Don't think I have ever seen a bay window with gutters. To stop the run off problem you put a small garden under the window, make sure the ground slopes away from the house, put in your flowers or small bushes and use white stone as a mulch. This allows the rain from the window to run away from the house without dirt splashes on the house during a heavy rain.

But our baywindow is on our deck and the rain run-off is hitting the deck and splashing back causing our siding to rot in places. I've never seen another baywindow with gutters, but it seems a necessity for us.
Thanks for your reply.

It depends on what type of eavestrough system you plan to install if you are using a 5 inch seamless aluminum gutter you can buy a strip mitre and you just have to cut the two ends of your gutter at 22.5 degress each then slip the mitre on and seal.
If you havn't done this yet and are using a differant type of gutter post back and I will let you know how.

It depends on what type of eavestrough system you plan to install if you are using a 5 inch seamless aluminum gutter you can buy a strip mitre and you just have to cut the two ends of your gutter at 22.5 degress each then slip the mitre on and seal.
If you havn't done this yet and are using a differant type of gutter post back and I will let you know how.

Simplest and least expensive might be to put a diverter on the roof above the bay window.