How Long Does It Take to Install a Flat Roof?

Wondering how long you’ll be living under tarps and dodging roofers? Good news, flat roof installation is typically quicker than pitched roofing. But how quick are we talking?

From prepping the surface to rolling out the waterproof membrane, the timeline can vary. The size of your roof, materials used, weather conditions, and even the crew’s experience all factor in.

This guide breaks down every major timing variable, so you know what to expect, without the guesswork.

Average Installation Times for Flat Roofs

How long does it take to install a flat roof? That depends on size, material, and project complexity. While many flat roofs can be completed within a few days, larger or more technical projects may take a week or more. Below, we break down timelines by roof size, roofing material, and property type.

Typical Duration Based on Roof Size

The larger the roof, the longer the install, simple enough. But it’s not just square footage that matters. More roof area means more surface prep, membrane work, and detailing like edges, penetrations, and drainage. Here’s a general guide:

Smaller roofs, like garages or extensions, can often be completed in a single day. Larger residential roofs may take several days, while commercial installations could extend into weeks depending on logistics.

Timeline by Roofing Material

Different materials come with different installation processes. Some, like EPDM, are quick to install with minimal seams. Others, like GRP or felt, involve layering, curing, and detail work that takes more time.

Material choice impacts not only timing but also cost, lifespan, and maintenance. GRP and felt systems take longer but may be preferred for durability or specific design needs.

Residential vs. Commercial Project Timelines

Project type also affects timing. Residential flat roofs are usually smaller and simpler, while commercial installations tend to be larger, with more equipment, safety protocols, and sometimes phased execution.

• Residential Projects

  • Generally 1–4 days

  • Faster due to smaller size and easier access

  • May include old roof removal and deck repair

• Commercial Projects

  • Typically 1–2 weeks or more

  • Larger surface area, complex detailing

  • May involve HVAC systems, skylights, or phased work

While commercial timelines are longer, they often allow for more precision and coordination—especially in high-traffic or operational spaces.

What Affects How Long Installation Takes?

Even with the best-laid plans (and the slickest crew), a few variables can extend or accelerate your timeline. Let’s explore the biggest influencers:

Weather and Seasonal Delays

Rain, snow, extreme heat, or cold can throw a wrench into the schedule.

• Torch-on felt? Not safe in high winds or rain.

• Resin systems like GRP? Need dry, mild weather to cure properly.

• Winter installs? Shorter daylight hours slow everything down.

🌤️ Ideal seasons: Late spring through early autumn offer the most consistent conditions.

Roof Access and Structural Condition

Easy access = faster work. Sounds simple, but you’d be surprised how often this is a problem.

• Steep scaffolding setups or limited vehicle access? Time-consuming.

• Weak decking or rotted timber? Adds extra repair steps before any new roofing goes on.

📏 Pro tip: Have your contractor inspect the existing structure first—it can prevent surprises (and delays).

Complexity of Roof Design

A flat roof isn’t always flat. Design features add time:

• Internal gutters, upstands, or skylights need detailed attention.

• Multiple drainage points, vents, or awkward angles = more cutting and sealing.

🎯 The simpler the layout, the quicker the install.

Contractor Experience and Crew Size

A seasoned flat roofing crew can work like a well-oiled machine.

• Larger teams can divide and conquer, while one team preps, another starts laying membrane.

• Less experienced installers may take longer to align, seal, and finish properly.

🧑‍🔧 Bottom line? You’re not just paying for the material, you’re paying for efficiency, too.

Flat Roof Installation: Step-by-Step Breakdown

Let’s lift the curtain on the process. Whether you’re just curious or planning your own project, here’s how the flat roof installation journey typically unfolds—from dusty start to polished finish.

Pre-Installation Inspection

Before a single nail is hammered or membrane unrolled, there’s groundwork—literally.

• A qualified contractor will inspect the existing roof (or deck if it’s a new build).

• They’ll check for signs of damage, water ingress, or structural weakness.

• Measurements are taken, materials are ordered, and a plan of action is mapped out.

📋 No surprises = smoother install. This step ensures accurate timelines and cost estimates.

Stripping the Old Roof (if applicable)

If it’s a replacement rather than a new build, the existing covering has to go.

• Old felt, EPDM, or asphalt layers are carefully removed.

• Any damp insulation or rotten decking gets flagged for repair.

• Debris is cleared to prep for the new system.

🚮 Heads up: Tear-off can add 1–2 days, especially if unforeseen issues pop up beneath the surface.

Repairing or Replacing the Roof Deck

Think of the deck as the canvas—and you don’t paint on a cracked one.

• Damaged or uneven decking? It’s either repaired or replaced with OSB or plywood.

• Structural fixes might include joist reinforcement or insulation upgrades.

• This step ensures the surface is solid, level, and ready to accept the membrane.

🪵 Skipping this? Don’t. A bad deck leads to pooling, leaks, and costly call-backs.

Laying the Waterproof Membrane

Here’s where the magic happens. The membrane is the hero of your flat roof—it keeps everything beneath it bone dry.

• EPDM is typically laid in large sheets and glued or mechanically fixed.

• GRP requires resin layers and fibreglass matting.

• Felt roofing involves torching or bonding several layers of bitumen-based material.

🔥 Precision matters: Seams are sealed, edges detailed, and flashings installed to prevent water ingress.

Final Touches (Gutters, Flashing, etc.)

Now it’s time to wrap things up (literally and figuratively).

• Drip edges, flashing, and upstands are installed around the perimeter.

• Gutters or internal drainage systems are fitted or tested.

• A final inspection ensures everything meets spec, and your contractor should give you a rundown or warranty.

🎯 Don’t underestimate the details: Most leaks happen where two systems meet, like walls and chimneys.

Each phase builds on the last, and the quality of one affects the outcome of the next. That’s why rushing? Not worth it. A meticulous install today saves headaches tomorrow.

How to Plan for a Smooth, Efficient Installation

Okay, you know the steps, the timeline, the materials. Now let’s talk strategy.

Want the installation to go like clockwork? It’s all about preparation. A few smart moves upfront can save you days of delay, extra costs, and general frustration.

Choosing a Reputable Roofing Contractor

Not all roofing companies are created equal.

Here’s what to look for when vetting your installer:

• Specialization in flat roofs, they’re a different beast than pitched systems.

• Verified experience with your chosen material (EPDM, GRP, felt).

• Positive reviews, solid portfolio, and clear contract terms.

• Licensing and insurance, always.

🗣️ Bonus tip: Ask how large their crew is and how many flat roof projects they handle monthly. Bigger teams can often finish faster, but only if they’re organized.

Scheduling Around Weather

We touched on weather earlier, but when planning, it’s worth emphasizing.

• Book during milder months: spring, early summer, or early fall are your best bet in most climates.

• Avoid seasons known for heavy rainfall, snow, or extreme temps (especially for GRP and felt systems).

• Keep buffer time built into your schedule, just in case Mother Nature isn’t feeling cooperative.

☁️ Reality check: Even a little unexpected drizzle can cause a full day’s delay, so don’t schedule a back-to-back patio party.

Ensuring Permits and Materials Are Ready in Advance

Red tape can stall even the fastest crew.

• Some flat roofing jobs (especially in commercial or conservation areas) require building permits or planning permissions.

• Confirm timelines with your local council or permit office, don’t leave it last-minute.

• Make sure your contractor has ordered and received all materials before the start date.

📦 Delays in membrane delivery or insulation stock-outs are more common than you’d think. No materials? No progress.

Final Takeaway: Setting Realistic Expectations

So, how long does it really take to install a flat roof? The truth is, it varies. Most residential projects finish within 2 to 5 days, but larger or more complex jobs, especially commercial installs, can extend to a week or more. Factors like roof size, material choice, weather, access, and the condition of the existing structure all play a role. More important than speed is quality, cutting corners to finish faster often leads to future leaks, drainage issues, or expensive repairs.

At Ex10, we take the guesswork out of the process. Our experienced teams provide honest timelines, efficient scheduling, and quality workmanship you can count on.

Call us today on 01202 618374 to get a tailored estimate and plan your project with confidence.

FAQs About Flat Roof Installation Time

Can Weather Really Delay the Project?

Yes. Rain, snow, or extreme temperatures can delay installation, especially with materials like GRP or torch-on felt, which require dry, mild conditions. Always factor in a 1–3 day weather buffer.

Is Installation Faster for a New Roof vs. Replacement?

Yes. New roofs are quicker since there’s no tear-off or deck repair. Replacements usually take 1–2 days longer due to removal of old materials and potential structural fixes.

Can You Stay Home During Installation?

Typically, yes. Most flat roof work is external. Expect some noise, but no major disruption. It’s safe to remain at home unless extensive structural work is involved.

Should You DIY or Hire a Professional?

Hire a professional. Flat roof systems require precise installation and sealing. Mistakes can lead to leaks, voided warranties, and costly damage. DIY is not recommended unless you’re experienced.