A flat roof is a roof that is almost level, with only a slight slope of usually less than 10 degrees to allow rainwater runoff. Flat roofs have been popular for commercial and industrial buildings for many years because they have a low profile, maximize usable interior space, are relatively easy to construct and provide flexibility for additional rooftop uses.

When it comes to residential homes, flat roofs have had cycles of popularity over the years. They experienced a surge of interest in mid-20th century modernist homes and are seeing renewed popularity today in contemporary and sustainable home designs. Flat roofs can be constructed with a variety of roofing materials, each with their own advantages and disadvantages.

Choosing the right type of flat roof material for your specific needs and climate will help ensure your roof stands the test of time. This guide provides an overview of common flat roof materials, their benefits and drawbacks, expected lifespans, and costs.

Built-Up Roofing

Built-up roofing (BUR), also known as tar and gravel roofing, is one of the oldest flat roofing systems. BUR roofs consist of alternating layers of bitumen (tar) and reinforcing fabrics that create a durable, waterproof membrane. The top layer is usually covered with gravel that provides protection from weather, sunlight and impacts.

Benefits:

• Very durable, can last 20-30 years or more
• With proper maintenance, can be repaired and additional layers added to extend lifespan
• Relatively low material costs

Drawbacks:

• Labor intensive installation
• Seams can be leak prone
• Needs regular maintenance
• Gravel top layer can lead to punctures if exposed to foot traffic

Cost:

• $4-$8 per square foot installed

BUR is a good option for inexpensive, low maintenance flat roofs in temperate climates. The use of hot asphalt makes BUR impractical for many do-it-yourself installations.

Modified Bitumen

Modified bitumen roofing is similar in construction to BUR, but the asphalt is “modified” with the addition of polymers to increase durability and flexibility. Modified bitumen can be installed in sheets or using a hot mopping technique similar to BUR.

The seams are usually heat welded together, avoiding the leak potential from overlaps in BUR. Modified bitumen is finished with a top coating that is available in various colors and can provide additional weather protection.

Benefits:

• More durable and longer lasting than BUR
• Easier repairs and maintenance
• Decorative finish options

Drawbacks:

• Still labor intensive to install
• Relatively short lifespan of 10-15 years
• Not ideal for cold climates

Cost:

• $5-$11 per square foot installed

Modified bitumen provides a longer lasting and easier to maintain roof than traditional BUR, at a moderate cost. The seams and top coat avoid common weaknesses of BUR.

EPDM

Ethylene propylene diene terpolymer (EPDM) is a synthetic rubber commonly used for flat roofing. EPDM can be installed as a single sheet or in strips that are adhered together with contact cement or tape. An adhesive primer is applied to bond the EPDM to the roof substrate.

Benefits:

• Extremely durable, with a lifespan of up to 50 years
• Seamless waterproof barrier when installed correctly
• Easy to repair small punctures
• Available in various thicknesses

Drawbacks:

• Difficult for DIY installation, requires skill to avoid seam leaks
• Vulnerable to punctures and tears if exposed to foot traffic
• Can be negatively impacted by ponding water

Cost:

• $4.50-$9 per square foot professionally installed

EPDM is one of the most popular and affordable choices for durable low slope roofs. Hiring professional installers is recommended to avoid potential leakage from seams. Overall an excellent option for a simple rubber roof.

TPO

Thermoplastic polyolefin (TPO) is a newer single ply roofing material that is rapidly gaining popularity. Similar to EPDM, TPO sheets are adhered together to form a seamless waterproof barrier. However, TPO uses heat welding to fuse the seams together, creating a monolithic membrane.

Benefits:

• Extremely durable and weather resistant, with a lifespan of up to 25 years
• Lightweight, flexible, and easy to install
• Energy efficient – can reduce cooling costs
• Available in white or light colors to reflect heat

Drawbacks:

• Relatively expensive compared to other flat roof materials
• Requires meticulous installation to avoid seam failures
• Not compatible for covering over some existing roofs

Cost:

• $6-$12 per square foot installed

TPO is an excellent choice for a long lasting, low maintenance flat roof. The heat welded seams and reflective surface make this an ideal choice in hot, sunny climates. Professional installation is critical.

PVC

Polyvinyl chloride (PVC) roofing membranes share many similarities with TPO as a single ply option. PVC sheets are fused together with heat welding to produce watertight seams. PVC membranes are typically white or light grey in color.

Benefits:

• Extremely durable, with a typical lifespan of 20+ years
• Excellent seam strength when properly fused
• Energy efficient and cool roof option
• Good fire resistance

Drawbacks:

• Relatively expensive compared to other flat roof materials
• Professionally installation is essential
• Not environmentally friendly compared to other membranes

Cost:

• $8-$14 per square foot installed

PVC is a top of the line flat roofing material that offers great performance and longevity. Like TPO, correct installation is key. The high cost may make PVC prohibitive for some budgets.

Spray Polyurethane Foam

Spray polyurethane foam (SPF) is a unique roofing option that is literally sprayed on as a liquid and rapidly expands to form a seamless, protective barrier. The foam bonds tenaciously to roof substrates. A protective coating is usually applied over the SPF membrane.

Benefits:

• Extremely durable and weather resistant, with a lifespan of 15-25 years
• Seamless application conforms to any shape and seals hard to waterproof areas
• Excellent insulator, can significantly reduce energy costs
• Highly reflective white coating options

Drawbacks:

• Very expensive material and specialized installation
• Can be damaged by UV exposure if coating fails
• Specific repair procedures must be followed

Cost:

• $6-$12 per square foot installed

SPF roofing excels in unique situations that are not suitable for other roofing materials. It is an excellent insulator but the high cost makes it prohibitive for many residential installations.

Liquid Applied Membranes

Liquid applied membranes consist of waterproof elastomeric polymers that are rolled or sprayed onto the roof surface. The liquid quickly cures to form a thick, monolithic coating. These roof coatings can be applied over existing substrates or new boards.

Benefits:

• Seamless application conforms to any shape
• Can extend lifespan of existing roofs when used as a coating
• Comparatively easy installation
• Good impact resistance

Drawbacks:

• Prone to damage from ponding water
• Short lifespan of 10 years or less
• Limited odor and temperature application constraints
• Requires priming for many substrates

Cost:

• $3-$7 per square foot installed

Liquid applied roof coatings provide an affordable waterproofing option for unusual roof shapes or over existing worn roofs. Their short lifespan makes them best for low budget installations.

Asphalt Shingles

Asphalt shingles are a familiar roofing material commonly used on sloped roofs. Specialty shingles are manufactured for use on low slope or flat roof applications. The shingles are applied in overlapping rows like siding, with small spaces maintained between each shingle.

Benefits:

• Very affordable roofing option
• Easy installation for DIYers
• Various color options available
• Can be used to shingle curved or unusual roof shapes

Drawbacks:

• Short lifespan of 10-20 years
• Not seamless, prone to leakage between shingles
• Not suitable for roofs with any standing water

Cost:

• $3-$5 per square foot installed

Asphalt shingles provide a basic and inexpensive roofing option for flat roofs on a budget. Their composition is not ideal for flat or low pitch surfaces, limiting their durability. Not a good choice for roofs prone to ponding water.

Metal Roofing

Metal roofing comes in large panels made from materials like galvanized steel, aluminum, copper, or zinc. The long panels are installed in an overlapping pattern and screwed into the roof deck. Joints are sealed with tape or caulk.

Benefits:

• Very long lifespan of 40-70 years
• Durable against all weather conditions including high winds
• Low maintenance and easy to clean
• Excellent rainwater runoff

Drawbacks:

• Comparatively expensive
• Noise from rain and hail impacts
• Standing seams can separate and leak over time
• Severe impacts can cause dents

Cost:

• $6-$20 per square foot installed

Metal roofs are an excellent choice for durability and longevity. The high cost and potential noise are the main downsides. Proper installation is critical to avoid seam leakage over time.

What Are the Key Factors in Choosing a Flat Roof Material?

There are several important considerations when selecting a flat roof material:

Climate and Weather

The climate and typical weather patterns for your location should influence your roofing material choice. For example, reflective membranes like TPO resist warm weather exposure better, while smooth rubber materials like EPDM perform well in cold climates.

Consider average rainfall, typical wind speeds, frequency of hail or snow, and annual temperature extremes when selecting a flat roof material rated to withstand your local conditions.

Roof Slope and Drainage

The slope and drainage characteristics of your specific roof should also be assessed. Materials like asphalt shingles and liquid applied coatings are not optimal for flat roofs with poor drainage or ponding water issues.

Installing a sloped substrate or tapered insulation prior to the roof covering can improve drainage for nearly flat roofs.

Building Use and Foot Traffic

The amount of regular foot traffic the roof surface will experience should influence the material choice. Materials like EPDM and spray polyurethane foam are more prone to damage from excessive foot traffic unless protective layers are added.

Consider how often the roof will be accessed for maintenance, repairs and recreational uses when selecting durable roofing materials.

Lifespan and Maintenance

The typical lifespan and maintenance requirements vary widely among flat roof materials. Asphalt coatings and shingles may only last 10-15 years, while EPDM or TPO can last 25 years or longer with proper care.

Consider the maintenance abilities of the owner and costs of hiring roofing professionals when choosing between long or short lifespan materials.

Energy Efficiency

For climate controlled buildings, selecting an energy efficient roof material can provide significant cost savings. Options like TPO, PVC, and spray foam offer good insulating properties that reduce cooling costs in warm weather.

Consider adding radiant barrier decking and extra insulation for optimal energy efficiency in extreme climates.

Installation Cost

Flat roof materials range in installed cost from $3 per square foot up to $20 per square foot or more. The labor intensiveness of the installation also affects total costs.

Realistic budgets and installation factors will help narrow the choices to affordable flat roof materials for your specific project.

Sustainability and Eco-Friendliness

Many modern flat roof materials offer sustainable benefits like durability, energy efficiency, and recyclability. PVC membranes have come under scrutiny for potential environmental impacts.

Prioritizing sustainability may steer your decision toward recycled rubber, thermoplastic membranes, or spray foam with high recycled content.

Common Flat Roof Concerns and How to Avoid Them

Flat roofs have some inherent vulnerabilities that differ from sloped roofs. Following best practices during design, installation and maintenance is key to maximizing the performance and longevity of a flat roof.

Ponding Water

Flat roofs require a slight slope to encourage positive drainage and prevent ponding water from accumulating. Ponding water can accelerate deterioration and leakage.

Solution:

• Design in a slope of at least 1/4 inch per foot
• Use tapered insulation boards to create slope
• Ensure proper roof drainage and scuppers

Excessive Foot Traffic

Regular foot traffic can wear down certain roofing materials over time. Membranes like EPDM and coatings are prone to punctures and abrasion from walking.

Solution:

• Install protective walkway pads in high traffic areas
• Restrict access to essential maintenance only
• Select durable materials like TPO for recreational rooftop uses

Seam and Edge Leakage

Leaking at membrane seams or perimeters is common without proper installation. Loose seams and poorly sealed edges are vulnerable to wind driven rain.

Solution:

• Use fully adhered or mechanically fastened membranes
• Employ heat or chemical welding for seamless bonds
• Reinforce perimeter edges and fully detail flashings

UV and Weather Exposure

UV rays, large temperature swings, wind, and hail can degrade roofing over time. Coatings and aggregates provide protection.

Solution:

• Use membranes containing UV inhibitors
• Maintain protective coatings and gravel coverings
• Install energy efficient materials in extreme climates

Lack of Maintenance

Regular roof maintenance enhances longevity by catching small issues before they become leaks. Neglect accelerates deterioration.

Solution:

• Conduct annual roof inspections and minor repairs
• Clear debris from drains and scuppers
• Reapply protective coatings like elastomeric acrylic

Choosing the Best Flat Roof System for Your Project

Selecting an optimal flat roofing system involves evaluating your specific needs, climate, budget and building functionality against the characteristics of each material. A few tips:

Hire Professional Installers

Many flat roof membranes require skill to install properly, especially around details and penetrations. Hiring an experienced roofing contractor is wise, even for DIY capable homeowners.

Consider Hybrid Systems

Combining different materials like a thermoplastic membrane with a liquid top coating can provide added durability and weather protection.

Include Slope and Drainage

Designing adequate slope into the roof structure and having proper drains and scuppers is critical, regardless of material.

Maximize Durability

For most homeowners, choosing the most durable roofing material within their budget makes sense. Lifespans of 20-30 years or more are ideal.

Regular Inspections and Care

Schedule annual inspections and perform preventative maintenance like clearing drains and making minor repairs to maximize the lifespan of your flat roof.

Key Takeaways on Flat Roof Materials

There are many excellent material options for flat roofing, each with their own strengths and weaknesses. Following are some of the key takeaways:

• Built-up and modified bitumen roofs are economical and durable, but require intensive installation.
• EPDM and TPO membranes provide long lifespans with little maintenance when properly installed.
• Liquid applied coatings are affordable and seamless but have shorter lifespans than other materials.
• Asphalt shingles offer very inexpensive flat roofing, but are prone to leakage and early failure.
• Metal panels are extremely durable but can be noisy in rain and are one of the most expensive options.
• Spray polyurethane foam provides a seamless insulating barrier but has high upfront costs.
• Careful material selection based on climate, usage, and budgets helps ensure optimal flat roof performance.
• Professional installation and regular maintenance are wise investments for any flat roof.

With the right material choice and proper care, flat roofs can provide decades of reliable performance. Consider both the initial and long term costs when selecting from the many excellent flat roofing options available.