I need to attach notched 4x4 posts to my deck. What size of lag screws should I use? 3/8 or 1/2. 4 or 5 in?

Question

I need to attach notched 4x4 posts to my deck. What size of lag screws should I use? 3/8 or 1/2. 4 or 5 in?

Answer 1

Bob is right, but one more thing I would do is to set the lags screws diagonal with each other not one on top and the other one on the bottom or side to side...just to reduce the risk of splitting the wood.

Answer 2

The engineers here are over killing the situation.  I go with the  diagonal placement of the holes, and 1/4 pre-drill, but for the 3/8 inch lags with washers.  These should go 1 and 1/2 inch into the underlying wood.

Answer 3

Not knowing the purpose of the notched 4x4's, weight bearing for the roof or railing posts, I would use the 1/2" lag bolts with washers under the heads. Add the thickness of the unnotched portion of the post plus the thickness otf the material you are attaching the post to and subtract 1/2". This is how long the lag bolt should be.  Be certain that you drill a 1/4" pilot hole through all material before using the lag bolts... this will keep the posts from splitting and use at least two lag bolts per post.  Good Luck.