Thoughts on single vs double nailing for hemlock siding?

Your foreman is right, even if he's got a short fuse. I’ve seen guys double-nail Western Red Cedar and Hemlock thinking they’re being thorough, only to come back a year later and see every board split right down the middle. Hemlock is temperamental. If you pin it in two spots, it has nowhere to go when the summer sun hits it.

The secret isn't more nails, it's the overlap. We usally run a 1-inch lap and drive a single 7d stainless ring-shank nail about an inch above that lap. You want the nail to just miss the top of the board underneath. That way, the bottom board can expand and contract behind the one above it without being pinned. I swear by Maze Nails becuase they don't bend as easily when you hit a knot in that tough hemlock grain.

If you're relly worried about cupping, make sure the back of the boards got a coat of primer before they went up. That does more for stability than a second nail ever will. Seeing a scrap over a nailing pattern is some old-school drama, but I'd rather have a grumpy foreman than a callback to replace a whole wall of split siding.

I disagree that a single nail is a catch-all solution for hemlock. I’ve seen more jobs fail beacuse the boards cupped so hard they pulled the single nails right out of the sheathing. Hemlock isn't cedar; it's got way more internal tension. If you're only using one nail near the top, you're asking for the bottom edge to flare out and look like a mess once the sun hits it. We switched to using 2-1/2 inch Maze stainless steel ring-shanks and we actualy nail thru the thick part of the board, just above the lap of the previous course. The key is making sure you don't pin the thin top of the under-board. This secures the "meat" of the siding while still letting the board behind it move. If you're worried about the wood splitting, just pre-drill your ends. It’s a bit more labor, but it prevents the boards from walking off the wall without needing to double-nail and cause those vertical cracks everyone is afraid of.

While the debate over mechanical restraint is valid, you really have to consider the hygroscopic equilibrium of the hemlock stock itself before it ever meets a driver, becuase installing timber with a moisture content exceeding fifteen percent virtually guarantees significant tangential shrinkage regardless of your nailing configuration. I’d wager the cupping mentioned earlier is often a result of cellular collapse during a botched kiln drying process rather than a failure of the fastening pattern, so ensuring your material is sourced from a mill that actualy understands the temperamental drying curve of Tsuga heterophylla is probably more critical than the specific gauge of your 304-grade stainless fasteners.

If you want to stop the cupping without splitting the wood, forget the standard nails and grab a box of GRK Trim Head Fin/Trim screws. They have those tiny heads that disappear but the holding power is insane. I use the 2-1/2 inch version for hemlock siding all the time. The reverse threading under the head pulls the board tight to the stud and keeps it from bowing out, even if you only use one per stud. I’ve seen guys go at it on site too, but ussually it’s beacuse they’re using cheap fasteners that pop out. Switch to GRK and you won't have to worry about the boards moving or the foreman losing his mind.