Extra waterproofing and feesCoating or extra layer, your permit figure
See full breakdown
To size a flat roof, multiply length by width in feet, counting any overhang or parapet turn up, and divide by 100 to get squares. Order about 10% extra for cuts and seam laps. A flat roof still needs fall: the code minimum is 1/4 inch per foot (about 2%), so the drop equals the run in feet × 0.25 inch, or 5 inches over a 20 ft run. Installed, most flat roofs cost $4 to $14 per sq ft: modified bitumen $4 to $8, built up $4 to $10, EPDM $4 to $13, TPO $4 to $14 and PVC $5 to $16, plus $1 to $5 per sq ft if the old roof comes off.
How to calculate flat roof area, slope and cost
This flat roof cost calculator works in two passes: measure first, then price what you measured. No pitch factor is needed, because a fall of 1/4 inch per foot adds well under 1% to the surface.
| Measure the roof area and order list | Price the roof installed cost | |
|---|---|---|
| Area | (Length + 2 × overhang) × (Width + 2 × overhang) | The measured area, not the order area with waste |
| Order and squares | Area × (1 + waste %), and squares = area ÷ 100 | Laps and offcuts are already inside an installed price |
| Slope | Drop in inches = run to the drain in ft × slope in inches per ft | New drains counted; tapered insulation quoted on site |
| Materials | Rolls = order area ÷ roll coverage; edge metal = perimeter | Area × installed price per sq ft, or your own rate |
| Result | Squares, rolls, adhesive, edge pieces and the drop | Membrane + Tear off + Insulation + Drains, curbs and pipes + Permit |
Worked example: 40 × 30 ft EPDM roof, old roof torn off, and a 20 ft run at 1/4 inch per ft for a 5 inch drop
When to use this flat roof calculator
Open the flat roof cost calculator any time a low slope roof needs a size, an order list or a price, from a porch roof to a small commercial building.
- Budgeting a new flat roof or a reroof: measure the outline, pick the membrane and you have a working range before anyone climbs up. Switch the old roof between removed and nothing to remove to see how much of the budget the tear off takes.
- Comparing TPO, EPDM and PVC: flip the membrane and watch the range move.
- Checking contractor bids: enter the same area, membrane, insulation and extras each roofer quoted, then line the bids up against the range. A price far above it with no wet insulation, deck repair or new drains listed deserves a question before you sign.
- Permit applications and financing: print the area and cost range for the paperwork.
- Ordering materials and planning delivery: the order list turns the area into sheets or rolls, gallons of bonding adhesive and 10 ft pieces of edge metal. That is enough to book the drop off and check the roof can take the load in one go.
- Garages, porches and extensions: small roofs where one EPDM sheet often covers the lot.
- Fixing ponding before the new roof goes on: enter the run to the drain and the slope you want. The drop tells you how much height tapered insulation or framing has to build, so the new membrane is not sitting in water two days after every storm.
- Takeoffs for flat roofers: squares, rolls, edge length and fall in one pass, ready to print.
How much does a flat roof cost per square foot by membrane?
Each price below is installed, so it covers both material and labor. Removing the old roof is often quoted on top, at $1 to $5 per sq ft.
| Membrane | Installed per sq ft | Typical life | How it goes on |
|---|---|---|---|
| Modified bitumen | $4 to $8 | About 20 years | Polymer modified asphalt sheet that is torched, mopped or self adhered. The lowest priced of the common choices and quicker to lay than built up. |
| Built up (BUR) | $4 to $10 | 15 to 30 years | Layers of felt and asphalt made on the roof. Easy to patch, but slow to build and needs regular checks. |
| EPDM rubber | $4 to $13 | Up to 30 years | Black rubber sheet with glued or taped seams. Shrugs off blown debris. |
| TPO | $4 to $14 | 20 to 30 years | White, reflective sheet with heat welded seams. Quality varies between makers, so the brand matters. |
| PVC | $5 to $16 | 20 to 30 years | Heat welded like TPO, with better resistance to grease, chemicals and fire. |
Other low slope options: fiberglass (GRP) at $4 to $6 per sq ft for small roofs, metal at $4 to $16 and up to $30 for copper, and planted green roofs at $10 to $30. Metal needs more fall than a membrane; price it with the metal roof cost calculator. For thickness and attachment choices on a white single ply roof, use the TPO roofing cost calculator.
What does a flat roof cost for 1,000 to 3,000 sq ft?
On a flat roof the area is close to the building's footprint. The national average replacement is about $8,000, for a 1,000 to 1,500 sq ft roof in modified bitumen.
| Roof area | Any membrane ($4 to $14) | Modified bitumen ($4 to $8) | PVC ($5 to $16) |
|---|---|---|---|
| 1,000 sq ft | $4,000 to $14,000 | $4,000 to $8,000 | $5,000 to $16,000 |
| 1,200 sq ft | $4,800 to $16,800 | $4,800 to $9,600 | $6,000 to $19,200 |
| 1,500 sq ft | $6,000 to $21,000 | $6,000 to $12,000 | $7,500 to $24,000 |
| 2,000 sq ft | $8,000 to $28,000 | $8,000 to $16,000 | $10,000 to $32,000 |
| 2,500 sq ft | $10,000 to $35,000 | $10,000 to $20,000 | $12,500 to $40,000 |
| 3,000 sq ft | $12,000 to $42,000 | $12,000 to $24,000 | $15,000 to $48,000 |
Installed prices on the measured area. If the quote leaves out removing the old roof, budget another $1,000 to $5,000 on an average sized roof.
How much slope does a flat roof need?
Building code sets a design slope of at least 1/4 inch per foot (a 2% grade) for built up, modified bitumen, EPDM, TPO, PVC, spray foam and liquid applied roofs. Only coal tar built up roofs may drop to 1/8 inch per foot.
| Slope per ft | Grade | Angle | Drop, 10 ft run | 20 ft run | 30 ft run | 40 ft run | Allowed for |
|---|---|---|---|---|---|---|---|
| 1/8 inch | 1.04% | 0.60° | 1.25 in | 2.5 in | 3.75 in | 5 in | Coal tar built up only |
| 1/4 inch | 2.08% | 1.19° | 2.5 in | 5 in | 7.5 in | 10 in | Code minimum for every other membrane |
| 3/8 inch | 3.13% | 1.79° | 3.75 in | 7.5 in | 11.25 in | 15 in | All membranes |
| 1/2 inch | 4.17% | 2.39° | 5 in | 10 in | 15 in | 20 in | All membranes |
Drop = run from the high point to the drain or edge in feet × slope in inches per foot. Water still standing 48 hours after rain is ponding, and it cuts the life of any membrane. From 2 in 12 upward, shingles become an option, so measure those roofs with the roofing calculator.
How many rolls, gallons and edge pieces does a flat roof need?
Divide the order area by what one roll or sheet covers. Bonding adhesive is worked out on the roof area itself, since it is kept off the seam laps.
| Material | Roll or sheet | Covers | Bonding adhesive, fully adhered |
|---|---|---|---|
| EPDM | 10 × 100 ft; also 20 ft wide, and up to 50 ft wide in lengths of 50 to 200 ft | 1,000 sq ft | 120 to 150 sq ft per gallon, rolled on the deck only; 60 to 75 on flashings, which take it on both faces |
| TPO | 10 × 100 ft, or 6 × 100 ft on mechanically attached roofs | 1,000 or 600 sq ft | 45 to 60 sq ft per gallon of contact adhesive, rolled onto both mating surfaces |
| PVC | 10 × 100 ft | About 1,000 sq ft | Check the maker's data sheet |
| Modified bitumen cap sheet | About 39 in × 33 ft | 1 square (100 sq ft) | None when torched or self adhered; mopped sheets go down in hot asphalt |
| Built up | Made on site from layered felts and asphalt | By ply count | Hot asphalt between plies |
| Edge metal or drip edge | 10 ft pieces | Perimeter ÷ 10 | Not used |
The 40 × 30 ft example: 1,320 sq ft to order is two EPDM sheets of 10 × 100 ft, 8 to 10 gallons of field adhesive, and 140 ft of edge in 14 pieces.
What do tear off, insulation, drains and flashing add to a flat roof?
Hold each extra on its own line so two bids can be compared like for like. Of the installed price, labor alone usually accounts for $3 to $7 per sq ft.
| Item | Typical cost | Notes |
|---|---|---|
| Whole flat roof, installed | $4 to $14 per sq ft | Most replacements land between $4,000 and $21,000 |
| Labor | $3 to $7 per sq ft | Or $30 to $130 an hour, about $75 on average. Anywhere from about 20% to nearly half the bill, highest for built up |
| Tear off and disposal | $1 to $5 per sq ft | About $1,000 to $5,000 on an average roof; heavy roofs such as concrete cost more to strip |
| Polyiso insulation | $1.40 to $3.15 per sq ft | Installed: 1 inch $1.40 to $1.75, 2 inch $2.00 to $2.50, 3 inch $2.80 to $3.15 |
| New roof drain | $180 to $250 each | The drain body and its flashing |
| Skylight, hatch or chimney curb | $250 to $500 each | By size and how tricky the flashing is |
| Pipe flashing | About $50 each | Plumbing vents and small stacks |
| Permit | $150 to $600 | More when the roof type changes or the structure needs work |
| Repair instead of replace | About $700 on average | Patches run $2 to $10 per sq ft; a drain repair $70 to $500 |
Only a few spots leaking? Price the patch with the roof repair cost calculator before committing to a new roof.
What makes one flat roof cost more than another?
Two roofs with the same footprint can come back thousands apart. Here is where the gap usually comes from.
| Factor | What changes the price | Typical effect |
|---|---|---|
| Roof size | The biggest single driver of the total. | 1,000 sq ft $4,000 to $14,000; 3,000 sq ft $12,000 to $42,000 |
| Membrane | Sets the per sq ft rate and how long the roof lasts. | Modified bitumen from $4 to $8, PVC up to $16 |
| Old roof removal | In many cases the old covering has to be stripped first. If two roofs are already stacked on the deck, code says both come off before a new one goes on. | $1 to $5 per sq ft |
| Hidden damage | Wet insulation, soft decking or tired underlayment found once the roof is open. | Hundreds to thousands of dollars |
| Installation method | Ballast (stone holding the sheet down) is the simplest. A fastened roof often costs the most, because every penetration has to be sealed, and fully adhered sits between the two. | Ballasted lowest, fastened highest |
| Parapets and penetrations | Parapet walls, vents and chimneys slow the crew down. | Curbs $250 to $500, pipes about $50 each |
| Drainage work | New drains, or extra height to reach a proper fall. | Drains $180 to $250 each |
| Insulation | New polyiso is priced by the inch, so each extra inch of board raises the rate. | $1.40 to $3.15 per sq ft |
| Location | Roofer pay follows the local cost of living, and labor heavy systems such as built up feel it most. | Crews bill $30 to $130 an hour |
How do roofers measure and price a flat roof?
Get the takeoff right and the rest of the bid follows. Miss a drain, a parapet nobody measured or a second layer hiding under the first, and the price is wrong before work starts.
- Measure the outlineLength × width of the main roof, plus any overhang. Break an L shape or an odd outline into rectangles and triangles, then add the pieces together.
- Trace the waterNote the run from each high point to its drain or scupper.
- Find out what is underneathCut a core to count the layers and look for wet board. Two layers already on means a full tear off down to the deck.
- Order with wasteAround 10% on a plain rectangle, more once curbs and pipes add cuts.
- Count every detailDrains, scuppers, curbs, pipes and the full perimeter for edge metal. Each one gets its own line, because each one is a leak point if it is rushed.
- Price and presentInstalled rate for the membrane, then tear off, insulation, details and the permit, laid out line by line.
The Measure the roof tab covers the first, second and fourth steps; Price the roof covers the rest.
Flat roofers: the ponding call comes the morning after the storm.
Water sitting on a garage roof or dripping through a porch ceiling sends a homeowner down the list of roofers in their phone. If you are up a ladder with a torch in your hand, that call goes to voicemail and the next name on the list gets the inspection. ServiceAgent answers every call, writes down the roof size, the membrane and where the water is getting in, and puts the inspection on your calendar.
Book a free demoFrequently asked questions
Straight answers on measuring, slope, materials and what a flat roof costs.
Book a free demoHow do I calculate the square footage of a flat roof?
Multiply the length by the width in feet, adding any overhang or parapet turn up to each side. For an L shape or an irregular outline, split the roof into rectangles and triangles, work out each piece and add them up. Divide the total by 100 to get roofing squares; no pitch factor is needed.
How much should a 1,200 sq ft flat roof cost?
About $4,800 to $16,800 installed at the usual $4 to $14 per sq ft. Modified bitumen keeps it nearer $4,800 to $9,600, while PVC can run $6,000 to $19,200. If the old roof has to come off, add $1,200 to $6,000 for tear off and disposal.
How many square feet of roof does a 1,500 sq ft house have?
On a flat roof, the roof area is close to the footprint of the building rather than its floor area. A single storey house with a 1,500 sq ft footprint has about 1,500 sq ft of roof plus any overhang; a two storey house with 1,500 sq ft of floor space may only sit on half that.
How accurate is this flat roof cost calculator?
The measurements are exact for the outline you enter. The price is a planning range built from national installed rates per membrane, tear off, insulation, counted drains, curbs and pipes, and the permit. Local labor rates, access, wet insulation and deck condition only show up once a roofer is on the roof.
What does the waste factor cover?
Seam laps, upturns at the edges and offcuts around curbs, pipes and drains. About 10% suits a simple rectangle; allow 10% to 15% when penetrations and odd angles add cuts. Waste applies to the material you order, not to the installed price per sq ft.
What slope does a flat roof need to drain?
Code calls for a design slope of at least 1/4 inch per foot, about a 2% grade, for built up, modified bitumen, EPDM, TPO and PVC roofs. Coal tar built up is the exception at 1/8 inch per foot. Multiply the run to the drain in feet by 0.25 to get the drop in inches: 7.5 inches over a 30 ft run.
Should I include permit and inspection fees?
Yes. Most building departments charge $150 to $600 for a flat roof replacement permit, and the calculator adds that range unless you enter your own figure. Changing the roof type or adding structural work can bring extra approvals and inspections.
Can this calculator handle multiple layers or a full tear off?
Choose Remove it under Old roof to add tear off and disposal at $1 to $5 per sq ft. Heavier and stacked roofs sit at the top of that range. A building that already carries two roofs has to be stripped to the deck before a new one goes on.
Which membrane is best: EPDM, TPO or PVC?
EPDM is the lowest priced single ply and easy to patch, but its glued seams are the weak point. TPO is white and reflective with welded seams, which helps cooling in hot climates. PVC costs the most and stands up best to grease and chemicals, which suits restaurants. All three last roughly 20 to 30 years when the roof drains properly.
Does the tool account for flashing, skylights and drains?
Yes. Count new drains ($180 to $250 each), skylight or hatch curbs ($250 to $500) and pipe flashings (about $50) in the Price the roof tab. The Measure the roof tab gives the perimeter and the number of 10 ft edge metal pieces. An extra waterproofing layer or coating can be added at your quoted price per sq ft.
Can I export the estimate for a proposal?
Yes. Print the result, keep it as a PDF with every input and line item, or send a link that opens the calculator with the same figures. Roofers can enter their own installed price per sq ft so the printout matches what they plan to charge.
What is the 25% rule in roofing?
Once repairs inside a 12 month window cover over a quarter of the roof, or a quarter of one section, many areas expect the whole roof or that section to be brought up to current code rather than patched again. It does not apply everywhere, so check with your local building department before planning a large repair.
Leads growth and product marketing for ServiceAgent at SaaS Labs, where he has worked in product marketing since 2020. Writes ServiceAgent's coverage of AI call answering, CRM and home services operations.
GAF Master Elite installer with 16 years in residential and commercial roofing. Former operations lead at Beldon Roofing, now reviews ServiceAgent roofing content.
Published by ServiceAgent, the AI front office for home service teams. Estimates only. What a flat roof really costs depends on what is under the old covering, how the roof drains, local labor rates and the warranty you choose. Measure on site and ask for a written, line by line quote before anyone starts.