot of weight, but this is a uniform load distributed over a wide area that is supported by the joists. If a concentrated load is applied between the joists using a 3"(75 mm) wide disk with 200 lbs (91 kg) applied to the same piece of plywood, the plywood can deflect, up to approximately .078" = 1/12.5" or 2mm. This calculation indicates that the single layer of plywood can have approximately twice the deflection acceptable for the installation of tile. In real terms, a concentrated weight of 200 lbs (91 kg) over a 3" (75 mm) wide area, is not all that difficult to attain in a situation such as a fridge or a stove being brought across a floor on a dolly.

With all of this being said, it can now be understood where the recommendation for two layers of plywood 5/8" (16 mm) thick or the use of other underlayments, is required by the TTMAC manual, Detail 313 F-98. Other underlayments such as ½" 13 (mm) cementitious backer board, or a pre-formed sheet applied membrane such as Schluter Systems, Ditra Matting can be considered.

Another consideration as well, is post-tensioned and pre-stressed concrete are often engineered with only L/240 deflection criterion. This means a slab 30 ft long, supported only at the ends, (L/ 360 = 360/360) could have deflection up to 1 " (25 mm). See TTMAC Detail 309F-98 for recommendations on installation over pre-cast concrete systems.

To give more food for thought, recently the Marble Institute of America has increased the deflection criterion to L/720, which is double that of tile. It is with all this in mind, that one should make sure that they are well acquainted with the new manual and its practices. Installation techniques and systems should be evaluated and be guaranteed to respect the L/360 criterion.

OK, got it?
You say this wasn't mentioned at the Home Center when you bought your new tile?

You should investigate your structures ability to support your new tile installation and to assure yourself that the installation will not self destruct in a short time.

Use the John Bridge Deflect-o-lator to help you determine your floor's suitability for tile and stone.

John Bridge 04-01-2002 05:29 PM

Floor Framing Definitions and Strengthening
A very good link submitted by Cami A., aka "da Hostess."


flatfloor 04-02-2002 05:16 PM

Oriented Strand Board (OSB) and waferboard are engineered, mat-formed panel products made of strands, flakes or wafers sliced from small diameter, round wood logs and bonded with an exterior-type binder under heat and pressure.

Strand dimensions are predetermined and have a uniform thickness. The majority of Structural Board Association (SBA) member mills use a combination of strands up to 6" (150mm) long and 1" (25mm) wide.

OSB panels consist of layered mats. Exterior or surface layers consist of strands aligned in the long panel direction; inner-layers consist of cross- or randomly-aligned strands. These large mats are then subjected to intense heat and pressure to become a "master" panel, then cut to size.

flatfloor 04-02-2002 05:25 PM

Particle Board . a grainless construction board of sawdust or wood particles with a resin binder.

Bud Cline 04-29-2002 12:38 PM


Registered: Not Yet
Posts: N/A

APA OSB and plywood subfloor sheathing

Hey Guys (Gals?), I’m an Engineered Wood Product Support Specialist working at the APA Help Desk. I am not an experienced tile setter. So why am I on your thread? Well Dog Paws asked us (APA) to read the conversation and comment on gapping issues as related to OSB subfloor sheathing. So here goes, hopefully I’ll add to understanding rather than confusion.

As stated in APA literature: 1/8 inch spacing is recommended at all panel edge and end joints (subfloor, wall and roof) unless otherwise indicated by the manufacturer. Spacing is an APA recommendation intended to minimize the risk of panel buckling when panels are exposed to moisture during construction or acclimatize to a higher moisture content after installation.

When inspecting an existing structure it may be very difficult to determine if panels were properly spaced at the time of installation. If the panels were dry at the time of installation and subsequently acclimatized to a higher moisture content the space (gap) will be diminished. The gap may even be fully closed if moisture conditions were high enough to cause significant panel expansion.

(Properly manufactured tongue and groove joints will provide a gap between the faces of adjoining panels when the tongue is snugly fitted in the groove.)

If the subfloor has been exposed to moisture during construction, there is no gap between panels and the panels are not buckled it is likely that the panels were properly gapped when installed and subsequently expanded to close the joints. Unless you expect the panels to further expand providing additional gaping is not necessary. If you choose to create a gap between panels care needs to be given to any tongue and groove edges. The tongue and groove assembly is a structural component of the floor system. If it is cut through blocking or structural underlayment (minimum 1/4-inch plywood or OSB) or 3/4-inch wood finish floor must be installed. It is also important to avoid cutting into underlying framing.

Ridging at panel joints may occur for a variety of reasons, edge swelling due to moisture exposure; twisted, uneven or misaligned framing, debris on the framing, over driving tongue and groove edges during assembly, compression of panel edges due to expansion, buckling, etc. In most cases ridged joints can be made flush by sanding.

I hope this information is helpful. Please let us know if we can supply you with further information. You can submit inquiries about engineered wood products to us at: help@apawood.org

Rob Z 08-05-2002 05:31 PM

A Cure For Bouncy Floors"-JLC article
We have discussed many solutions that can be employed to help prepare a substandard floor system for tiling.

One that has been mentioned before is the following article from Journal of Light Construction. The method detailed by the author, Robert Randall, P.E., is relatively simple and yieds good results. I think it is well within the abilities of a motivated DIY\'er to try this if they need it for their floor.

The article \"A Cure For Bouncy Floors\" can be found in the June, 1997 issue of Journal of Light Construction.

Contact http://www.jlconline.com for access to this article if you cannot find it in a library.

davem 11-18-2002 09:29 PM

WWPA Notching and boring guide for floor joists & stud walls in conventional light-frame construction (Courtesy Western Wood Products Association).


John Bridge 12-05-2003 10:40 AM

Plywood Floors -- CDX vs. Flooring Grades
This is a must read for anyone considering the use of CDX sheathing for subflooring material.


John Bridge 03-05-2004 06:32 PM

X-Bracing and Joist Sistering

Mike2 08-27-2005 03:18 PM

Plywood Deflection and Grain Orientation
Plywood Deflection and Grain Orientation

Here is a visual intended to demonstrate the importance of laying plywood sub-flooring and underlayment perpendicular to the joist runs.

Relevant materials used in this demonstration include:
  • Two strips of 3/8” (actually 11/32”) ACX Douglas Fir plywood cut into 5 ¼” strips. One was cut/ripped with the face grain. The other was cross-cut against the grain. Both strips were cut from the same panel which has 3 plys (face, core, and back).
  • Two 2"X6" 'joists' spaced 16” on center.
  • Six bricks (the load) weighing in at 29.9 pds. total.
  • One dial indicator for accurate measurement of deflection.

Results are shown in picture below:

Case #1 - Plywood laid perpendicular to joists
Depicted on the left, this plywood strip is laid in the right direction with the face grain running perpendicular to the joists. Actual deflection under load was .069” or slightly more the 1/16”

Case #2 - Plywood laid parallel to joists
On the right, this strip is laid in the wrong direction with the face grain running parallel to the joist. Actual deflection was .380” or 6/16” (3/8”), six times greater than Case #1.


In summary:
Tile needs a stiff floor, disliking movement of any kind.
  • Pay attention to your joist structure - i.e., joist size, spacing, and length of unsupported span (See DEFLECTO tool towards the top of this screen).
  • AND make sure your plywood is laid in the correct orientation, perpendicular to the joist runs.

Note: Actual results (deflection amounts under a given load) will vary of course depending upon plywood thickness, number of layers, grade, species of wood, etc. However having said all that, orientating the face grain perpendicular to the joist runs will always yield the least deflection given the common grades of building plywood readily available.

Mike2 11-05-2005 10:02 PM

Here is another article on plywood co-authored by Frank Woeste, P.E., and Peter Nielsen, Schluter Systems L.P. : Position of Underlayment to Prevent Cracked Tile and Grout

Mike2 04-24-2007 11:00 PM

1 Attachment(s)
JerseyReef (Mike) a member here recently posted purchasing some plywood Sheathing panels stamped PRP-108 (see picture below). With no veneer grades on this stamp such as C-D or C-C, he contacted the APA (American Plywood Association) for additional information on this panel.

APA response was that Exposure 1 Sheathing manufactured under either PRP-108 or PS 2-92 standards is equivalent to a DD grade. The full text of that response is in this post here: http://johnbridge.com/vbulletin/show...1&postcount=56

Clearly then, plywood stamped APA Rated Sheathing, Exposure 1, PRP-108 or PS 2-92 is not suitable as sub-flooring or underlayment under a tiled surface.

Mike2 07-04-2008 09:25 PM

Plywood grades
Plywood veneers are graded into five levels, as follows:
A – Highest grade level. No knots or knotholes. Some patches allowed
B – Small round knots. Patches and round plugs allowed.
C Plugged - An improved C grade, found in Sturd-I-Floor and some of the better underlayments.
C – Small knots and knotholes allowed. Lowest grade allowed in Exterior rated type plywoods.
D - Larger knots and knotholes. Limited voids/pockets allowed in wall and roof sheathing panels. This grade is not allowed in Exterior rated panels. Furthermore, it should not be used under a tiled surface
Example: When two grades are used together as in AC , the panel has grade A veneers on the face, C grade on the backside.

If you are interested in more technical information, the APA (Americal Plywood Association) publishes many free documents on plywood. This one here provides a thorough discussions of the various grades. APA Panel Handbook & Grade Glossary

John Bridge 03-31-2009 03:27 PM

Wood Framed Subfloors
1 Attachment(s)
Investigating Failures Frank Woeste & Peter Nielsen

This article was originally published in Tile Letter in two parts. Please see the footnote at the bottom of page one.

John Bridge 02-12-2010 10:24 AM

Plywood Underlayment Direction
1 Attachment(s)
New article on plywood floors by Frank Woeste, Virginia Tech and Peter Nielsen, Schluter Systems. The article appears in Tile Letter, February 2010. A draft copy appears here:

All times are GMT -6. The time now is 10:36 AM.

Powered by vBulletin® Version 3.7.4
Copyright ©2000 - 2022, Jelsoft Enterprises Ltd.
Copyright 2018 John Bridge & Associates, LLC