How to replace deck railing (and why was it rotten?)

[jarko]

My house had a deck when I purchased it which was built in 2017.

One of the railings began growing fungus a few months ago, and has become rotten and discolored (can easily stick a screw driver into the wood).

The railing adjacent and the supporting posts do not appear to be affected:

I removed the railing today and will give the top of the posts some time to dry out, before I seal them and replace the railing.

I have some questions about the replacement:

 - What could have caused this railing to rot, while all other railings are fine?

 - This railing had 4 bugle screws in the top driven into the posts, in addition to the metal brackets underneath - why would the screws also be installed? Most other railings do not have bugle screws, and these appear to have allowed water to penetrate the treated wood more easily (the bugle screws are rusted). The only purpose I can think of is to try to prevent movement since the railing is mitered on one side? Should I use bugle screws again on the replacement or just accept the miter will separate outside anyway?

Thanks

[JacobZ]

Hi @jarko,

I'd say the bugle screws are the main cause of the rotting. Their location on top of the timber has allowed an entry point for the water.

You should avoid having any screws on the top of an exposed railing like this as the water will pool on top of the timber and eventually find its way into the screw holes. Once the water has found its way in, there is no airflow to dry it out, so it will sit in there slowly breaking down the treatment and eventually the timber. 

Fixing from underneath is the way to go, as the water can't pool on the hole and slowly find its way in.

Due to the exposed nature of the timber, expansion and contraction will occur, and I would expect the mitre to open up slightly. If you wanted, you could screw horizontally through the join, however, this would still be creating an entry point for water, it would just be on a better face to avoid water pooling. If you can live with the mitre opening up, only screwing from underneath is going to be a lot better long term.

Let me know if you have any further questions. 

Jacob