Knotty pine

That would be a tough thing to quantify. Sometimes knots actually make the wood stronger. Think of splitting firewood. In the shipyard we used to call tight, hard knots in Yellow Cedar God’s bolts.

Never though of it as I always avoid them.

Give the board a good bend. If it breaks or feels like it will break you have your answer.

I have seen good and bad knots. They can strengthen or weaken a board. What I am referring to here tho is knotty white pine with the knot passing perpendicular through the wood. As well if the knot is cut thru on the edge of a board the knot itself has a tendency to pop out of the wood. But for argument sake lets just say that the knot is stable and encircled with regular grain wood. In knotty pine the knots will tend to weaken not strengthen the board. The question remains by how much is it weakened?

The first question you probably want to ask yourself is what is the intended structural design requirement. If something really needs to be sturdy, perhaps it should be made or reinforced with hardwood. I use pine 1×6 for door frames. You can hang a pretty heavy door from it so long as its shimmed and fastened properly to the 2x rough framing.

On the other hand, in a project I recently worked as owner’s rep, I saw a contractor use 2×4s ripped down to about 3/4 × 3/4 spindles for replacing stair railing balusters (they needed to raise the railing height). This was fine for the pieces that were clear of knots, but you could see a few with knots in the middle that are sure to break with very little force. Hopefully it doesn’t involve a child. I protested this shoddy work, but nothing came of it.

If you use your guidelines. Chris, I would say not much weaker. It would be like a truss. The top (regular wood grain) would be in compression and the bottom (regular wood grain) would be in tension. The middle (where the knot is) would just be a spacer and as long as the knot wasn’t huge it should have almost no effect. I’m thinking of the bed rail you referred to originally.