When I was in the Boy Scouts, we would sometimes set up tents at a camp outside of town. One night it rained really hard and water accumulated on the top of our canvas tents. I reached my hand to feel the bulging roof and my scoutmaster pulled my hand back. He said that the tent was only water-resistant, so if I touched the canvas, the water would have a way to wick through the cloth and drip into the tent.
I didn't try it that time, but later on I did put my hand on the roof of a water-resistant tent and the water did indeed drip through the canvas.