Designers often worry that the connection between the subfloor and the floor joists is insufficient and they strengthen this this connection even though there is not a single recorded instance of this being a failure point.  This is why none of the extant seismic retrofit codes and guidelines strengthen this connection.    Furthermore, Appendix A or CHAPTER A1 of The Guidelines For The Retrofit of Existing Buildings, tells us if the typical older home subfloor is made of 3/4″ diagonal sheathing with 5/16″ finished wood flooring on top of it and has a capacity of a whopping 1800 lbs. per linear foot.  This flooring configuration is what we see in almost all houses built before ~1955.

A Common Detail Used All Too Commonly

Some designers question the adequacy of the joist to sub-floor connection and want to augment it.  We will be looking at a very common way of doing this.  Look at the detail below and notice that it is attaching a joist to what appears to be a 3/4″ plywood subfloor with Simpson A35s.

We do not advise using A35s.  The nails are closer together than the L90s and they split dry framing consistently.  In addition, even though Simpson has tested this connection for  A35s, they only had them tested with Simpson screws so what you see here is an untested system.  This detail specifies #6 x 3/4″ pan-heard screws be used.  The screws will penetrate the 1/16″ thick  A35 and then penetrate the subfloor another 11/16″.  Let’s look at what what happens when we analyze this connection with the American Wood Council Load Calculator. 

Common Construction Detail Showing An A35 Subfloor Connection

Here are the basic parameters.  We are looking at lateral loading and the fastener is a screw.
When we click submit initial values this is what we see.  I have already filled in the required information; gage and length of screw, thickness of the floor, steel for the A35, etc.


The calculator does not even have a setting for screws less than 1″ long.  The only thing we can say for sure is that that it should be more than a 3/4″ screw.

Once we calculate the capacity of our A35 subfloor connection this is what we get.

Is There A Solution?

If there is reason to suspect the connection between the subfloor and the floor joists is insufficient (such as when the sub-floor is made of plywood) adhesive shear blocking can be installed on one or either side of the joist. A good Subfloor adhesive has a tremendous amount of shear strength.

Many designers have shied away from structural adhesive because they were worried the connection would be brittle (some adhesives have very low ductility). This means, just as is the case with stucco, once the failure it is sudden a complete.  In light of this fact, this connection should always be over-designed so that the glued connection is so strong there is no question of any movement at the glued connection.  Modern adhesives will resist something like 250 pounds per square inch.  If a 2 x 2 x 8 is used in a shear block you would have around 35,520 lbs. of shear resistance.  Put that on either side of joist and you have doubled that.  So, you have a choice, either use pan-head screws and A35s that give you zero capacity or use shear blocking that provides 35,000 lbs.