I have a gate mounted to a 6x8 post where the center has rotted out of the post. I am looking to fill the center of the post, rather then replace it if I could. The post itself is still solid, just hollow so the mount bolt have nothing to connect to anymore. I am thinking to try the expanding post foam inside the post, but have not worked with it and I have no reference if it would be stong enough to hold a bolt.
This sounds like a bad idea. Just replace the post.
I’d replace the post.
The foam might work for a hot minute but I’m not even convinced of that. I’d think the stress from the weight of the gate will cause it to fail rather quickly.
If the hollow is up in an area where the hinges attach you might split the post with the foam expanding as it cures.
If you’re in the US, replace it with a western red cedar post. It costs more but lasts much longer than standard SPF 4x4s. Let your kids replace the next one.
Or a 3" thick gauge steel pipe, if you’re not concerned with looks. Your great grandkids can replace that.
Time to get a new post… This sounds like the equivalent of duct taping your fence together.
The thing about the expanding foam is it may force you to get a new post anyway.
When that stuff expands, it can push cracks apart and if you have enough hollow in that center, it may be enough to provide decent compression power to the foam.
On the positive side, the foam will keep water from going in there, which is causing/exacerbating the damage.
Will foam provide support? Marginally. Bolts won’t get much added strength, but the entire post may flex a tiny bit less, assuming it doesn’t split during the curing period.
Silicone may work better, but I’m betting the quantity needed would surpass the cost of having someone replace it all.
You won’t be out much if it doesn’t work, but if it does end up splitting the wood, you may want to be prepared to replace it.
