We often get asked by customers whether or not they need a rain fly for their new wall tent. Most customers assume that since their tent is treated, a rain fly is not necessary. However, there is more than one reason to get a rain fly for your tent.
First of all, your new tent is treated to be water resistant, not water proof. Cotton canvas can be made 100% water proof, but only if it is coated with PVC, heavy wax, or polyurethane. While these treatments keep the water out, they also keep the tent from breathing. The reason we still build our wall tents with cotton canvas is because they breathe, and make for a more comfortable experience as a result. So, using a rain fly will ensure that your tent remains 100% water proof, while still allowing it to breathe.
Another reason to use a rain fly is to protect your tent from sparks. Wood stoves generate sparks which often end up on the roof of wall tents. Over time, tiny pin holes are created by these sparks. If you use a rain fly, it will catch the sparks before they hit the tent, and thus protect the tent from these pin holes.
Lastly, UV radiation can break down the cotton fibers in your tent. It's not that big of a deal if you only use your tent 1-2 weeks a year. But if you want to set it up for longer time periods, a rain fly will protect your tent from the UV radiation.
The bottom line is that a rain fly will usually result in a tent that lasts longer over time. It's always much cheaper to replace a rain fly than it is a tent. However, the ultimate decision is up to you.