Flat Roof Replacement Yonkers, NY — Completed by RH Renovation
Project Overview
RH Renovation NYC completed a full flat roof replacement on an approximately 8,000-square-foot commercial building in Yonkers, NY, part of Westchester County. The project called for more than a straightforward membrane swap: it required a two-ply APP modified bitumen system reinforced with a bleed-trap base coat and silicone-coated detail work at every skylight, rooftop A/C unit, and perimeter edge — the exact spots where large flat roofs most often fail.
Flat roofs of this size carry a lot of ponding stress, foot traffic from mechanical service, and thermal movement across the membrane. Rather than treating the field membrane and the penetrations as one uniform system, our crew engineered the field of the roof and the critical transition points as two separate lines of defense — a durable modified bitumen base, and a flexible, high-adhesion silicone seal wherever the roof plane meets a skylight curb, an HVAC unit, or the building edge.
Project Scope at a Glance
- Location: Yonkers, NY — Westchester County
- Roof size: approximately 8,000 square feet
- Roofing system: two-ply APP modified bitumen (Elevate APP 160 base sheet + APP 180 Black Cap Sheet)
- Critical-area sealing: A-4271 Bleed-Trap Acrylic Base Coat at perimeter, skylights, and A/C units
- Waterproofing topcoat: GacoFlex S4200 White High-Adhesion 100% Silicone Roof Coating
- New counter flashing installed around rooftop A/C units
- Full site cleanup and debris removal on completion
Surface Preparation and Priming
Before any membrane went down, the crew cleaned and prepared the entire 8,000-square-foot roof surface, clearing away dirt, dust, and loose or deteriorated material. On a roof this size, skipped prep is the most common reason a new membrane fails early — trapped debris and dust create voids that prevent proper adhesion.
Once the deck was clean, we applied primer across the full roof area. Priming gives the modified bitumen membrane a consistent, properly bonded surface to adhere to, which is what keeps a large flat roof system watertight through years of temperature swings, foot traffic, and standing water.
Two-Ply APP Modified Bitumen Membrane System
APP (atactic polypropylene) modified bitumen is a heat-fused asphalt roofing membrane built for exactly this kind of large, low-slope commercial application. Unlike single-ply membranes, APP systems are installed in layers, and each layer plays a distinct role.
We installed one full layer of Elevate APP 160 membrane across the entire roof as the base ply. This layer establishes the primary waterproof barrier and gives the roof its structural reinforcement. Over that, we installed a second full layer of Elevate APP 180 Black Cap Sheet, which serves as the finished, UV-resistant wear surface.
Running two fully adhered plies over the same 8,000 square feet — rather than a single layer — builds in redundancy: if the top layer is ever compromised by foot traffic, hail, or age, the base ply is still there doing its job. That redundancy is a major reason modified bitumen remains a preferred system for larger, mechanically-loaded flat roofs like this one, and it pairs well with the property’s existing roofing needs; see our full breakdown of flat roof repair and replacement systems for how we match membrane types to different Bronx and Westchester properties.
Reinforced Detail Work at Skylights, A/C Units, and Roof Edges
Flat roofs almost never fail in the middle of an open field — they fail at the seams, edges, and penetrations. This roof had multiple skylights and several rooftop A/C units, plus the full perimeter edge, all of which needed a different level of protection than the open membrane field.
At every one of those locations, we applied A-4271 Bleed-Trap Acrylic Base Coat, a product formulated specifically for asphalt-based roofing that’s about to receive a silicone topcoat. Asphaltic membranes naturally contain oils that can “bleed” up through a silicone coating over time, staining it and weakening the bond. The bleed-trap base coat blocks that migration, so the silicone layer applied on top stays fully adhered and free of discoloration for the long term — a detail that’s easy to skip and expensive to fix later.
GacoFlex Silicone Roof Coating for Long-Term Waterproofing
Once the base coat had cured, we applied GacoFlex S4200 White High-Adhesion 100% Silicone Roof Coating to the perimeter, skylight curbs, and A/C unit penetrations. Silicone coating brings two advantages that modified bitumen alone doesn’t offer at these detail points: it forms a seamless, flexible, high-adhesion barrier that moves with the roof rather than cracking, and its white reflective finish helps reduce surface temperature and UV degradation right where the roof is most exposed.
Using GacoFlex on the vulnerable transition areas, rather than across the whole field, targets the coating where it adds the most protection per square foot — around penetrations and edges — while the two-ply APP system handles the open roof plane. Building owners looking into silicone roof coatings for their own property can find more detail on our professional flat roofing services page, where we cover coating and membrane options for Bronx and Westchester flat roofs.
New Counter Flashing at Rooftop A/C Units
Rooftop A/C units are one of the most common leak sources on a commercial flat roof. Where the unit’s curb meets the roofing membrane, water can work its way behind old or corroded flashing and travel straight into the building envelope.
As part of this project, we installed new counter flashing around the A/C units on this Yonkers roof. Counter flashing overlaps the base flashing at the unit curb and directs water away from the seam entirely, rather than relying on sealant alone to hold back moisture — a longer-lasting fix than caulking around an aging flashing detail.
Debris Removal and Site Cleanup
With the new roofing system installed and every detail sealed, the crew removed and disposed of all roofing debris generated during the project and left the site clean. For a commercial building in active use, a clean, hazard-free rooftop and grounds at close-out is part of finishing the job correctly.
Final Result
The Yonkers property now has a fully replaced, 8,000-square-foot flat roofing system built on redundant APP modified bitumen protection, with silicone-reinforced detailing at every skylight, A/C unit, and roof edge. New counter flashing around the rooftop units closes off one of the most common flat roof leak points on commercial buildings, and the primed, two-ply membrane gives the property a durable, weathertight roof built to handle Westchester County’s seasonal extremes for years to come.
RH Renovation NYC is a GAF Master Elite® contractor serving flat roof, gutter, skylight, and chimney projects throughout the Bronx and Westchester County, including Yonkers, White Plains, Scarsdale, and New Rochelle. If your commercial or residential flat roof needs the same level of detail this project received, see our Westchester County roof repair and replacement services, check project costs with our roof replacement cost calculator, or get a free estimate for your property.
Frequently Asked Questions
What is APP modified bitumen roofing?
APP (atactic polypropylene) modified bitumen is an asphalt-based roofing membrane engineered for flat and low-slope roofs. It’s installed in heat-fused or self-adhered plies, giving it strong resistance to weathering, UV exposure, and foot traffic, which is why it’s widely used on larger commercial and multi-family flat roofs like this Yonkers project.
Why install two layers of APP modified bitumen instead of one?
A two-ply system — a base sheet like Elevate APP 160 topped with a cap sheet like APP 180 — builds redundancy into the roof. If the top layer is ever punctured or wears down, the base ply is still providing waterproof protection underneath, which extends the roof’s service life compared to a single-layer installation.
What does a bleed-trap base coat do, and why is it needed under a silicone coating?
Asphalt-based membranes contain oils that can migrate, or “bleed,” through a silicone topcoat over time, staining it and weakening adhesion. A bleed-trap acrylic base coat, like the A-4271 used on this project, blocks that migration so the silicone coating applied over it stays fully bonded and free of discoloration.
How long does an APP modified bitumen roof with silicone-coated detailing last?
A properly installed two-ply APP modified bitumen system with reinforced silicone detailing at penetrations and edges is typically built to last 20 years or more, with the silicone-coated critical areas adding extra protection at the points most likely to see early wear.
Why is new counter flashing important around rooftop A/C units?
Counter flashing overlaps the base flashing at an A/C unit’s curb and directs water away from the seam rather than relying on sealant alone. On older roofs, worn or missing counter flashing is one of the most frequent causes of leaks around rooftop equipment, so replacing it is a key step in preventing future water intrusion.
Does RH Renovation handle flat roof projects like this elsewhere in Westchester and the Bronx?
Yes. This Yonkers project follows the same detail-first approach we use across Westchester and the Bronx, including a similar flat roof replacement completed in the Bronx. You can see more completed work on our project portfolio or reach out for a free estimate on your own property.
