Sheet Metal Screws vs. Wood Screws

BobsYourUncle wrote:
A sheet metal screw has the threads go right up to the head.

A wood screw has the threads stop short of the head, leaving some unthreaded shank.


You can get a variety of heads on both. Pan head, flat head and others.
The application dictates what's best to use.

Fastening a bracket to a roof through only a membrane, you want a pan head sheet metal screw. The wood is immediately below the membrane. Just be sure the shank of the screw is smaller than the hole in the bracket so the screw will rotate freely in the bracket in order to secure it tightly to the roof. You want maximum threads in the wood below. And DON'T overtighten it and spin the screw in the wood. Turn it until snug contact only.

Using a wood screw in that application will have a portion of unthreaded shank in the wood you want the screw to hold to. It won't hold as well. In 3/8 plywood, the threads of a wood screw will be below the wood and will not hold or tighten the bracket in place. You will have nothing but screw shank in the thin plywood.

And yes, caulk the whole screw before running it in. Shove the shank of the screw in a tube of sealant, then run it in, in addition to a blob on the roof through the bracket.

I would use some self levelling Dicor over it all once the brackets are screwed into place. Cover the base of the bracket and the screws.

Windrivers suggestion is a good one. Screws can pull out of 3/8 plywood even if installed properly. Big winds when you're doing 60 down the highway.
Or even use toggle bolts.

Sir you are CORRECT!

I would use #10 sheet metal screws and pre drill a hole for each screw the diameter of the thinnest part of the screw. Be careful running them in as not to strip them out. I would add extra screws if possible to the brackets. Caulk the holes before installing screws and also sealant under the brackets.

No one uses true "WOOD" screws any longer. 35 years Carpenter/Construction Superintendent.