Modified Bitumen vs Built-Up Roofing (BUR): Which Flat Roof System Holds Up in Chicagoland?
Built-up roofing and modified bitumen are cousins, not rivals. Both are asphalt systems, both work on flat roofs in our climate, and both fail for the same reasons when they are installed badly. Here is the honest comparison from a crew that has torn off and replaced both since 1996.
Start with what they have in common
Modified bitumen and built-up roofing are both asphalt-based, multi-layer flat roof systems. Both rely on redundancy: several layers of waterproofing rather than one, so a single flaw does not become a leak right away. Both are heavy compared to single-ply membranes like TPO, both handle foot traffic well, and both have decades of track record on commercial buildings in Mount Prospect, Schaumburg, Palatine and Arlington Heights, plus plenty of flat-roofed homes, garages and additions in Glenview and along the North Shore. If you are choosing between the two, you are choosing between two versions of the same idea, not between two different philosophies. That matters because the installer and the details (drains, edges, penetrations, flashings) will decide how long the roof lasts far more than the label on the system.
What built-up roofing (BUR) actually is
A built-up roof is the older of the two. The crew alternates layers of hot asphalt (or a cold-applied adhesive) with reinforcing felts, usually three or four plies, then finishes the top with a flood coat of asphalt and a layer of gravel, or with a mineral-surfaced cap sheet. The gravel protects the asphalt from sunlight and gives the roof its familiar tar-and-gravel look. This is the system on many older commercial buildings and on a fair number of 1950s and 1960s flat-roofed homes in the northwest suburbs. Done properly, a BUR is thick, tough and very forgiving of foot traffic. The hot asphalt kettle is also why fewer crews do it today: it is hot, it smells, it needs a fire watch and a place to stage the kettle, and many buildings and neighbors do not want it.
What modified bitumen actually is
Modified bitumen (mod bit) takes the same asphalt and adds polymers to it so it behaves better in the cold and the heat. It comes on rolls, in two families: APP (more plastic-like, usually torch-applied) and SBS (more rubber-like, applied with torch, hot asphalt, cold adhesive, or self-adhered peel-and-stick). A typical mod bit roof is a base sheet plus a granulated cap sheet, sometimes with an extra ply in between. Because it arrives as a factory-made sheet, the thickness is consistent and the crew does not have to build the waterproofing on the roof one mop at a time. That consistency is the main reason mod bit took over most of the work BUR used to get. For a much deeper look at the product family, see our modified bitumen roofing page.
Where BUR still wins
Built-up roofing has real advantages, and we would not want a property manager to think of it as obsolete. The multiple plies make it the more redundant of the two; a four-ply BUR has more waterproofing in it than a two-ply mod bit roof. The gravel surface takes abuse from HVAC technicians, dropped tools and hail better than a granulated cap sheet. On large, simple, open commercial roofs with few penetrations, BUR can be very cost-effective because the material itself is inexpensive. And on a building that already has a sound BUR, a recover or partial repair of that BUR is often the smartest money, which is exactly the kind of thing we will tell you when a replacement is not needed.
Where modified bitumen wins
Mod bit wins on flexibility, which is the whole point of adding polymers to asphalt. Our winters take a flat roof from below zero to bright sun on the same February afternoon, and the roof expands and contracts with it. Plain asphalt gets brittle in that cycle; SBS mod bit stays pliable. Mod bit also wins on installation options. On a residential garage or a small addition where nobody wants an asphalt kettle in the driveway, a self-adhered or cold-applied SBS system goes down with no open flame at all. Mod bit is easier to detail around a cluttered roof with many curbs, pipes and skylights, and it is easier to inspect, because a granulated cap sheet shows you its seams and blisters while a gravel surface hides them. Repairs are simpler too: a mod bit patch is a piece of the same cap sheet; a BUR patch means cutting through gravel and rebuilding plies.
Lifespan in our climate, honestly
Both systems can reach twenty years and beyond when they are installed correctly, kept drained and looked at once or twice a year. Both die early for the same reasons: ponding water that never drains, clogged scuppers, failed flashings at walls and curbs, and nobody looking at the roof until a tenant reports a stain. We inspect a lot of flat roofs in Schaumburg and Elk Grove that would have had ten more years with a $300 to $10,000 repair done at the right time. Our position, on either system, is that maintenance buys more roof life than the choice of material does. That is why we push a maintenance plan on every commercial flat roof we touch, whether we installed it or not.
Residential flat roofs: garages, additions, porches and mid-century homes
This is not just a commercial question. Glenview, Arlington Heights, Mount Prospect and Highland Park all have flat-roofed homes, flat garages, and low-slope additions and porch roofs tucked onto pitched houses. For these we almost always recommend SBS modified bitumen, usually self-adhered or cold-applied, because the sections are small, they sit right next to wood framing and living space, and nobody wants a kettle or a torch against the house. If the existing roof is a BUR that is still sound, we will say so and repair it. If it has been recovered two or three times and is now several inches of layered asphalt, the deck underneath needs to be checked before anything new goes on.
Commercial buildings and property managers
On commercial work, the decision usually comes down to the building, not the brochure. Large, open, low-traffic roofs on industrial buildings in Elk Grove Village or along the Schaumburg corridor: BUR or a hot-applied SBS mod bit both make sense, and the price difference is often small. Retail strip centers and office buildings in Palatine, Mount Prospect or Arlington Heights with many rooftop units and penetrations: mod bit, because the detailing is cleaner and the repairs are cheaper for the life of the roof. Buildings where open flame is prohibited by the owner or insurer: cold-applied or self-adhered mod bit. Buildings where a light-colored, reflective roof is required: neither, look at TPO or a reflective coating over mod bit instead.
What it costs, and what moves the number
For a commercial or flat-roof replacement in our area, most projects land between $25,000 and $75,000. Within that range, the system choice moves the price less than these things do: how many existing layers have to come off and hauled away, whether the deck has wet insulation or rot, how much tapered insulation is needed to fix drainage, how many curbs and penetrations need new flashings, and whether the parapet walls and coping need work. Repairs on either system run $300 to $10,000 depending on scope. We do not price by the square foot on this site because a per-square-foot number without the tear-off, insulation and flashing scope is a guess, and a guess is not a proposal.
Our honest recommendation
If you are replacing a flat roof today, modified bitumen is our default for most residential and commercial buildings in the northwest suburbs and North Shore: SBS, two or three plies, with the application method chosen for the building. Built-up roofing is still a legitimate choice on large, simple commercial roofs where a crew set up for hot work can do it well, and an existing BUR in decent shape should be maintained, not torn off. If someone tells you one of these systems is junk and the other is the only answer, ask them how many of each they have installed. Both have been keeping water out of buildings around here for longer than most roofing companies have existed.
Talk to us
Leaders Roofing has installed, repaired and torn off both built-up and modified bitumen roofs across Cook and Lake Counties since 1996. If you own or manage a flat-roofed building, or a home with a flat section, and you are not sure whether it needs a repair or a replacement, call (708) 847-5418 or book a roof health check. We will tell you which one it is, and we will explain the reasoning in plain English or Polish.