Modified bitumen roofing: what it is, how it goes on, and what it costs
Modified bitumen is asphalt roofing made in rolls, reinforced and modified with polymers, for flat and low-slope roofs. It goes on hot-mopped, torched, set in cold adhesive or self-adhered, often as a cap sheet over a base sheet. The code calls for at least a 1/4:12 slope. A 600 sq ft garage roof typically comes to $4,250.
Anthony Hicks30+ years in construction. Updated November 5, 2026.
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What it is
Modified bitumen (“mod bit”) is asphalt with polymers added, made into reinforced rolls for flat and low-slope roofs. It can go on in two layers, a base sheet under a cap sheet with granules on top; the code allows a base sheet to ASTM D1970, ASTM D4601 or the International Building Code under a modified bitumen cap sheet (IRC R905.11.2.1).
How it goes on
| Method | How the sheet is bonded |
|---|---|
| Hot mopped | Set in hot asphalt mopped on the roof |
| Torch applied | The back of the sheet melted with a torch and pressed down |
| Cold adhesive | Set in adhesive applied cold |
| Self-adhering | A sticky back under a release film |
Whichever method, the roof goes on by the code and the maker’s instructions (IRC R905.11.3).
What the code says
- Slope: a design slope of at least 1/4 inch per foot (2%) for drainage (IRC R905.11.1).
- Materials: the membrane meets ASTM D6162, D6163, D6164, D6222, D6223 or D6298, with the adhesives, cements, coatings and primers to their own standards (Table R905.11.2).
What a modified bitumen roof costs
Priced the way a contractor’s estimate is priced: modified bitumen roofing with the old roof torn off, 10% over the roof for laps and turn-ups at the edges.
| Roof | Range | Typical |
|---|---|---|
| A porch or small addition, 300 sq ft | $1,850 to $2,600 | $2,150 |
| A two-car garage, 600 sq ft | $3,750 to $5,200 | $4,250 |
| A flat-roofed house, 1,500 sq ft | $9,300 to $13,000 | $10,600 |
Totals include the contractor’s overhead and profit (20%) and the state’s average sales tax. Each range runs from the lower-priced parts of the country to the higher-priced ones. Edge metal, insulation, new decking and drains are extra.
Modified bitumen, EPDM or TPO
On the same roofs, priced the same way:
| Roof | Modified bitumen | EPDM, 60 mil | TPO, 60 mil |
|---|---|---|---|
| A two-car garage, 600 sq ft | $3,750 to $5,200 | $5,500 to $7,600 | $4,450 to $5,900 |
The EPDM guide and the TPO guide cover the other two.
Common questions
How much does modified bitumen cost per square foot?
About $6.25 to $8.75 a sq ft of roof, with the old roof torn off, overhead, profit and sales tax.
Is modified bitumen the same as roll roofing?
No. Both come in rolls, but modified bitumen is a polymer-modified membrane laid in plies, while mineral-surfaced roll roofing is a single asphalt sheet.
What’s the lowest pitch for modified bitumen?
1/4 inch per foot, a 2% slope (IRC R905.11.1).