How Much Does a Flat Roof Cost?

Flat Roof Replacement Cost per Square Foot

Flat roof replacement typically costs $4 to $9 per square foot for labor and materials, but installers may charge $250 to $350 per square instead. Each square equals 100 square feet. The installation includes removal of a single layer of old roofing and disposal fees. Ventilation adds $300 to $600, while roof drains are $40 to $100 apiece. Extra underlayment could increase your total by $70 to $2,000.

New Flat Roof Costs by Type

Installed, flat roofs cost anywhere from $4 to $30. At the higher end of the range, you can expect materials with better structural integrity, longer lifespans, and better weather resistance.

Fiberglass Flat Roof Costs

Fiberglass, or GRP,  flat roofs cost an average of $5 per square foot, or $500 per square. You'll pay between $4 and $6 per square foot. A fiberglass flat roof is usually double-layered to add strength and durability, and they have a lifespan of 20 years or more. 

Although a budget-friendly material, fiberglass is lightweight and fire- and weather-resistant. However, fiberglass roofs aren't recommended for large structures, as they're prone to ponding across large areas, which puts strain on the roof and risks cracking and leaking.

Polyurethane

Most people pay around $5.50 per square foot, installed, for polyurethane roofing, or between $4 and $7 per square foot, with an average price of $550 per square. Polyurethane roofing, also known as SPF, can last for up to 50 years if properly maintained. It's a liquid product that expands into a dense, waterproof foam and is a popular choice because of its longevity, energy efficiency, and easy maintenance. However, it requires multiple layers to be truly waterproof, which increases cost, and each layer has to be applied quickly; in its liquid form, SPF has a short pot life.

Single-Ply Membrane

Single-ply membrane cost around $5.50 per square foot, or between $4 and $7 per square foot, installed. A roof installer heats and stretches PVC, plastic, rubber, or EPDM to fit across your roof and adheres it to your home with a solvent or bitumen or melts it to the roof structure to keep it in place. 

These roofs are the most popular flat roof option and can last 40 years or more. Seamless models are better than seamed ones, as the seams are weak points that can leak. Also, be sure to choose a UV-resistant material so it doesn't degrade with continued exposure to sunlight. 

Flat Concrete

For a flat concrete roof, expect to pay around $6, all-in, with most people paying between $4 and $8 per square foot. These roofs are highly durable and can last for 50 years or more. They're weather- and fire-resistant and require little, if any, maintenance. However, concrete can play host to very determined plant life that takes root in its uneven surface.

Modified Bitumen

Modified bitumen roofs cost around $6 per square foot, all-in, ranging from $4 to $8 per square foot. This flat roofing type is notable for its light hue that reflects heat and its 30-year lifespan. This type of roof is popular for its ability to reflect heat in hot climates. Because modified bitumen installs in rolls, a lot of seams can be potential leak points, so proper installation is essential.

Built-Up Tar

Expect to pay around $7 per square foot, or $4 to $10 per square foot, to have a built-up tar roof installed. This type of roof is fire-resistant and easily lasts about 30 years. Consisting of multiple layers of bitumen and waterproof membrane, this type of roof is more common on commercial properties than residential. They are easy to install, but last only 10 to 15 years.

Rubber Flat Roof Costs

Rubber flat roofs cost $4 to $13 per square foot, including labor and materials. A rubber, or EPDM roof, resists sun damage, is lightweight, and lasts up to 50 years with proper care. The rubber layer installs over felted insulation boards and, although it's weather- and waterproof, it's prone to shrinkage over time.

Metal

Metal flat roofs cost an average of $27 per square foot, installed, but you can pay anywhere from $4 to $30 per square foot. Low-cost metal roofs, like corrugated tin, fall at the lower end of the range. Once a popular choice, tin roofs are now less common because they need a lot of maintenance and regular painting to keep them in good shape, and they're prone to rust. 

At the higher end of the range are options like zinc. These are popular because of their ability to reflect heat, self-heal, and require minimal maintenance. While tin roofs last between 10 and 20 years, a zinc roof can last 60 to 100 years.

TPO

TPO, or thermoplastic polyolefin, flat roofs typically cost $8.75, or $3.50 to $15 per square foot, all-in. A type of modified rubber, TPO requires minimal maintenance and can last 30 years or more.

It's reflective, which helps keep home interiors cooler and lowers energy costs in warm climates and, because it's made from a high percentage of recycled materials, it's relatively eco-friendly. TPO is lightweight and easy to install, too. However, pros can only install it in dry weather because it requires heat to bond the material to the roof deck.

Flat Roof Replacement Cost Factors

Aside from the size and the material of your flat roof, several other elements influence how much you'll pay, including drainage and sealing. 

Labor

Contractors typically charge $3 to $7 per square foot to install flat roofs. This price includes removal and disposal of the old roofing. Expect to pay $1 to $2 more per square foot to install additional roofing layers.

Residential vs. Commercial

Commercial flat roofs cost $4 to $9 per square foot to install, depending on the material. They’re much bigger than residential roofs at 10,000 square feet, on average. Expect to pay commercial roofers near you $40,000 to $90,000 for the complete removal of the old roofing and to install the new materials.

The materials used most often for commercial flat roofs include:

• EPDM: A synthetic rubber membrane that seals and waterproofs the surface

• TPO: Single-ply rubber blend with the ability to weatherproof the roof and reflect heat

• Modified bitumen: Waterproof asphalt-based sheets that bond to the roof using heat

• Built-up tar: Features several layers of roofing felt along with hot tar, fiberglass, and gravel

Drainage

Installing or rerouting a flat roof drainage system costs $600 to $1,000. It's vital because a flat roof has no (or very minimal) slope, so unless your drainage system directs water away from your roof and home, it'll pool on the roof or run down the sides of your home and damage the foundation.

Sealing

Sealing a flat roof costs $50 to $500 extra. The process involves adding a layer of sealant (usually acrylic) to the top roofing layer to make it more impervious to water, weather, and penetration.

If you have penetrations such as a chimney, skylights, or vents, you'll also need to install adequate flashing to prevent water from working its way inside. Installing flashing costs anywhere from $500 to $1,000. 

DIY Flat Roof Replacement vs. Hire a Pro

You should only attempt a DIY flat roof replacement if you have the time, tools, and experience to get the job done right. Otherwise, leave it to the professionals since they can open up your roof without causing damage. They are also skilled in laying down the roofing materials so that they won’t leak in the near future.

Just make sure to find a licensed, bonded, and insured roofing contractor with a great track record of success. Get quotes from at least three flat roof contractors near you to weigh your options. You’ll want to speak with their references and check out pictures of their past projects before selecting the best one.

FAQs

How do you maintain a flat roof?

The cost to seal the roof surface is $400 to $1,800, but it’s well worth the expense. The sealant creates a thick barrier that makes all the rest of your maintenance tasks a breeze. With that done, you just have to keep the gutters and surface clean and routinely check the flashing for damage.

Clear the gutters after storms or at least every six months. Homes with many pine trees nearby may need their gutters cleaned every four months. In addition, it’s important to sweep off the roof periodically. Flat roofs are much more likely to collect leaves, dirt, and other debris that can accumulate in piles and trap standing water, resulting in leaks.

Finally, be sure to inspect the flashing around chimneys and other roof joints for cracking. Apply caulk to the cracks or replace pieces of the flashing if necessary to prevent leaks.

How much do flat roof repairs cost?

Patching small tears and other minor flat roof repairs cost $300 to $600. Repairing storm damage, rerouting drains, and other major overhauls could cost over $1,000. Remember to regularly take a close look at your roof for tears, holes, and leaks. Promptly addressing these issues through DIY repair or by contacting a flat roof repair pro near you is the best way to extend the life of your roof.

What are the advantages and disadvantages of a flat roof?

Flat roofs are no longer just for commercial space. They have grown in popularity in recent years, and for good reason. Flat rooflines have a modern appeal that beautifully complements a wide variety of home designs.

Other benefits include:

• Better energy efficiency than traditional pitched roofs, according to Energy Star

• Much more versatility and ease of transforming into a rooftop patio or greenspace

• Completely safe to walk on when properly sealed and treated

Flat roofs do come with their downsides, however. For starters, they do not work on every home design, and installation and repairs are difficult to DIY. Also, flat roofs are not ideal for homes in colder climates with heavy snowfall. The low-grade slope may slow drainage, leading to pools of water and ice dams during the winter months.