If you have a long roof line you'd like to keep pristine in a watercolor...usually an architectural feature but it could be a mountain line or water, you can use regular packing tape (I like Ace brand) to mask it off. Since doing straight lines with mask is rather difficult...this is a honey of an idea that I've used many times. The tape is translucent and you can see your drawing lines lightly through it. Using a straight edge and a very sharp craft knife, you lightly and gently cut through the tape (not cutting paper is trick) and then pull off as you see above. Then I snug it down with my thumbs a little to make sure it's adhering properly. It is absolutely the best mask for a straight edge I've ever found. Generally, it works best on absolutely dry unpainted paper. I've tried it over painted and it's not so good and you risk loss of some color when it pulls up. But you could experiment on a scrap paper and see.
This is a drawing of our little church up here in Lac du Flambeau, WI. It is the Community Presbyterian church.