Build Single Story Flat Roof Extensions

If you add a single storey flat roof extension to your Brooklyn home, do you want just more square footage-or a space that stays bright, dry, and comfortable in both February slush and August heat? The answer lives in five decisions you make before construction starts: how big the extension will be, what load the flat roof has to carry (deck, equipment, nothing), how it will drain, how warm or cool you expect the new room to feel year-round, and how it ties into the existing building. Get these right and your single storey flat roof extension becomes the best room in the house. Miss them and you’re fixing leaks and fighting condensation for years.

Turn a Single-Storey Flat Roof Extension from Sketch to Structure

A single-storey flat roof extension is often the simplest way to get the kitchen, dining, or family room you actually need in a Brooklyn home. It can stretch out into the yard without adding a full new floor, bring in more light, and improve the way your home connects to outside. But the “flat” roof over that extension isn’t a box to fill in later-it’s a structural, thermal, and waterproofing system that has to be designed along with the walls and openings.

This guide is for you if you want to:

• Understand how a single-storey flat roof extension is put together from the roof’s point of view
• See where roof structure, insulation, and drainage fit in the build sequence
• Know what to ask your architect and contractor so the extension roof is done properly for Brooklyn weather

Start with How You’ll Use the Space and the Roof Above It

On a single storey flat roof extension I worked on in Prospect Lefferts Gardens, the family wanted a kitchen with big glass doors to the yard. They weren’t sure whether they’d ever use the roof. I asked them to decide now whether that possibility mattered, because building the structure to hold a future terrace costs maybe 15% more today-but retrofitting it later means ripping out the roof and spending three times as much. They chose to build for the terrace option. Two years later they added pavers and railings, and the membrane we’d selected handled it without a single change.

Clarify a few basics before thinking about the roof build-up:

• Is the extension mainly kitchen/living, a bedroom, or a mixed space?
• Will the roof above be purely a weathering surface, or do you want a terrace or green roof later?
• Do you want rooflights or a lantern to bring light deep into the plan?
• Are you trying to keep the extension low-profile from the street or neighbors’ side?
• How important is energy performance and sound insulation in that new room?

Why Single-Storey Flat Roof Extensions Suit Brooklyn Houses

Typical setups we see:

• Rear extensions off brownstones and rowhouses, filling part of the yard at the garden or parlor level
• Single-storey additions to semi-detached homes in outer neighborhoods, often along the side or back
• Ground-level extensions under existing apartments, where structural headroom is tight and flat roofs keep heights down

Brooklyn constraints that affect the extension roof: Zoning limits on how far you can build into the yard, party walls shared with neighbors, landmark or block context for rear elevations, and structural capacity of older walls all shape the extension’s footprint and how high and thick the flat roof build-up can be. In a Bed-Stuy rowhouse, we had 18 inches between the top of the new joists and the bottom of an existing window above-so the entire warm roof build-up (deck, vapour control, insulation, membrane, and pavers) had to stay within that envelope while still giving us R-30 insulation and proper drainage slope.

Structural Skeleton: Supporting a Single-Storey Flat Roof Extension

Think of the extension roof as a shallow bridge: Your single-storey extension roof spans from one support to another-usually from the back of the existing building to a new rear wall, or between side walls. It must carry its own weight, insulation and membrane, any snow, and potentially people, planters, or future pavers if you plan to use it as a terrace.

Structural questions your engineer or architect answers (we then build to that design):

• What span do the roof joists or beams need to cover, and in which direction?
• Are steel beams or LVLs needed at wide openings (e.g., big sliding doors)?
• How will the new roof loads be transferred into existing walls and foundations?
• Do we need to design for extra load from pavers, guardrails, or a possible green roof?

On a Bay Ridge side extension, the span from the existing house to the new wall was 16 feet. We used engineered lumber joists at 16″ centers, which gave us the depth to run services and enough stiffness to support a future terrace. The engineer specified a steel beam at the big slider opening to keep the header shallow and the door head at a comfortable height.

Choosing the Right Flat Roof Build-Up for a Single-Storey Extension

Most single-storey extensions in Brooklyn end up as warm flat roofs. The insulation sits on top of the structural deck, keeping the joists and everything below at room temperature. This cuts condensation risk dramatically and makes the junction where the roof meets walls much simpler to insulate and seal. If you’re planning a terrace, we add a protection layer (boards or pavers on pedestals) above the membrane so foot traffic never touches the waterproofing itself.

Membrane and Finish: What Actually Goes on Top

Common membranes we use on single-storey flat roof extensions:

• EPDM (rubber) – Good for small/medium simple roofs, repairable, often under decking. We used it on a Carroll Gardens extension under ipe decking; membrane has been invisible and leak-free for six years.
• TPO/PVC (white single-ply) – Reflective, nice for larger exposed roofs but needs careful detailing at parapets and penetrations.
• Modified bitumen – Layered system well-suited to complex edges and occasional service traffic. Slightly heavier but very forgiving at junctions.
• Liquid-applied – For intricate junctions or overlays, usually as part of a hybrid solution when tying into existing weird geometry.

If the extension roof will be used as a terrace:

• Concrete or porcelain pavers on pedestals (allows water to drain under pavers and easy access to membrane if service ever needed)
• Wood or composite decking on sleepers with a protection layer under sleepers
• Guardrails or parapets integrated into the structural and waterproofing design-code requires 42″ guards where there’s a drop

Tying a Single-Storey Flat Roof Extension into the Existing House

This is where most leaks happen on poorly built extensions. The junction between new flat roof and old wall is a three-dimensional puzzle: you have to manage water running down the wall, water sitting on the roof, thermal movement between materials, and the reality that old brick walls may not have modern cavity trays or damp-proof courses where you need them.

Key tie-in points we pay attention to:

• Where the new roof meets the rear wall of the house-brick, block, or siding
• Connections to existing flat roofs (e.g., main roof above or side returns)
• Any sloped shingle roofs that drain onto the new flat roof
• Existing or new parapets forming the edge of the extension roof
• Door thresholds from existing rooms onto a terrace roof

Principles at these joins: Roof membranes must turn up walls to a safe height-usually at least 8 inches above the roof surface, more if there’s any chance of snow drifting or paver build-up-and tuck under or behind proper flashings or cavity trays. Any existing damp-proof courses or weep holes in cavity walls must discharge over the new roof upstand, not behind it. When tying into old roofing, we extend the new membrane to logical breaks-parapets, joints, or drains-instead of making arbitrary mid-field seams that will move and leak.

On a Kensington extension, the existing rear wall was solid brick with no cavity. We had to cut a reglet (a narrow slot) into the mortar joint, tuck the membrane flashing into it, seal it with mastic, then cover the whole upstand with a capping flashing mechanically fastened above the reglet. That detail has handled five winters of freeze-thaw without a drip.

Designing Fall and Drainage on a One-Storey Flat Roof

Even small roofs need a real drainage plan. A single-storey extension roof might look small, but if it takes rain from upper roofs, gutters, or neighboring properties, the water volume is not trivial. Slope and outlet placement have to be planned, not left to chance.

Choices we make on extensions:

• Direction of fall: Toward the yard, side return, or internal drain? On tight city lots we often slope to an internal drain that ties into existing plumbing, avoiding the need to run new downspouts along party walls.
• Method of fall: Sloped joists vs tapered insulation vs local crickets. Tapered insulation is cleanest for warm roofs-the structure stays level and the slope is built into foam panels.
• Outlet type: Internal drain (tying into plumbing) vs scuppers through a parapet vs edge gutters. Scuppers are code-compliant overflow routes even if you have a main drain.
• Overflow: Where water goes if a drain clogs-overflows, secondary scuppers, or controlled spill points. Never assume the primary drain will always work.

Minimum slope is typically 1:80 (about 1/4″ per foot) to get water moving, but we aim for 1:60 where possible. On a Flatbush extension we used tapered polyiso insulation to create a 1:50 slope toward two internal drains, with scuppers in the parapet as emergency overflows. In a heavy rain the drains handle everything; if leaves ever clog them, water spills through the scuppers into the yard instead of ponding and finding a way into the ceiling.

Integrating Rooflights and Lanterns into a Single-Storey Flat Roof

Natural light transforms a single-storey extension from “extra room” to “the place everyone wants to be.” But every opening you put in the flat roof is a penetration that has to be framed, flashed, and drained correctly.

Common light-opening strategies:

Flat rooflights: Clean modern look; positioned over kitchens, islands, circulation zones. Curb-mounted units with membrane upstands; plan framing and drainage around them so water doesn’t pond against the upstand. We typically use fixed triple-glazed units with internal blinds for solar control.

Pyramid or rectangular lanterns: Feature over dining or living in the extension; adds height and drama. Require a robust curb and careful integration where lantern upstand meets flat roof membrane and any parapets. On a Park Slope extension we built a 4′ × 6′ lantern over the dining table; it brought south light deep into the plan and gave the flat ceiling some visual relief.

Short clerestory band at house wall: Letting light in where extension roof meets existing rear wall. Requires junction detail that handles water at both the clerestory and the roof/wall line. This is tricky but worth it when the existing building blocks light from overhead.

Single-Storey Flat Roof Extension: Build Sequence from Your Perspective

How the roof fits into the extension build:

Design & approvals: You work with an architect/engineer to fix footprint, wall locations, window/door sizes, and, on drawings, the roof type, insulation, drainage, and any rooflights. We’re often brought in at this stage to comment on roof build-up and tie-ins so details are constructible, not just pretty on paper. Typical design phase: 4-8 weeks. DOB approval in Brooklyn: 2-6 months depending on complexity and whether you’re in a landmark district.

Shell & structure: Foundations and walls of the extension go in. Beams and roof joists are installed, ready for the flat roof build-up, and any openings are framed. For a typical single-storey extension this phase is 3-5 weeks. You’ll see the space take shape and get a real sense of size and light.

Roof deck & envelope: Roof deck is laid (usually plywood or OSB), vapour control layer installed (for warm roofs), insulation set (including any tapers for drainage), and membrane applied with all flashings, upstands, and terminations. At this point the extension becomes properly watertight and interior trades can start without weather delays. Roofing phase: 1-2 weeks depending on complexity and weather windows.

Openings & finishes: Skylights or lanterns are fitted and flashed, parapets/railings completed if it’s a terrace, and interior ceilings, lights, and finishes under the roof are done. If you’re adding pavers or decking on the roof, that happens after the membrane is fully cured and inspected, typically in the final week or two of the project.

Your Role in Roof Decisions vs What the Pros Take On

Decisions you lead:

• How much you want to use the roof (never, sometimes, or as a regular terrace)
• Your appetite for skylights/lanterns versus solid ceilings
• Priorities around energy efficiency vs pure initial cost
• Where doors and windows to the garden should be and how they align with the roof

Decisions we/your design team lead:

• Choosing the appropriate roof build-up and membrane for your use case
• Sizing structure to carry roof, snow, and any terrace loads safely
• Detailing drains, parapets, and wall tie-ins so water sheds correctly
• Coordinating roof design with NYC code, zoning, and any landmark constraints

The sweet spot is when you tell us what you want the extension to do and feel like, and we translate that into a roof system that delivers it without surprises. On a Windsor Terrace project the owner said “I want to eat breakfast in here in January without it feeling like a cold porch.” That single sentence told us we needed continuous insulation, thermal-break details at the structure, and enough ceiling height to tuck in recessed lights without compressing insulation. The result: R-32 warm roof, radiant heat in the slab, and a room that feels like the original house year-round.

Single-Storey Flat Roof Extensions – Questions People Ask

Are flat roofs on single-storey extensions more likely to leak than pitched roofs?
Not if they’re designed and built correctly. A well-detailed warm flat roof with proper fall and outlets can be just as reliable as a pitched roof. Leaks tend to come from poor detailing at wall/roof junctions, drains, and skylights-not from the concept of a flat roof itself. I’ve seen pitched roofs over extensions leak at valleys and flashings while the flat roof next door stayed dry, because someone took the time to think through the flat roof details.

Can I add a roof terrace on my single-storey extension later?
Often yes, but it’s much easier and cheaper if we design for that possibility now. That might mean extra structure (joists sized for 100 psf live load instead of 40 psf), a more robust membrane choice (something that won’t be damaged by paver adjustments), and thought about stair/door access and railings, even if you don’t build the full terrace right away. Retrofit costs can be 2-3× more because you’re undoing finished work.

Should the extension roof use the same membrane as my existing flat roof?
Matching systems where practical makes detailing and maintenance easier-same manufacturer, same installation crew, same warranty structure. If your existing roof is old or not ideal for the new use, we may choose a better-suited system for the extension and tie them together with compatible transition details. On a Cobble Hill extension the main roof was 20-year-old built-up bitumen; we used modified bitumen on the extension because it tied in cleanly and gave us a fresh 15-year warranty on the new work.

How much height will the flat roof build-up add to my extension?
A typical warm roof build-up (deck, vapour control, 4-6″ polyiso insulation, membrane) adds 5-8 inches above the structural joists. If you’re adding pavers on pedestals, add another 3-5 inches. We coordinate this with door/window head heights and parapet/rail heights so everything feels proportionate and meets code. On constrained sites we sometimes use higher-R-value insulation in a thinner profile to stay within tight height limits.

Can I keep my existing rear wall as-is and just bolt the extension roof to it?
Usually we need to form proper flashings and sometimes cavity trays at the junction, which means some work to the existing wall-cutting a reglet, installing through-wall flashings, or building out a curb. Simply bolting into brick without a designed joint is a recipe for leaks. We aim to respect historic walls while still creating a robust connection. On landmark projects we coordinate details with LPC so interventions are minimal and reversible where required.

Plan the Flat Roof for Your Single-Storey Extension in Brooklyn

We help make the roof side of your extension predictable, not risky:

• Review of your extension concepts or drawings with a roof-focused eye, before you commit to a design that’s pretty but problematic
• Site visit to assess existing structure, drainage, and tie-in conditions so we catch issues early
• Recommendations on build-up, membrane, drainage, and openings that suit your home, your budget, and how you’ll actually use the space
• Coordination with your architect/contractor to integrate details before construction starts, not as expensive surprises mid-project

Ready to build a single-storey flat roof extension that lasts? Request an Extension Roof Consultation and we’ll walk through your plans, your site, and the five decisions that quietly determine whether your new room becomes the best space in your home or a source of regret.

We work on single-storey flat roof extensions behind Brooklyn brownstones, rowhouses, and semis-from simple kitchen bump-outs to more complex living spaces with roof terraces and lanterns-so you get the room you need on a roof that can handle New York weather, today and twenty years from now.