Why weight decides before volume
On almost every residential job, volume is in charge: you fill the box, it leaves. A roof is the exception. Shingles are dense and they stack flat — 25 bundles weigh one ton. A shingle dumpster already weighs tons while it is barely a third full. It is the only house project where the eye is that wrong.
Two numbers never to confuse. Capacity is what fits: a 20 yard swallows up to 175 bundles, which is 7 tons of shingles. The included tonnage is what the rental has already paid for: 2 tons on the 20 yard, 4 tons on the 40 yard. Shingles therefore reach the included tonnage long before they fill the dumpster. The included tonnage is not a capacity limit — it is a billing threshold.
Beyond it, the excess is billed from the disposal centre's weigh ticket, never rounded up: the real weight of your load, nothing estimated. The Pricing page lists everything that is included and the only possible extras. Fill to the rim, never above the walls.
From 50 bundles on, both sizes work, and you are the one who decides. The 20 yard if the roof is all you are throwing out. The 40 yard if you want to use the leftover tonnage to empty the garage or the basement in the same rental: the box is already in the driveway, it may as well work twice. The dumpster size guide compares the three sizes in detail.