Install Built Up Flat Roof System

If you’re deciding whether to install a built up flat roof system on your Brooklyn building or go with a modern single-ply membrane, here’s what most people miss: many of the longest-lasting flat roofs in Brooklyn-the ones that hit 35, 40, even 50 years with just recoating and minor repairs-are old-school built-up systems with multiple plies of felt and asphalt. These roofs are essentially redundant waterproofing stacks, layer over layer, so a small puncture or lap failure doesn’t immediately mean a leak in your ceiling. Single-ply systems can be excellent, but they’re one layer doing all the work. When you’re standing on a roof that sees HVAC guys, satellite installers, and snow shovels every winter, that difference matters.

At FlatTop Brooklyn, we’re the crew building owners call when they want that multi-layer durability-when they’ve seen too many thin roofs fail at five or ten years and they’re willing to invest in something built to outlast the next decade of ownership. I’ve been laying built up flat roof systems since I was hauling mop carts for my father’s crew in Borough Park, and I still spec them today when the building, the budget, and the situation line up right. This guide walks through exactly what a built-up system is, when it makes sense in Brooklyn, and how we actually install one from deck to surfacing.

What Is a Built Up Flat Roof System (and Why “Built Up”)?

A built-up roof-BUR in the trade-is a flat roof constructed from alternating layers of reinforcement fabric (typically fiberglass or polyester felt) and bitumen (hot asphalt or cold-applied adhesive). You start at the deck, add a base sheet, then mop or adhere additional plies on top, staggering the seams so they don’t align. The final surface is either a flood coat of hot asphalt embedded with gravel, a reflective coating, or a modified bitumen cap sheet. The name “built up” is literal: you build the roof in place, one ply at a time, creating a thick, multilayer waterproof membrane.

Why this method still exists in 2025: Redundancy. If you puncture or split the top ply, you’ve still got three or four more layers below doing their job. Compare that to a single-ply TPO or EPDM roof where one bad seam or puncture can leak immediately. BUR also handles foot traffic and abuse better when properly surfaced-those embedded stones or a tough cap sheet spread impact loads across multiple plies instead of stressing one thin membrane.

In Brooklyn, you’ll still see built up flat roof systems on older walk-ups, industrial buildings, and many pre-war co-ops. Some of those roofs were installed in the 1980s or even earlier and are only now being replaced-not because the concept failed, but because the top surfacing wore down and wasn’t maintained. When we tear off a 30-year-old BUR that’s still mostly dry underneath, that tells you something about the layered approach.

When Does a Built Up System Make Sense for Your Brooklyn Roof?

Not every flat roof should be a built-up system. Here’s the decision framework we use on site visits:

• Heavy traffic or abuse: If your roof sees regular HVAC maintenance, window washers, or tenant access, BUR with aggregate surfacing is very forgiving. You’re not babying a single membrane.
• Complex details and penetrations: Buildings with lots of parapet walls, chimneys, skylights, and shared party walls benefit from the way BUR flashing can be built up in multiple plies at transitions. You’re creating redundant waterproofing at every seam.
• Fire rating requirements: Certain occupancy types and building codes in NYC demand Class A fire ratings. A BUR with gravel surfacing can meet that without additional layers.
• Matching existing systems: If half your roof is already a built-up system and you’re only replacing one section, tying new BUR into old BUR is much cleaner than trying to marry incompatible materials.

On the other hand, you might choose single-ply membranes if:

• You need maximum solar reflectance for energy rebates or cool roof mandates-white TPO or PVC reflects more heat than dark aggregate.
• Your building has very tight access (narrow stairs, no crane access) and hauling kettles or dozens of rolls of felt isn’t practical.
• You’re in a high-sensitivity situation-restaurant tenants below, medical offices-where hot asphalt odor or open flame is a dealbreaker.
• You want the fastest possible install with minimal disruption; single-ply can go down in days where BUR takes a week or more for plies to be built and surfaced.

On a 4-story mixed-use in Sunset Park last year, the owner chose BUR because the roof was the building’s shared social space-tenants went up to hang laundry and kids played up there in summer. A thin membrane would’ve been shredded. We installed a four-ply built up flat roof system with gravel surfacing, and two years later it still looks new despite constant foot traffic.

The Layer Stack: What You’re Actually Getting in a Built Up Flat Roof

Understanding the anatomy helps you read proposals and catch where contractors might be cutting corners. Here’s the full stack from bottom to top:

Each of those layers has a job. The most common mistake we see: owners or low-bid contractors skipping the cover board or going with just two plies instead of three or four to save money up front. That’s the difference between a 20-year roof and a 35-year roof.

Hot-Applied vs. Cold-Applied: How Bitumen Gets Between the Plies

When you hear “built up flat roof system,” most people picture a smoking kettle of hot asphalt. That’s hot-applied BUR, and it’s still very common on industrial and commercial buildings. We bring a kettle on site, heat asphalt to around 400-450°F, and mop it onto each felt ply as we roll it out. The hot asphalt flows into the felt fibers and creates an extremely strong, waterproof bond between layers.

Advantages of hot-applied:

• Immediate, aggressive bond-no cure time, no waiting for adhesive to set.
• Time-tested; this is how most of Brooklyn’s longest-lasting roofs were built.
• Works in cooler weather better than some cold adhesives.

Challenges:

• Odor and smoke-can be a problem in dense residential blocks or near schools.
• Fire safety protocols-open flame, hot materials, requires experienced crew and proper site setup.
• Logistics-you need space for the kettle, truck access, and a way to haul hot buckets to the roof.

Cold-applied BUR uses proprietary mastics or adhesives instead of hot asphalt. You spread the cold adhesive, roll the felt into it, and let it cure. No kettles, no smoke, much lower fire risk. We use cold systems on schools, healthcare buildings, and tight residential sites where hot asphalt just isn’t practical or allowed.

On a pre-war co-op in Crown Heights with tenants on the top floor and no elevator, we went cold-applied. The building’s insurance and the board’s concerns about odor made hot asphalt a non-starter. The cold system added a day to the schedule for proper cure time, but the final roof performs just as well-and the residents didn’t smell tar for a week.

Modified bitumen hybrids are also popular in Brooklyn: you build the base plies in traditional BUR style (hot or cold), then cap the whole assembly with a tough modified bitumen sheet-either torch-applied, cold-adhered, or self-adhered. This gives you the redundancy of multiple plies underneath and a clean, durable cap sheet on top that’s easier to detail around penetrations and easier to repair later if needed.

Installation Sequence: Deck to Surfacing

Here’s how we actually install a built up flat roof system, step by step, on a typical Brooklyn building:

1. Tear-Off and Deck Inspection. We remove all existing roofing down to the structural deck. On older buildings, that might mean pulling off two or three generations of roof that were installed over each other. Once the deck is exposed, we inspect every board or panel-replace any rotted wood, rusted metal deck sections, or spalled concrete. If the structure can’t hold the new roof’s weight (BUR is heavy), we bring in an engineer to reinforce or redesign the assembly.

2. Slope and Drainage Corrections. Flat roofs aren’t truly flat-they need minimum 1/4″ per foot slope to drains. If the existing deck is dead flat or sagging, we’ll add tapered insulation or re-frame sections to create positive slope. Ponding water is the enemy of any roof, and with BUR’s multiple plies, you don’t want standing water sitting on seams for weeks at a time.

3. Vapor Barrier and Insulation. Install the vapor/air control layer where the building’s energy design requires it, then lay rigid insulation boards across the deck. We secure them with mechanical fasteners or adhesive depending on the deck type and wind load calcs. Then a cover board goes over the insulation-this protects the soft insulation from the hot mop operations and gives us a smooth, stable surface to start the waterproofing plies.

4. Base Sheet. The first felt ply-the base sheet-is rolled out and bonded to the cover board in hot asphalt or cold adhesive. We’re establishing clean, straight lines at parapets, drains, and edges here, because every ply that follows will build off this one. Laps are sealed fully; no fishmouths or air pockets.

5. Interply Felts. Now we add the redundant plies-typically two to four additional layers of fiberglass felt. Each one is mopped in hot asphalt (or cold adhesive) and rolled out with laps staggered from the layer below. We’re building a thick, waterproof sandwich where even if one seam fails, the layers above and below keep the roof dry. On a standard commercial job, we’ll do three or four plies total; on high-traffic or long-warranty projects, we might go to five.

6. Flashing. Parapets, walls, curbs, drains, and penetrations get their own built-up flashing sequences-multiple plies of felt turned up the vertical surface and embedded in bitumen, often with a metal counterflashing or reglet at the top. In Brooklyn, where most buildings have masonry parapets and shared party walls, the flashing details are where leaks start if you rush or skip plies. We don’t.

7. Surfacing. The final layer depends on the building’s needs:

• Gravel surfacing: A flood coat of hot asphalt, then 400-600 lbs per square of smooth river stone or slag. This is the classic look, excellent for foot traffic and fire rating, but dark and heavy.
• Reflective coating: A white or aluminum coating rolled or sprayed over the top ply. Lighter color, helps with cooling, but needs recoating every 5-10 years.
• Modified cap sheet: A granular-surfaced cap sheet torch-applied or adhered as the final ply. Tough, easy to repair, available in light colors for reflectivity.

On a 6-story industrial building in Bushwick, we went with gravel surfacing because the owner wanted maximum durability and the roof was going to see heavy equipment deliveries via crane. The aggregate surface spreads those loads and protects the plies underneath. On a residential building in Park Slope, we used a white reflective coating to meet the building’s energy goals and keep top-floor units cooler in summer.

Built Up Flat Roof Systems vs. Modern Single-Ply: The Honest Comparison

Owners ask us all the time: why not just go with TPO or EPDM? Here’s the real answer, no sales pitch:

BUR wins on:

• Impact and puncture resistance. Multiple plies and aggregate surfacing can take a beating. You can drop tools, drag equipment, shovel snow aggressively-the roof forgives it.
• Redundancy. One bad seam in a single-ply roof can mean a leak. One bad seam in a four-ply BUR is usually still protected by the layers above and below.
• Longevity on high-traffic roofs. We see 30+ year BUR installations regularly; single-ply on a heavily used roof often shows wear and tear at 15-20 years.

Single-ply wins on:

• Speed and simplicity. A single-ply roof can go down in 2-3 days. BUR takes a week or more because each ply needs time and care.
• Weight. TPO or EPDM with minimal ballast is much lighter than a four-ply BUR with gravel. Matters on older buildings with marginal structure.
• Reflectivity and energy performance. White TPO roofs reflect 70-80% of solar heat right out of the box. BUR with dark aggregate absorbs heat unless you add a reflective coating.
• Cleanliness and low-odor install. No kettles, no smoke, easier to work around sensitive occupants.

The truth? Both systems work when installed correctly by skilled crews. We install plenty of single-ply roofs at FlatTop Brooklyn. But when a building owner wants maximum durability, expects the roof to last 30+ years, and isn’t worried about a few extra days of construction or the look of a gravel surface, we’re going to recommend a built up flat roof system every time.

Common Installation Mistakes (and How to Avoid Them)

Most built up flat roof failures aren’t because the system is outdated-they’re because someone cut corners or didn’t understand the details. Here’s what we see on tear-offs of failed BUR projects:

Installing over wet or damaged decks. If the old roof leaked and the deck is rotted or moldy, you can’t just cover it up with new plies. The rot continues underneath, and the new roof delaminates or sags within a few years. Always tear off to the deck, dry it out, and repair or replace bad sections before starting the new system.

Accepting dead-flat or negative-slope areas. Ponding water on a flat roof is slow death. Even BUR, with all its redundancy, will fail at seams that sit underwater for weeks at a time. Use tapered insulation or re-framing to create positive slope to every drain. No excuses.

Rushing the plies and leaving voids. Each felt ply has to be fully embedded in hot asphalt or adhesive-no dry spots, no wrinkles, no air pockets. When crews rush to finish before weather or at the end of the day, you get fishmouths (unsealed edges) and voids that turn into leaks. Good BUR work is methodical.

Skimping on flashing plies. The field of the roof-the big, flat center area-is usually fine. Leaks start at parapets, walls, drains, and curbs where the built up flat roof system transitions from horizontal to vertical. You need the same number of plies (or more) turned up those walls and properly terminated. Don’t stop at two plies when the field has four.

Ignoring structural load. BUR with insulation, four plies, and 500 lbs/square of gravel can add serious dead load to an old building. If the framing was marginal before, adding that weight without an engineer’s review is asking for sagging or even structural failure. Always run the numbers.

How Long Will Your Built Up Flat Roof Last?

With proper installation and reasonable maintenance, a built up flat roof system in Brooklyn typically lasts 25-35 years. The high end of that range comes when you:

• Design and maintain positive drainage so water doesn’t pond.
• Recoat or re-surface the roof every 10-15 years to protect the top ply from UV breakdown.
• Fix small punctures or flashing issues promptly instead of letting them become bigger leaks.
• Keep drains and scuppers clear so water exits the roof quickly after rain or snow.

The shortest-lived BUR installations we tear off are the ones that were installed flat (no slope), never recoated, and left to bake in the sun with clogged drains for 20 years. Even then, many still make it to 20-25 years before catastrophic failure. That says something about the system’s inherent durability.

Compare that to low-bid single-ply roofs we’ve seen fail at 8-10 years because one seam opened up or the membrane was punctured and never caught. The difference is that redundancy-when BUR starts to wear, it warns you slowly. When single-ply fails, it often fails fast.

Is a Built Up Flat Roof System Right for Your Brooklyn Building?

If you’re managing a building in Brooklyn and considering a roof replacement, here’s my straight answer: a built up flat roof system makes sense when you value proven, long-term durability over speed and aesthetics. It makes sense when your roof sees regular use or abuse. It makes sense when you’re planning to own or manage the building for the next 20-30 years and you want a roof that matches that timeline.

It’s probably not the right choice if you need the roof done in three days, if your structure can’t handle the weight, or if you’re chasing maximum energy rebates that require white reflective surfaces (though reflective coatings over BUR can help there).

At FlatTop Brooklyn, we’ll walk your roof with you, talk through your building’s actual needs-traffic, access, budget, fire rating, future plans-and give you an honest recommendation. Sometimes that’s BUR. Sometimes it’s TPO or a hybrid system. We’re roofers, not salespeople, and we’ve been doing this long enough to know that the best roof is the one that fits the building, not the one that fits a sales pitch.

Want an assessment of your flat roof and a real conversation about whether built-up is the right system? Call FlatTop Brooklyn or fill out our contact form. We’ll schedule a site visit, measure your roof, check your existing structure and drainage, and give you a detailed proposal that explains exactly what you’re getting-ply by ply, layer by layer. No surprises, no shortcuts, just a built up flat roof system designed to last as long as you own the building.