The best thing to ever happen to wall tents is a poly tarp. You can replace cheaply every year. Snow slides off it, it stops UV damage , and your tent is just about guaranteed to be water proof.
Canvas tents are not really waterproof they are only repellent. Capillary action makes the water too big to fit through the small holes in the weaving. But if something like a finger or even a tent pole touches the tent in the right spot, the surface tension is disturbed and water begins to flow through the fabric.
Another reason a tent may leak is because its brand new and it has not weatherized. The needle holes where it was stitched together at the seams are stretched and are larger then the thread, allowing water in through the holes.
To prevent leaky seams purchase a fly or a tarp to place on top. The treated canvas is breathable on all sides. Allowing air to escape preventing condensation to be trapped inside even if the fly/tarp is directly on top of the wall tent.
* UV Rays: Long exposure to the sun's UV rays will damage all canvas wall tents. A fly is recommended to prevent UV damage if you set up your tent 2-3 weeks or longer annually. A fly also keeps your roof clean, snow slides off the roof better and acts as another weather barrier.
* Zipper Maintenance: Always have zipper unzipped when placing/removing tent over frame to prevent too much stress on zipper. As your roof shrinks the tent will fit tighter on the frame and increase stress on the door zipper.
* Mildew: If you have mildew on your canvas wall tent mix one or two cups of bleach in a 5 gallon bucket. Wash only the area with mildew. This will help clean the mildew spots without affecting the treatment on the canvas. Use the bleach chemical at your discretion.
* Washing Tent: Use only mild detergent like "Woollite". Use a hose and scrub lightly.
* Storage: Never store a damp or wet tent. Always let it dry naturally. Your tent will get mildew if stored wet regardless if you have a mildew treated canvas.
* Cleaning: Always brush off the dirt or mud (let mud dry) with a soft brush.
* Set up Location: Be aware of which trees emit resins and avoid setting up your canvas wall tent under these types of trees.