Roof Tear-Off Dumpster Sizing: Squares by 10-40 Yard

I swung a hammer for eleven years before I ever wrote anything for a living, and the question I heard more than any other — from homeowners, from adjusters, from my own green helpers on their first solo tear-off — was some version of "how big a can do I need for this roof?" Nine times out of ten the person asking is thinking about it wrong. They're picturing a pile of shingles and trying to guess how much space it takes up. That's not the number that matters. Asphalt shingles are heavy, dense, and they stack flat. You will hit the weight limit on a roll-off long before you fill it to the rails.

So let's do the math the way a roofer does it standing in the driveway, then talk about what's specific to roofs here in Rock County — ice dam rot on the north slope, the hail runs, and the fact that everybody in Janesville wants their roof done between June and the first hard freeze.

First: what a "square" is

A square is 100 square feet of roof surface. That's it. It's a roofing unit, not a math unit, and it exists because a bundle of shingles is sized so that three bundles cover one square. When a contractor tells you "it's a 28-square roof," he means roughly 2,800 square feet of actual sloped roof surface — not the footprint of your house.

That distinction trips people up constantly. A 1,600 sq ft ranch does not have a 16-square roof. The roof is bigger than the floor plan because it's tilted, and because it hangs past the walls at the eaves and rakes.

Estimating squares from your footprint and pitch

Here's the driveway method. Take the outside dimensions of the house — including the garage, including any additions — and multiply for footprint square footage. Add roughly 6 to 10 percent for eave and rake overhang. Then multiply by the pitch factor below, and divide by 100.

Worked example. A 40 x 30 ranch with an attached 22 x 24 garage. That's 1,200 plus 528 = 1,728 sq ft of footprint. Add 8 percent for overhang: about 1,866. It's a 6/12, so multiply by 1.12: about 2,090 sq ft of roof. Divide by 100 — call it 21 squares. Round up, always. Nobody in the history of roofing has been mad about ordering slightly too much dumpster.

If your roof is cut up with dormers, valleys, and hips, add another 10 percent. Complicated roofs generate more debris per square foot than simple gables.

What a square of tear-off actually weighs

This is the number that decides your container size.

• 3-tab asphalt (the flat, older style): 230–250 lb per square per layer.
• Architectural / dimensional / laminate: 280–350 lb per square per layer. Most roofs installed in the last twenty-five years around here are architectural, and the heavier premium lines push the top of that range.
• Wood shake: 350–450 lb per square dry — considerably more if it's been soaking up spring rain.
• Concrete tile: 900–1,100 lb per square. Rare in Wisconsin, but it shows up.
• Clay tile: 600–900 lb per square.

Take that 21-square example with architectural shingles at 300 lb per square. One layer is 6,300 lb — more than three tons — before a single sheet of rotten decking goes in the box.

How many squares actually fit: the weight-limited table

Every roll-off has a volume and a weight allowance, and roofing debris is the classic case where the weight allowance runs out first. A 30-yard container will physically swallow 45 squares of shingles. It will be far past legal haul weight long before that. The numbers below assume a typical residential weight allowance and single-layer tear-off. Confirm your exact tonnage when you book — allowances vary.

Two things jump out of that table if you've been on a roof. First, the jump from 20 to 30 yards buys you less than you'd think — the extra volume is real, but the extra tonnage is what caps you — and the jump from 30 to 40 buys you almost nothing at all. Second, the 20-yard is the sweet spot for most tear-offs, and not only because of capacity. A 20-yard sits lower than a 30, and when a guy on the ground is pitching bundles over the rail eight hours straight, six inches of wall height is the difference between a good day and a sore back.

Why a 40-yard is almost never the right box for a shingle roof

We run 40-yard containers too, and I have almost never put one on a shingle tear-off. A 40 measures 22 feet long by 8 feet wide by 8 feet tall — the exact same footprint as a 30 yard. All of that extra ten cubic yards is two more feet of wall height, for 14 to 16 pickup loads total. The tonnage barely moves: 5 to 6 tons against the 30's 4 to 5. You are paying for a third more volume that shingles will never use, because you hit the weight cap with the box barely half full either way. Look back at the table — a 40 carries a handful more squares than a 30, not a third more.

Worse, those eight-foot walls work against a roofing crew. On a 20 yard, with sides around four and a half feet, a crew can pitch debris over the rail from a ladder or the roof edge. At eight feet that stops being possible — everything goes through the rear door or gets tarped and carried. That's why the 20 yard is still the roofing workhorse even though bigger boxes exist. If a 30 won't carry your roof by weight, the answer is a second container or a planned mid-job swap, not a taller box.

The one roof that genuinely wants a 40 is a large commercial re-roof where the tear-off is insulation board, EPDM or TPO membrane and other ballast-free material rather than asphalt. That debris is light and bulky — it fills volume long before it nears the tonnage cap, and the height finally earns its keep.

Layers: the thing that blows up every estimate

Wisconsin housing stock is old, and for decades it was legal and normal to shingle right over the existing roof. Plenty of homes in Janesville's older neighborhoods — the Fourth Ward, Look West, the near-east side — carry two full layers, and I have personally pulled three off a 1920s foursquare more than once.

The rule is brutally simple: multiply your tonnage by the number of layers. Two layers of architectural on that 21-square example is 12,600 lb. That's not a 20-yard job anymore. That's a 30-yard, or a 20-yard with a planned swap-out mid-job.

How do you know before you start? Pull up a shingle at the rake edge and count the courses. From the street, a two-layer roof usually sits noticeably proud where it meets fascia and step flashing. If you're not sure, budget for two — getting a second container out on short notice in August isn't always possible.

Everything else that goes in the box

Shingles are the bulk of it, but they aren't all of it. A realistic tear-off load also includes:

• Felt or synthetic underlayment — light, but bulky and it takes up air space.
• Ice-and-water shield — sticky, heavy, and it usually comes off with a layer of decking attached.
• Rotten decking — plan on a few sheets on any Wisconsin roof; more if there's been ice damming.
• Flashing, drip edge, valley metal, pipe boots, old vents.
• Nails. Thousands of them. A full tear-off can generate 40–60 lb of nails alone.
• Sometimes gutters, if they're going in the same trip.

Add 10–15 percent to your shingle tonnage to cover it. If you're replacing more than a handful of decking sheets, add more — plywood and OSB are dense and add up fast. Plywood and OSB are dense and they add up fast.

Wisconsin realities

Ice dams. Our north-facing slopes see the worst of it. Snow melts from attic heat, refreezes at the cold eave, and water backs up under the shingles. By tear-off day that lower three or four feet of decking is often soft, and the ice-and-water shield underneath is welded to it. Budget extra weight for the north side.

Hail and wind. Rock County has taken real hits — the 2019 derecho and multiple hail runs through the Janesville–Milton–Edgerton corridor. Storm-damage roofs frequently come with an insurance claim attached, which means you want to document the load: photograph the roof before tear-off, photograph the debris in the container, and keep your weight tickets. Adjusters ask, and a scale ticket showing 4.2 tons of shingles is hard to argue with.

The season. Asphalt won't seal reliably below about 40 degrees, which means our real roofing window runs from late spring to roughly late October. Every roofer in the county is fighting for the same twenty weeks, and container availability gets genuinely tight from June through September. Book ahead. For a Monday start, calling the Wednesday before is not early — it's normal.

For disposal rules and what can't go in a mixed load, the Wisconsin DNR is the authority, and the City of Janesville can tell you whether your placement needs a right-of-way permit.

Placement: protect the driveway, save your crew's legs

Set the container on the gable end or eave side closest to the biggest roof plane, as tight to the house as the truck can safely place it. Every extra step a loader takes with a shovel full of shingles is multiplied by a few hundred trips. If you're using a chute, position the box so the chute lands in the middle of the container, not over one end — a can loaded heavy at the back is a problem for the driver.

On asphalt, put something down. Two layers of 3/4-inch plywood or a few 2x10s under the wheels and rear rollers spreads the load and keeps steel off your driveway — Janesville asphalt goes soft in July heat. Keep the box off sprinkler heads, septic lids, and soft lawn edges.

If you're not sure which size fits your roof, walk it with us — details are on our roofing dumpster service page.

Ready to price your tear-off?

Tell us the footprint, the pitch, and how many layers you think are up there, and we'll tell you the right box — no guessing, no surprise overage. Call Little J's Dumpster Rental Janesville at (608) 531-8367 for same-day and next-day delivery across Janesville and Rock County, or get in touch here. We'd rather size it right the first time than send a second truck.