Learn to Felt Your Garage Flat Roof

The biggest mistake I see homeowners make is throwing a single cheap felt layer straight onto a soft, uneven garage roof deck on the first warm spring day-no primer, no proper laps, no edge details-then wondering why water’s dripping onto their lawnmower by the first Brooklyn nor’easter. Your garage roof is small, but it’s still a real flat roof system that needs deck prep, correct overlaps, edge work, and drainage. Get those basics right, and felting a garage flat roof is absolutely a DIY-friendly project. Skip them, and you’ll be back on that ladder next year peeling off soggy felt and rotten plywood.

In this guide, you’ll learn:

• What “felting” a garage flat roof actually means today-materials, layers, and modern alternatives to hot tar.
• How to assess whether your existing garage roof deck and structure are ready for new felt.
• The main stages of felting a garage roof, with clear checkpoints for safety and quality.
• Brooklyn-specific considerations-drainage to alleys, neighbor walls, fire risk-and when to call a pro instead.

What Do We Mean by “Felt” on a Garage Flat Roof?

Historically, felting a flat roof meant hot asphalt and heavy mineral-surface rolls. Today, most garage roofs use modified bitumen felt-either torch-on or cold-applied-built up in one to three layers over a solid deck. The principle is still the same: overlapping sheets to form a continuous, watertight membrane that sheds water toward gutters or edges.

Common garage flat roof felt systems:

• Torch-on modified bitumen: Heated with a propane torch to melt the bottom layer and bond to the deck or underlay. Fast, durable, but higher fire risk and requires experience.
• Cold-applied modified bitumen: Adhered with cold adhesive or self-adhesive backing. Safer for DIYers, no open flame, slightly longer install time.
• Built-up felt systems (BUR): Multiple layers-base sheet, intermediate layer, mineral cap sheet-usually installed by pros for longer-lasting garage roofs or those supporting decks above.

All three work. The choice comes down to your comfort with torch work, local fire codes, and whether you want a 10-year quick fix or a 20-year roof.

Pre-Check: Is Your Garage Roof Ready to Be Felted?

Before you buy a single roll of felt, walk the roof and answer these questions honestly. I’ve stripped more failed DIY garage roofs than I can count, and 90% of them never should have been felted in the first place-the deck was rotten, the structure was sagging, or there was no fall for water to drain.

Questions to answer before you even buy materials:

• Is the existing deck (plywood, OSB, or tongue-and-groove boards) solid-no bounce, sag, or soft spots when you walk on it?
• Does the roof have at least a 1/4-inch per foot fall toward a gutter, scupper, or edge, or is water currently ponding in the middle?
• Are there cracks in the garage walls, bowing beams, or other signs the structure needs an engineer, not a new felt layer?
• Is there safe ladder access and enough room to work on all sides, or will you be squeezed between fences, neighbors’ walls, and power lines?
• Are you allowed to use a torch (local fire code, proximity to wood siding or dry trees, your own comfort level)?

Important safety point: Torch-on work over old timber garages or within six feet of neighboring buildings in dense Brooklyn lots is a serious fire risk. I’ve seen garage roofs catch mid-install and spread to fences, sheds, even a neighbor’s back porch. Cold-applied or professionally supervised torch work is often required by insurers in those situations.

Materials and Tools Used to Felt a Garage Flat Roof

A typical single-layer cold-applied garage roof for a 12×20-foot garage runs about $400-$650 in materials if you’re buying retail at a Brooklyn roofing supplier. Torch-on systems cost roughly the same for materials, but you need to own or rent the torch, regulator, and gas bottles.

Typical components in a felted garage roof system:

• Roofing felt rolls-one base layer, one cap sheet for a two-layer system.
• Deck primer (bitumen or acrylic, depending on system).
• Cold adhesive or torch equipment.
• Galvanized roofing nails or mechanical plates for perimeter and upstands.
• Metal drip edges at eaves and rake edges.
• Compatible sealant for detail work at pipes, vents, or wall junctions.
• Optional: rigid insulation boards if upgrading to a warm roof, vapor control layer for condensation-prone garages.

Tools commonly needed:

• Heavy-duty utility knife with hooked blades for felt.
• Measuring tape, chalk line, carpenter’s square.
• 100-pound weighted roller or hand roller to embed felt into adhesive.
• Hammer or impact driver for edge fixings.
• For torch-on only: propane roofing torch, spare gas bottle, ABC fire extinguisher within arm’s reach, heat-resistant gloves, and closed-toe boots (not sneakers).

Main Stages of Felting a Garage Flat Roof

On a narrow alley garage in Bay Ridge a few years back, the homeowner had already stripped the old felt and called me in just to check the deck before he felted it himself. We found three rotten sheets of plywood hidden under sound edges-water had been ponding in the center for years. Replaced those three sheets, added a tapered firring strip down the middle to create fall, primed, and he cold-applied the new felt over two weekends with zero leaks since. That sequence-strip, inspect and fix, prep, layer, detail, test-is how every garage roof should go.

From bare deck to finished felted roof:

1. Strip and inspect. Remove old felt, shingles, or coatings down to the deck. Don’t assume the deck is good because the top layer looked okay. Check every sheet of plywood or board. Replace any that are soft, discolored, or delaminating. If there’s no fall, now is the time to add tapered firring strips or new joists to create at least 1/4-inch per foot slope toward the low edge.

2. Prep the surface. Sweep and vacuum the deck. Pull any protruding nails or screws. If the system requires primer, apply it evenly and let it dry per the manufacturer’s instructions-usually 1-4 hours depending on temperature. Install metal drip edges at eaves and rakes so the new felt can lap over them and shed water cleanly off the roof.

3. Lay base layer / underlay. Roll out your first felt layer perpendicular to the fall (so water runs across seams, not along them). For cold-applied, spread adhesive with a notched trowel and press the felt into it, rolling firmly to eliminate air pockets. For torch-on, heat the underside of the felt evenly, watching for a small bead of melted bitumen to squeeze out at edges-that’s your bond indicator. Overlap side laps by at least 3 inches, end laps by 6 inches minimum.

4. Add main cap sheet layer. Stagger the cap sheet seams so they don’t align with base layer seams-offset by at least 18 inches. Use the same adhesive or torch technique, paying extra attention to the edges and seams. You want full contact, no voids, no wrinkles. If torching, don’t overheat and burn through the reinforcement mat; if cold-applying, don’t skimp on adhesive at seams.

5. Detail upstands and penetrations. Turn the felt up at least 6 inches at any wall, parapet, or higher structure. Mechanically fix the top edge with roofing nails and a termination bar, then cover with metal flashing or a compatible wall capping. Around vent pipes, cut a neat slit, wrap the felt, and seal with mastic or a pre-formed boot. Corners are where most DIY jobs leak-take your time, use extra adhesive or heat, and don’t leave any loose edges.

6. Final inspection. Walk the entire roof looking for unbonded spots, fish-mouth bubbles, or gaps at seams. Run a garden hose for 10 minutes near high points and watch the low edge-water should flow freely to the gutter or drip edge, not pond. If it ponds, you may need to add more fall or install a scupper to drain that zone.

Common DIY Felting Mistakes on Garage Roofs

I get called to fix DIY garage roofs almost as often as I install new ones. Here are the mistakes I see again and again, usually within the first year:

• Felting over a rotten or spongy deck instead of replacing bad plywood first. The felt looks great for six months, then the deck collapses under the first snow load.
• Ignoring fall-the new felted roof is dead flat or even reverse-sloped in places, so water ponds exactly where it did before (or worse).
• Too little overlap at seams, or running seams parallel to water flow instead of across it. Water finds those gaps fast.
• Overheating torch-on felt and melting through the fiberglass mat, or underheating and leaving cold, unbonded patches that peel up in wind.
• Sloppy detailing where the garage meets house walls or parapets. No termination bar, no flashing, just felt tucked into a crack and hoped for the best. That’s where 80% of garage roof leaks start.
• Using random sealants-silicone, latex caulk, whatever’s in the truck-instead of bitumen-compatible mastic. Those fail in UV and temperature swings within months.

Details That Matter on a Garage Flat Roof in Brooklyn

Brooklyn garages aren’t standalone sheds in a field-they’re squeezed between fences, alleys, neighbors’ walls, and often under or beside decks, fire escapes, and second-story additions. Pay extra attention to these local details:

• Where the garage roof drains: Into a narrow alley, a shared yard, the sidewalk, or onto a neighbor’s driveway. Badly placed scuppers or overflowing gutters can cause disputes or code violations. Make sure your drainage plan doesn’t dump water onto someone else’s property or create ice hazards on a public walk.
• Low upstands where the garage abuts a taller building. If your garage roof meets a house wall or neighbor’s wall, that junction needs at least 6-8 inches of felt upturn, mechanical fixing, and proper counter-flashing from above. These zones leak first and hardest in Brooklyn’s freeze-thaw cycles.
• Snow and ice loads. A flat garage roof in Brooklyn can hold 6-10 inches of wet snow, and if your drainage freezes or clogs, meltwater backs up under the felt at edges. Shallow slopes and clear scuppers are critical.
• Visible edges from the street or alley. Neat metal trims and clean felt terminations look better and protect felt edges from wind lift, animal damage, and UV degradation. A messy edge is also a flag to inspectors and buyers that the work was amateur.

Deciding How Much of the Felting Job You Should Do Yourself

Not every garage roof should be a DIY project, and not every garage roof needs a full professional crew. Here’s how to decide where you fit:

You might DIY more if:

• The garage is detached, single-story, and you can safely access all four sides with a ladder.
• You’re comfortable working at height, following detailed instructions, and handling heavy rolls (50-70 pounds each).
• You choose a cold-applied or self-adhesive felt system designed for homeowner installation-no torch, no hot work permit needed.
• You accept that you may need a pro to fix complex junctions, add insulation, or correct mistakes if things go sideways.

You should strongly consider a pro roofer if:

• The garage adjoins your house, supports a deck or walkway above, or shares a wall with a neighbor’s building.
• You’re in a dense part of Brooklyn (Bed-Stuy, Bushwick, Sunset Park) where a roof fire or major leak would quickly affect others.
• The existing roof has three or more old felt layers, hidden rot, or drainage problems you can’t diagnose on your own.
• Local fire code, your homeowner’s insurance, or your own comfort level says no to torch work or structural alterations.

Your Role vs Our Role on a Garage Flat Roof Project

When homeowners do hire us for a garage roof, it’s often a collaboration-they handle the simple stuff, we handle the technical and safety-critical work. Here’s a typical split:

You can:

• Decide upfront whether you want a budget single-layer repair or a long-term two-layer system with insulation.
• Clear the garage interior, move bikes and tools, and trim back tree branches or vines near the roof edge.
• Strip old roofing felt (if it’s safe, single-layer, and we agree ahead of time on disposal).
• Handle final cleanup, paint touch-ups, and gutter reinstallation once the roof is watertight.

We can:

• Assess the deck and structure, replace rotten plywood or damaged joists, and confirm or create proper fall.
• Design and install drainage-scuppers, edge outlets, or integrated gutters-so water actually leaves the roof.
• Install multi-layer felt systems (torch-on or cold-applied) with correct overlaps, edge details, and upstand flashings that won’t leak in five years.
• Integrate the garage roof with house walls, parapet caps, deck supports, or neighboring buildings in a code-compliant, watertight way.

Garage Flat Roof Felting – Common Questions

Do I have to strip the old felt off my garage roof before refelting?
Not always, but it’s usually best. Stripping lets you inspect and replace bad deck boards, adjust fall with firring strips, and avoid building up too much height at edges (which can trap water or interfere with door clearances). If there’s only one sound existing layer and the deck underneath is confirmed good, you can sometimes felt over it-but multiple old layers, trapped moisture, or any doubt about the deck means strip it first.

Is torch-on felt safe for a timber garage?
Torch-on can be used safely over timber with proper technique-flame shielding, a fire watch, an extinguisher on hand, and experienced control of the torch. But in tightly packed Brooklyn lots with dry wood siding, overhead lines, and neighbors six feet away, it’s riskier. Cold-applied or self-adhesive systems, or hiring a pro who’s insured and knows Brooklyn fire code, often make more sense.

How long should a newly felted garage roof last?
A well-installed two-layer modified bitumen system on a simple garage roof with decent slope and regular gutter cleaning should last 15-20 years, sometimes longer if it’s a light-colored cap sheet in a shaded location. Single-layer or poorly detailed DIY systems may only give you 7-10 years before seams open up or ponding water degrades the felt.

Can I add insulation when I refelt the garage roof?
Yes, and it’s a smart move if you use the garage as a workshop, studio, or future living space. Adding rigid insulation boards above the deck (warm roof design) improves comfort and prevents condensation, but it raises the finished roof height-so you’ll need taller edge trims, extended flashings, and sometimes new door or window head clearances. Plan that before you start felting.

Will refelting my garage stop damp on the inside walls?
It can, if the roof was the moisture source. But internal damp can also come from rising groundwater, poor ventilation causing condensation, or cracks in the walls themselves. Part of any good roof assessment is confirming that roof leaks-not other moisture paths-are actually the problem before you spend money on new felt.

Need Help Felting or Re-Felting a Garage Flat Roof in Brooklyn?

Look, I’m all for capable homeowners doing their own garage roofs-I’ve walked dozens of people through the process, loaned tools, even stopped by mid-project to check their laps and edge work. But I also know when a garage roof is beyond a safe or smart DIY attempt: rotten structure, complex drainage, tight urban lot, or a homeowner who’s handy but doesn’t want to spend a weekend torching felt in July heat.

We can step in where you need us most:

• Full garage roof replacement with two- or three-layer felt systems designed for 15-20+ year service in Brooklyn weather.
• Structural and deck repairs-replacing rotten plywood, reinforcing sagging joists, adding fall where there is none.
• Professional detailing at house walls, parapets, neighbor walls, and deck supports-the junctions that leak if done wrong.
• Honest advice and transparent quotes if you’re unsure whether to DIY, hire out the whole job, or split the work.

Want a second set of eyes on that garage roof before you commit? We work on flat-roofed garages all over Brooklyn-behind brownstones in Park Slope, rowhouses in Bed-Stuy, and small commercial buildings in Sunset Park-whether that means installing a new felt system from scratch, fixing a past DIY attempt that’s leaking, or just giving you a clear plan and materials list so you can do it safely on your own.