I purchased 6’ 4×4’s to put up a 4’ fence. I learned about digging below the frost line after. So now the non returnable 4×4’s I have are too short.
Is it possible to join two 4×4’s together to make my six foot posts into eight foot posts? If so can the joint be below ground?
Best Answer
Is it possible, yes. Is it practical (.vs. just buying the right size posts), not really. Should it be below ground - no.
To really do it right you want a router bit or shaper cutter that will put tapered "fingers" on the end of the joint which interlock for maximum glued surface - which, unsurprisingly, is exactly what you'll see in the end joints of engineered lumber - but if making a 4x4 from engineered lumber, there would be 2 offset 2x4's that were end jointed, so that the end joints were not all at the same point on the resulting timber.
You can, of course, do it to a lower standard. It may work well enough, or it may fail.
Another option would be to:
Set steel posts and bolt the wood to them. Either cut them off or just let the posts stick 2 feet above the main fence (bolt them to the steel posts above ground level - don't half-bury them.) Depending on the purpose of the fence those extra 2 feet can be useful, i.e to string trellis wire for plants or monfilimaent to annoy deer above your solid fence.