Install Underlayment on Flat Roof

On a flat roof in Brooklyn, the membrane you see is often just the last line of defense-most leaks start because the underlayment underneath was wrong, missing, or badly installed. I watched a “new” roof on a Bed-Stuy three-family fail after three winters because the contractor skipped a proper base sheet to save a day on the schedule. Water migrated under the membrane, blistered the cap layer, and by year four the owner had ceiling stains in all the top-floor units. The visible membrane looked perfect, but the assembly underneath was a time bomb.

I’m Brendan Kelleher. I’ve spent 21 years installing and fixing flat roofs across Brooklyn, and I run training crews for FlatTop Brooklyn now. What most people don’t realize is that flat roof underlayment installation isn’t about slapping down extra paper-it’s about creating a full moisture, vapor, and movement management system between the deck and the final membrane. Get that layer wrong, and no amount of expensive top membrane will save you.

What “Underlayment” Means on a Flat Roof (It’s Not the Same as Shingles)

If you’ve heard the word “underlayment” on pitched roofs, forget that definition. On a shingle roof, underlayment usually means felt or synthetic sheet between the deck and shingles-a single role, one material. On flat roofs in Brooklyn, underlayment can mean base sheets, vapor barriers, slip sheets, cover boards, or secondary moisture layers, depending on the system. Each one does a different job: managing water from above, blocking vapor from below, protecting insulation, or stabilizing the membrane.

This matters because a flat roof is really a sandwich of materials, and each layer affects the others. If you install a vapor-impermeable base sheet on the wrong side of the insulation, you trap moisture. If you skip a cover board in a high-traffic area, foot traffic telegraphs down and damages the membrane over time. Underlayment is part of a full assembly, not just extra insurance you toss under the visible roof.

Why does Brooklyn care more than most places? Freeze-thaw cycles in winter stress every joint and seam. Summer humidity drives moisture upward from kitchens, bathrooms, and laundry rooms-especially in older multifamily buildings with poor ventilation. Steam heat, tight construction, and dense occupancy all push water vapor toward your roof deck. A good underlayment layer controls moisture from above and below, protects the insulation, and can extend membrane life by ten years. A bad or missing underlayment leads to trapped condensation, blistering, chronic leaks, and rotted decks-problems that show up years after the roofer is gone.

Know Your Starting Point: Existing Flat Roof Type and Deck Condition

Before you touch underlayment, categorize what you’re working with. Most Brooklyn flat roofs fall into three deck types:

• Wood joists with plank or plywood decks on brownstones, rowhouses, and older small buildings-common in Park Slope, Crown Heights, and Ditmas Park.
• Concrete slabs over masonry or steel frames on larger walk-ups, mixed-use buildings, and post-war multifamily construction.
• Metal deck with insulation and membrane on light commercial, warehouse conversions, and some modern extensions.

Next, identify the existing membrane: modified bitumen, built-up roofing (BUR), EPDM, TPO, PVC, coatings, or pavers. Also note where the insulation sits-above the deck (typical in newer installs) or below it (older buildings sometimes insulated from the interior). That changes your dew point and vapor drive, which directly impacts underlayment needs.

If you’re not certain, have a roofer cut a small test opening away from drains and edges. I’ve found saturated insulation, rusted metal deck, and three layers of old felt in places the owner thought was “one roof.” Underlayment only works on a solid, dry substrate. Rotten wood, spalled concrete, or rusted metal must be repaired or replaced first. Chronic ponding areas often hide saturated insulation or decayed deck underneath-fix the cause, not just the surface.

Flat Roof Layer Cake: Where Underlayment Fits in Different Assemblies

To understand what you’re installing, you need to see the whole stack. Here are three typical assemblies on Brooklyn flat roofs, each with underlayment playing a different role:

Assembly A: Conventional warm roof (insulation above deck)

• Deck (wood, concrete, or metal)
• Vapor retarder or air barrier (sometimes called underlayment in specs)
• Rigid insulation boards (polyiso, EPS, XPS)
• Cover board or base sheet (often the “underlayment” a roofer talks about)
• Main waterproofing membrane (mod bit, EPDM, TPO/PVC)

Assembly B: Inverted/PMR roof over extensions and terraces

• Deck and structural slab
• Main waterproofing membrane applied directly to deck
• Protection layer or slip sheet (acts like underlayment for pavers or ballast)
• Insulation and filter fabric above the membrane
• Pavers, decking, or ballast as the walking surface

Assembly C: Recover/overlay on old flat roof

• Existing small-slope membrane or BUR (kept in place after inspection)
• Separation board or base sheet as underlayment between old and new system
• New insulation (if adding) and new membrane, flashed to code
• Note: NYC DOB restricts when a second layer is allowed-check before assuming recover is an option

In practice, “underlayment” can be a peel-and-stick base sheet under mod bit, a high-density cover board over insulation, or a vapor barrier directly on the deck. The rest of this article focuses on the practical install steps for common mod bit and single-ply flat roof underlayment installations in Brooklyn-the two systems I see most on rowhouses and small multifamily buildings.

Choosing the Right Underlayment for Your Flat Roof System

Start with the manufacturer’s system spec. Every major membrane brand-GAF, Firestone, Carlisle, Johns Manville-publishes compatible base sheets, vapor barriers, and cover boards for their membranes. Ignoring those specs can void warranties and create condensation problems. I’ve seen contractors mix a GAF base sheet with a Firestone cap just because they had leftover stock. That’s not a system-it’s a gamble.

Common flat roof underlayment materials include:

Brooklyn-specific factors matter. Older buildings with high interior humidity-think steam radiators, small bathrooms with poor exhaust, and shared laundry rooms-benefit from robust vapor control at the roof deck. Roofs that double as terraces need underlayment that can handle pedestals, planters, and frequent foot traffic. Harsh UV and temperature swings (I’ve measured 160°F surface temps in July on black mod bit) push you toward underlayment that tolerates exposure during construction without degrading.

Prep Work: Before You Roll Out a Single Sheet

Surface prep makes or breaks underlayment performance. Sweep and remove all debris, loose aggregate, nails, and sharp objects-underlayment telegraphs every bump underneath. The deck or existing roof must be bone dry. Installing over damp substrates traps moisture and causes blisters later, sometimes months after the job. In Brooklyn humidity, that means watching the weather. I won’t lay self-adhered base sheets after a rainstorm until I’ve confirmed the deck is dry, even if it means coming back the next day.

Check slope and ponding areas before you start. Underlayment is not a fix for structural ponding-depressions should be addressed with deck repair or tapered insulation first. Mark low spots, drains, scuppers, and transitions to parapets or walls. Laps should be oriented to shed water downhill, not trap it.

Safety and access on Brooklyn roofs require planning. Establish safe ladder access, fall protection around edges, and clear material staging areas. In tight rowhouse corridors and walk-up buildings, figure out how underlayment rolls will get to the roof without damage or injury. I’ve carried 75-pound rolls up three flights of narrow stairs more times than I can count-it’s not glamorous, but it’s part of the job in Brooklyn.

Installation Sequence for Mod Bit-Style Flat Roof Underlayment

Here’s the layer-by-layer sequence for a typical modified bitumen flat roof with a self-adhered base sheet over wood or concrete deck:

1. Prime the deck (if required)
Apply asphalt primer or manufacturer-specified primer to concrete or aged surfaces if the spec calls for it. Let it cure properly-don’t set underlayment into tacky primer unless instructions explicitly say so. On porous concrete, primer helps adhesion. On wood, it can seal knots and resin. Skip it only if your base sheet is approved for unprimed application.

2. Snap lines and plan sheet layout
Plan roll direction to minimize seams against water flow and avoid short, awkward starter pieces. Snap chalk lines to keep rolls straight. On long Brooklyn rowhouse roofs, visual wandering shows up later-chalk lines keep you honest. Start at the low point or drain so successive courses lap “shingle style” uphill.

3. Install the first course at the low point
For self-adhered base sheets, peel the release film gradually and press from the center outward to avoid wrinkles and fishmouths. Work with a helper-one person unrolls and peels, the other presses and smooths. For mechanically fastened base sheets, follow the nailing pattern: edge spacing, in-field spacing, and approved fasteners (usually cap nails or plates). Don’t overdrive fasteners into wood or you’ll create dimples that collect water.

4. Lap, seal, and detail seams
Maintain required side and end lap widths-typically 6 inches side, 6 inches end for base sheets, but check your spec. Stagger end laps so they don’t line up in one row (that creates a weak seam line). Use manufacturer-approved mastics or heat to seal laps where required, especially in cold or windy weather. Pay extra attention to seams near drains, scuppers, and parapet bases-those are Brooklyn leak hot-spots. I’ve pulled back membranes years later and found open seams right at the drain because someone rushed that corner.

5. Turn up walls and curbs as needed
Extend underlayment up parapets, hatch curbs, or penetrations as specified, creating a continuous base for final flashings. Cut neat inside and outside corners-avoid over-cutting into the deck. Reinforce corners per spec, usually with extra patches or strips. A well-detailed corner at the parapet is worth ten minutes of careful work now and saves you from a callback later.

Base sheets can serve as temporary waterproofing, but only for a limited time and under specific conditions. In Brooklyn weather, don’t leave underlayment uncovered longer than the manufacturer allows-UV and foot traffic damage it quickly. I’ve seen base sheets exposed for two weeks turn brittle and crack under the next rain.

Underlayment in Single-Ply Systems (EPDM, TPO, PVC)

Single-ply systems use different logic. In most single-ply assemblies, a cover board over insulation acts as the functional underlayment beneath the membrane. Gypsum or high-density polyiso cover boards provide a firm, smooth surface for adhering or mechanically attaching the membrane and improve impact resistance, fire rating, and hail performance.

Basic cover board installation over insulation:

1. Stagger joints relative to insulation
Lay boards so seams do not align with insulation seams, reducing thermal bridging and weak points. Keep joints tight, but don’t force boards together so they buckle when temperatures change. In Brooklyn’s 100°F summer-to-winter swings, that matters.

2. Fasten or adhere as specified
Use the fastener and plate pattern called for in the wind uplift calculations for your building. Brooklyn doesn’t have hurricane wind loads, but DOB still requires proper fastening. Where fully adhered, use compatible adhesives and watch manufacturer temperature and humidity limits-adhesive performance drops outside the application window.

3. Treat edges and penetrations carefully
Cut boards cleanly around curbs and pipes, leaving proper gaps where slip or movement is required. Support edges near drains and corners so they don’t crack under foot traffic before the membrane goes down.

In recover work or over certain substrates, a thin separation sheet may be installed under the single-ply membrane to prevent chemical incompatibility or sticking. These are often loose-laid and must be kept wrinkle-free. They’re delicate, so staging and foot-traffic control matter during install.

Moisture and Vapor: Getting the “Invisible” Layer Right

This is the most misunderstood part of flat roof underlayment installation, and it’s especially important in dense, humid Brooklyn buildings. A vapor barrier is a low-permeability layer installed on the warm-in-winter side of the assembly (usually directly on the deck under insulation) to stop interior moisture from migrating up into the roof and condensing in the cold layers.

When do you need a true vapor barrier? Buildings with high interior moisture almost always do: restaurants, commercial kitchens, laundromats, indoor pools. Many multifamily Brooklyn buildings with steam heat and poorly vented bathrooms also benefit from robust vapor control at the roof. I’ve opened roofs on Prospect Heights three-families and found insulation soaked through, not from leaks but from years of upward moisture drive.

Basic installation logic: vapor barriers go on the warm side, seams must be taped or sealed (random overlaps don’t stop vapor migration), and penetrations through the barrier must be detailed as carefully as roof penetrations. Moisture will find the path of least resistance.

Avoid trapped moisture by not placing multiple low-perm layers on both sides of insulation without an engineer’s or building scientist’s input. Trapped moisture can rot wood decks, corrode metal, and cause blistering in membranes-often years after a seemingly “good” install. I don’t design vapor profiles; I follow the manufacturer’s system or call in a consultant on complex jobs. That’s not a weakness-it’s professionalism.

Brooklyn-Specific Constraints: Permits, Recover Limits, and Tight Sites

NYC DOB limits how many roof layers you can have. Often only one recover is allowed, and only if the existing roof is sound and the structure can handle the additional weight. Adding underlayment over a failing roof doesn’t “reset the clock”-structural and fire code issues still apply. I’ve walked jobs where the owner wanted to save money with a recover, but the old roof had three layers already and DOB would never sign off.

Logistics on narrow Brooklyn roofs are harder than most contractors admit. Material delivery, staging, and debris removal are a puzzle on rowhouse blocks with no side yard access. Underlayment installation must be sequenced so areas are made watertight before afternoon storms and so access paths remain safe. I’ve staged rolls on fire escapes, hauled them through interior stairwells, and coordinated with neighbors to use shared access-all part of working in Brooklyn.

Working over shared party walls requires care in tying new underlayment into neighbors’ older roofing without creating weak links. In landmarked districts, insulation thickness and parapet detailing around the underlayment/membrane stack may face extra scrutiny from Landmarks. Plan for it, don’t fight it.

Quality Checks: How to Tell if Your Underlayment Was Installed Properly

Even if you’re not doing the work yourself, you can spot good underlayment installation:

1. Straight, even courses and laps: Rolls run parallel with consistent side laps, not wavy or wandering toward parapets.
2. Tight, wrinkle-free adhesion: No big wrinkles, fishmouths, or bubbles. Minor wrinkles should be flattened or cut and patched per manufacturer rules.
3. Clean, reinforced corners and transitions: Extra pieces at inside/outside corners, drains, and scuppers-not just one layer bent sharply.
4. No exposed fasteners in critical water paths: Where fasteners are allowed, they’re properly driven and covered by overlaps, not left exposed near low points.
5. Dry substrate and no obvious trapped moisture: No squishy spots underfoot or visible moisture bleeding from seams during installation.

Ask your roofer which specific underlayment products were used and request data sheets. Take progress photos of layers before the top membrane goes on-they’re invaluable if problems emerge later. I hand clients a photo set at the end of every job showing deck condition, underlayment, insulation, and membrane layers. It’s documentation and education rolled into one.

DIY vs. Pro: Where Flat Roof Underlayment Crosses the Line

Very small outbuildings or sheds not tied into the main building envelope might allow basic underlayment work by skilled DIYers, but even then compatibility with the chosen membrane and drainage patterns must be respected. Most Brooklyn flat roof underlayment should be professional work. Structural questions, vapor control, and tie-ins at parapets or party walls are not weekend-project issues.

Warranty-backed systems require certified installers in many cases, especially on multifamily or commercial roofs. Mistakes on shared or stacked buildings can damage neighbors’ units and escalate quickly into legal and DOB problems. Use this knowledge to ask sharper questions, review proposals, and understand photos-not to cut corners on professional labor where it counts.

Working With a Brooklyn Flat Roof Contractor on Underlayment

When you’re getting estimates, ask these questions:

• Which underlayment, vapor barrier, and cover board products will you use on my roof, and why?
• How will you handle existing ponding areas, wet insulation, or failed sections before installing new layers?
• Are you installing a manufacturer-approved system, and will I receive a material and/or labor warranty?

Your written scope of work should include a clear description of deck prep, moisture checks, and where repairs or replacement will occur. It should list every layer: primer (if any), vapor barrier, insulation, underlayment or cover board, and membrane. Details on how parapets, drains, skylights, and hatches will be integrated into the new underlayment and membrane matter.

A Brooklyn-based specialist like FlatTop Brooklyn understands common failure patterns in brownstones, walk-ups, and converted warehouses. We’re familiar with DOB expectations, typical landlord-tenant concerns, and logistical constraints on busy Brooklyn blocks. We’ve fixed the mistakes other contractors made, and we know what it takes to get underlayment right the first time.

If you’re in Brooklyn and planning a flat roof project, schedule an inspection with a contractor who specializes in full flat roof assemblies-not just quick patches. Use this article as a checklist during your walkthrough. Ask them to explain how they’ll handle layers, vapor, and drainage from deck to membrane. A well-designed flat roof underlayment installation now can save you from leaks, blistering, and expensive interior repairs for many Brooklyn winters to come.