I have a deck about 6FT off the ground with old 4x4 posts that stick up about 24" above the existing deck. I replaced the decking and when installing the railings, I noticed the posts on the deck were about 12" too short. Is there any way to extend the posts about 12" so when i screw the top railing in, it would actually go into the post? Want it up to code as much as possible, in VA. Thanks.
What I did was drill axial holes in the center of the 4 X 4 ; push in one foot lengths copper tubing ( or steel conduit) halfway. Then drill matching holes into the extension 4 X 4 and push them onto the tubing . When connecting railings are added it has good strength. Select a hole size to give a tight fit with whatever tubing you use.
So what you want to do is extend the existing posts by splicing on another 2' or so. Fortunately it's just the above - deck part of the post, so you don't have to worry about load bearing, just that it's strong enough to support your railing.
I think the simplest way to extend the posts would be with a half lap joint:
You'd want to make the lap section at least six inches long, and longer would be better. Use two 1/2" bolts to bolt the joint together using large washers on both sides of the joint. If you cut the joints so the "cheeks" of the lap are perpendicular to the edge of the deck, the joint will be stronger against people leaning on the rail.
I'd also use a good strong glue that can fill the gaps on the short faces. The better you cut your joint, the smaller the gaps, the better the glue will work. The worst glue to use would be a polyurethane glue such as Gorilla Glue, that will not work well. A good outdoor rated wood glue like Titebond III will probably work best for you. There are wood epoxies that are very strong too but they are more expensive and harder to find, and a bit of a learning curve to using them.