This is an existing pool deck that I would like to add 4x4 posts to strengthen the handrails. The wood railing on the top is bowing and it's not very sturdy. The square sections of the deck seem pretty straightforward. How would I attach posts around the round section? Won't the posts not sit flush over the mitered joists?

 

Welcome to the forums!

Unless the 4x4 continues to the ground I don't see how it will do much to stiffen the railing. Are all the bolts secured tightly?

 

All of the bolts are tight. The worst parts are the straight sections. The top rail there is bowed pretty bad.

 

There is no core structure for your railing system. There is one board run vertically along the top. Great for getting it to bend to match the curve of the deck but that bend-ability also means it doesn't have much strength to remain straight on your straight sections. Then holding it up are a series of small wood balusters. In your last photo the top mounting screw/bolt for the balusters appears to be right at the top of the framing member, probably pulling loose or breaking out and offering little strength. I wonder in the area where your straight section is really bowed if it's top screws are also missing the framing below.

Unfortunately I can't think of an easy solution or what that doesn't significantly alter the appearance of what you have. On the long straight sections I would consider attaching a pressure treated 2x6 horizontally to the bottom of your top member. This would hold the straight sections straight and put the new wood inside where it would be less visible. It would also form a small shelf... perfect for sunglasses and beverages so it wouldn't look so much like a fix.

Then as an added step I would consider totally new 4x4 posts sunk outside your existing posts and going all the way up to the top of your railing. You could attach these new posts to the deck structure with brackets for additional supports. With the 2x6 on the straight sections you could space the new posts further apart. Then on the curved part of the deck I would run new 4x4 where the existing posts are located and come up and support the railing at the joints.

 

I think I know what you're envisioning but I don't have the skill set to know if it will work.

To do it, I would remove the balusters at the joints and replace with a 4x4 which is carriage bolted through the structure at the bottom. You will have to shape the bottoms a bit to fit the angles.

 

I was thinking of something like this. It you scroll down a bit there is a picture where they are bolting the posts on like stickshift mentioned. I just had concerns of the posts rocking or having gaps where I bolt them on. That tutorial on the link is almost exactly how my deck was built minus the posts added in to give strength to the rails. The curved sections aren't too bad because they are short sections. But the long sections are at least a 10 foot span supported by just the ballasters.

How to Build a Deck Around an Above Ground Pool

 

More photos.

 

I'll just say that I don't like those deck building "instructions". Their attachment of the railing posts looks problematic.