Modified Bitumen Roof Cost Guide (Updated 2024)

By Jack Gray, Roof Online Editor • Published June 30, 2024

To learn more about modified bitumen roof systems, please see our Introduction to Modified Bitumen Roofing.

modified bitumen roof with tan granule surface.
A modified bitumen roof on a bank that has a tan granule surface

Introduction

As professional roof consultants, we work out roof replacement budget projections for our clients, review roofing bids, and award roof installation contracts to roofing contractors. This means we have to know current material and labor costs for all kinds of roofs, including modified bitumen roof systems. We have extensive first-hand experience with real-world roof pricing and all the factors that go into it.

This article should give you a pretty accurate idea of what the typical total installed cost will be for various types of modified bitumen roofs in 2024.

You should be aware that the cost of any individual modified bitumen roof will depend on several variables, including roof size, roof complexity, local labor costs, the type of modified bitumen being used, the number of plies installed, and the local insulation R-value requirements. The insulation requirements on low-slope roofs can be a major cost factor, by the way.

If roof recovery (putting the new roof over the old roof) is an option (and it may not be, for a number of reasons, not just code restrictions) this can significantly reduce the cost of a roofing project by taking insulation out of the equation.

On average, the cost of a new modified bitumen roof in 2024 ranges from around $9 per square foot for a 2-ply system with no added insulation to around $20 per square foot for a 3-ply system that includes enough above-deck insulation to meet an R-30 energy code R-value requirement.

What is Modified Bitumen Roofing?

Modified bitumen roofing is a low-slope or “flat” roof system in which sheets (that come in rolls) of modified bitumen are joined together to form the waterproof surface of the roof (the roof membrane). These sheets are made of a reinforcement fabric coated with thick layers of asphalt mixed with synthetic rubber or plastic. Modified bitumen roof membranes are typically installed using two or three layers, or plies.

A modified bitumen roof membrane is tough; it’s far more puncture-resistant than single-ply membranes such as EPDM or TPO. It also tends to be more expensive, but it’s usually the best roofing solution for smaller apartment buildings where tenants have access to the roof and you need something that can stand up to the extra foot traffic without springing a leak.

Modified bitumen is “modified” when the synthetic rubber or plastic is mixed into the hot asphalt (the “bitumen”) during the manufacturing process. Modified bitumen is also known as “polymer-modified bitumen” or “rubberized asphalt”. It’s commonly called “mod bit” in the roofing industry.

Modified bitumen roofing is typically produced in rolls that are 39 inches (1 meter) wide and between 25 feet and 33 feet (10 meters) long. These rolls can be torched down, installed in cold process adhesive, or come in self-adhering rolls (peel-and-stick). SBS-modified bitumen can also be installed using hot-mopped asphalt.

A proper modified bitumen roof system will always have at least two plies of modified bitumen sheets installed. One of the big attractions of a modified bitumen roof is the redundancy, strength, and toughness produced by the multi-layer installation. If you see strangely low cost estimates for a modified bitumen roof on the internet, you can usually assume whoever wrote the article doesn’t know much about roofing and simply calculated the cost using only one layer of modified bitumen., no cover board, and no insulation. Which is not what you want.

A typical modified bitumen roof system will include several inches of board insulation (such as polyisocyanurate) as required by the local energy code, a layer of cover board made of a harder, fire-resistant material (typically gypsum board with a special facer), a modified bitumen base sheet, and a modified bitumen cap sheet. The surface of the cap sheet will usually have an embedded layer of mineral granules like asphalt shingles do, although it may be smooth-surfaced and then coated with a protective roof coating.

Since modified bitumen is an asphalt-based roofing product, it’s compatible with other asphalt-based roofing products, such as roof cement, asphalt-based roof coatings, etc. Because of all the the easy-to-use asphalt roofing products available, this means a lot of roof maintenance tasks and minor repairs can be successfully performed by building maintenance technicians without calling a roofer.

Types of Modified Bitumen

Modified bitumen is referred to by the type of polymer added to the asphalt. Almost all modified bitumen roofing is either APP-modified or SBS-modified.

As far as the end-user (you) should be concerned, the most significant difference between the two is that APP-modified bitumen is preferred in warmer climates and SBS-modified bitumen is preferred in colder climates.

There aren’t any code restrictions that require the use of one type and not the other simply because of the local climate. Both types of mod bit can be found wherever mod bit is commonly used as a roofing material.

APP-Modified Bitumen

APP (atactic polypropylene) is a plastic mainly used for the purpose of modifying roofing asphalt. APP mod bit is less flexible or elastomeric than SBS-modified, but it’s stronger and more resistant to UV radiation and high temperatures. APP mod bit should not be installed in hot asphalt (the hot-mopping technique); it doesn’t melt and bond properly when applied this way.

SBS-Modified Bitumen

SBS (styrene butadiene styrene) is a synthetic rubber with many uses. Modifying roofing asphalt is just one of the things it’s used for. SBS mod bit is more elastic than APP-modified and does not become as brittle in cold temperatures. SBS mod bit can be hot-mopped (installed in a layer of regular hot asphalt).

Modified Bitumen Roof Cost in 2024

These cost estimates should be fairly accurate; we try to keep them up-to-date using current or recent material prices and labor data and we verify their general accuracy by checking them against actual contract prices of real roofing projects.

The costs listed below are national averages, and costs can vary quite a bit according to location.

To get a more accurate idea of what a new modified bitumen roof will cost in your area, see the local cost factor table in our article “Relative Construction Costs by U.S State” and apply your state multiplier to the national average cost provided on this page.

Note that 3-ply modified bitumen roofs will typically last 25% longer than 2-ply modified bitumen roofs, everything else being equal. See our article on roof life expectancy to compare the lifespans of various roof systems.

Table 1: Modified Bitumen Roof Cost per Square Foot

The cost estimates given in this table cover the installation of various types of modified bitumen roof systems. All of them include the cost of the modified bitumen membrane and related materials, the cover board installed under the membrane, and the labor required for the installation.

1. The second column shows current price estimates for the average installed cost per square foot with no new insulation installed. This type of roof is appropriate when a new modified bitumen roof is installed on top of an existing roof and no insulation R-value increase is required (a roof recover). It can also be used on roof areas where insulation is installed below the roof deck, or on roofs over unheated interior spaces, such as sheds or some garages..

2. The third and fourth columns provide price estimates for full roof systems that include insulation that meets the code-required total system R-value. R-25 is the typical R-value requirement in the southern half of the U.S., and R-30 is common in the northern half of the country. Some places will have different roof R-value energy code requirements, so always check your local code.

Our calculations use current prices for polyisocyanurate insulation, although a modified bitumen roof system may use other types of insulation.

The estimates include the removal of the existing roof in the third and fourth columns and we have added $1.00 to $2.00 per square foot for that.

Average Cost of a Modified Bitumen Roof System
per Square Foot by System Type
Type of Modified Bitumen Roof System Average Installed Cost
Per Square Foot
No Insulation
Average Installed Cost
Per Square Foot
Insulation R-25
Average Installed Cost
Per Square Foot
Insulation R-30
APP-Modified Bitumen Roof
2 Plies
$9.13 $15.63 $16.73
APP-Modified Bitumen Roof
3 Plies
$11.88 $18.38 $19.48
SBS-Modified Bitumen Roof
2 Plies
$9.63 $16.13 $17.23
SBS-Modified Bitumen Roof
3 Plies
$12.63 $19.13 $20.23

About the Author

Jack Gray is a principal roof consultant and vice president at the Moriarty Corporation, an award-winning building enclosure consultant firm founded in 1967. He is also the editor of the Roof Online website.

Mr. Gray has worked in the roofing industry for over 25 years, with training and practical experience in roof installation, roof inspection, roof safety, roof condition assessment, construction estimating, roof design & specification, quality assurance, roof maintenance & repair, and roof asset management.

He was awarded the Registered Roof Observer (RRO) professional credential in 2009.

He also served as an infantry paratrooper in the 82nd Airborne Division and has a B.A. from Cornell University. Read full bio.


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