While the fabric may provide good waterproofing, water will initially enter via stitching lines, along seams, necklines and around fittings. Canvas rugs use are sewn using a specially developed cotton covered nylon thread. This is designed to swell when wet, to completely fill the stitch holes. But you will never stop water entering along any stitching seams. Canvas has lots of benefits - but if you need a rug that will keep your horse dry for long periods, avoid canvas and look at using synthetic deniers.
A wool blend lining creates a membrane between the canvas and horses coat to extend water penetration. Canvas will absorb moisture from exposure to rain and from condensation under the rug. If you use canvas in wet conditions, you will need to remove canvas regularly and allow to dry out completely. During heavy rain or longer periods of light rain, canvas will eventually 'wet out' and the fabric will become saturated. This is part of the breathability of canvas. Provided you have a wool layer or wool lining between the canvas and the horse, the horse should remain dry for longer periods, even though placing your hands between the wool and canvas it may appear wet. The wool will repel the moisture and keep it at bay between the wool and inner canvas. As soon as the rain stops, the horses body heat will start drying the rug.
After around 12 months of constant heavy use, the rug may require re-waterproofing. This will be evident when water is leaking through the actual canvas, not just the joins. We then recommend that you use a canvas reproofer: