Opinions...Is glue enough?

[Tom Bender]

A few thoughts;

11' is a long fall, cut no corners

The treads are 5 3/4 wide but make the tenons 4" to leave more wood in the stringer

Make double tenons 1 1/2" wide leaving 1" of stringer wood in the center

Now you can go back to 1/2" deep

Put the rod across under the tread, make it decorative with a fancy nut then paint it. A spray can of gold paint is cheap and will look good. Now you can forget about the plugs.

What will the user hold when climbing? Be sure he never gets a splinter, even when the ladder is old and no longer in great condition

[Al Launier]

My initial thought was also to use a metal rod (~3/16" dia.) under the treads to pull the sides together. If preferred, a slot (square or preferably rounded. i.e. bull nose router bit) could be milled to the bottom of the tread to accept the rod since the dado will be supporting the tread vertically. Since the sides are 5/4 there is ample thickness to c'bore & plug the holes at each end of the rod. This would add stiffness while hiding the rod.