7 Types of Flat Roof Systems

Having a flat roof gives you usable space on top of your house for large air conditioning units, solar panels, or in the case of many apartments and corporate buildings, a garden terrace. But flat roofs also come with unique challenges and multiple styles. So what are the different types of flat roofs, and which is best?

What is a Flat Roof?

Roofs come in two classifications - low-slope or steep-slope. Flat roofs classify as low-slope due to a pitch of less than 3:12, whereas steep-slope roofs have an angle greater than 3:12.

This ratio defines the roof's slant or pitch. The first number - 3 - refers to the number of inches that the roof rises (vertical rise) for every 12" in depth towards the roof peak (the run or horizontal span) - the second number. The higher the slope, the more visible the pitch will be when looking at the roof.

The construction of a flat roof allows for the use of multiple types of roofing materials, which we'll discuss shortly. But it also can cause numerous problems. The biggest concern about flat roofs is standing water. Pitched houses have an advantage, as the slope allows water to drain off. Although a flat roof with a ½ "1" gradient will drain most of the precipitation, it will not drain all of it.

Pooling Water Problems

A professional contractor must be sure to install the proper drainage to ensure the roof doesn't collect water since standing water can cause a lot of trouble for flat roofs. 

Water puts additional weight on the affected areas which can cause too much strain to the structure. Areas that hold water for too long are at a higher risk of developing collapsing, cracks, and leaks. 

Leaks can also cause structural damage and potentially lead to mold, mildew, or wood rot. And due to the design of most flat roofs, it's often hard to locate the damaged area to make repairs. These problems can lead to potentially huge repair costs..

Picking the correct type of material for your flat roof reduces the chances of these dire travesties. So, let's look at which material might be suitable for your home.  

Built-up Roofing (BUR)

Built-up Roofing - referred to as tar and gravel - is one of the oldest covering for flat roofs. Modern advancements have made BUR roofs less common, although they are still used on commercial buildings and residential homes.

Built-up roofing consists of layers of roofing felt covered with modified hot asphalt. Each layer of materials can add five years of life to your roof for up to 30 years. You can apply a BUR roof in three, four, or five-layer systems.

A layer of coarse gravel goes on top of the materials to protect against UV damage, increase thermal regulation, and help with energy efficiency. It's also a fire-retardant material for fire safety. Modern techniques include using additional top layers, such as field applied reflective coating or a granular cap sheet. These materials increase the roof's UV protection and thermal insulation.

Pricing

The cost to install a tar and gravel roof can cost between $3,750 and $6,750, or $2.50 to $4 a square foot ($250 to $400 a roofing square - 100 square feet). 

Attention: Harmful odor installation.

The application of a BUR roof requires a labor-intensive application using hot applications, which means using a blowtorch or a hot mop covered with asphalt. But, unfortunately, this method of installation leaves foul odors that can be harmful to people. Many businesses with frequent customers and most homeowners will shy away from BUR roofs for other alternatives due to the chemical release by torch install.

Pro Tip: Multiple factors play a role in the cost of any of the roofing materials we discuss in this list. Things like the size of your roof, the complexity of the roofline, your location, obstructions like chimneys, air conditioning units, vents, and other items that you can't remove, and if your old roof needs to be torn off first.

Modified Bitumen Roofing

Unlike BUR roofs, which have multiple layers (plies), modified bitumen consists of a single material layer - single-ply. The single layer of Modified Bitumen consists of polymer-modified asphalt bitumen reinforced with a durable material like fiberglass, polyester, or a combination of the two. 

The ease of installation makes modified bitumen an excellent material for DIY roof jobs. MB comes in a rolled sheet with a 3-foot width and up to 36 feet long. After laying a base sheet, you roll out the modified bitumen sheet and seal it down.

Older types of MB required using a torch to heat the material to create a solid, impenetrable seal. But, newer designs have adhesive strips that use a peel-and-stick application technique, allowing for a safer, faster installation. It's not uncommon to see hot-mopped asphalt or cold-applied adhesive applications either. The torch-down method is rarely recommended due to being a fire hazard.

Pricing

You can expect to be in the range of $4 to $8. If you are hiring a contractor, plan to spend $3 to $7 a square foot for labor and an additional $1 to $2 per square foot if you want more than one layer.

Attention: Large color selection.

Modified bitumen also comes in many different lighter colors, which gives it a higher reflectivity. So, you'll see savings on your energy bills as a result. MB is leak-proof, tear-resistant, seamless, and weather-resistant (due to the five-layer composition). And not only is it easy to install, but it's also easy to make repairs using bitumen patches.

Pro Tip: Modified bitumen will have two types of polymers - SBS (Styrene Butadiene Styrene) or APP (Atactic Polypropylene).

• SBS (Styrene Butadiene Styrene)

SBS gives MB more of a rubberized texture, giving it greater flexibility. In addition, this synthetic rubber modification provides better resistance to high winds, aging, oxidization, and temperature fluctuations. And it won't crack from the stress caused by contraction or expansion. 

Application of SBS-modified bitumen can be through the use of hot-mopped, cold adhesive or with self-adhesive backing. This material requires less heat to melt for installing, making for faster installation for all three methods.

• APP (Atactic Polypropylene)

APP makes the asphalt have a more plastic quality that melts into a liquid wax with a torch then mopped over the roof during installation. Due to high-temperature tolerance and easy melting, APP is a popular choice for smaller residential roofs and commercial buildings.

APP-modified bitumen has more flexibility in lower temperatures, allowing for resistance to stress caused by temperature changes. And it has a higher UV resistance and an insulating layer for better energy efficiency.

Single-Ply Membrane (EDPM, PVC, TPO)

A single-ply membrane goes directly on top of an approved substrate, which allows for faster installation and cheaper costs. Single-ply products have a longer lifespan and perform better than BUR roofs, making them a popular choice for flat roofs. There are 3 frequently-used types: PVC, TPO, and EPDM.

PVC - Polyvinyl Chloride

PVC - also referred to as vinyl - is a mixture of chlorine and ethylene to create a fire-resistant, waterproof, recyclable roof covering. It was the first single-ply membrane to receive ASTM International (ASTM D4434) safety standards.

This single-ply membrane has a reinforcement scrim covered with two layers of PVC. The top layer offers flexibility and UV protection, while the bottom layer of black or gray PVC has a higher concentration of plasticizers. 

PVC application can be ballasted, adhered, or mechanically fastened with a lifespan of twenty to thirty years with minimal to no maintenance. And since PVC is resistant to fire, punctures, precipitation, impacts, with 80% solar reflectance, it's one of the most durable materials you can use for roof protection.

Pricing

If you choose to install a PVC roof, you can expect costs between $4 and $7 for installation. But the price can go up to $8 to $12 or higher a square foot, depending on thickness.

TPO - Thermoplastic Polyolefin 

TPO is a white fabricated membrane of polypropylene and ethylene-propylene - two rubbers - to create a flat, single-ply sheet. The white color makes TPO better at reflecting solar heat, which allows for easier cooling of the interior.

Installation is a simple process using mechanical attachments or adhesive strips. And because of the flexibility, TPO is an ideal product for roofs with odd angles and obstructions. 

The single membrane of a TPO piece consists of a cap layer and a core membrane. The cap layer has 75% TPO polymer mixed with 25% to 35% fire retardants, UV stabilizers, and a small portion of pigments. The core is also TPO, but with 15% recycled material and no UV protection (not needed). 

Pricing

TPO can cost you $5.50 to $6 a square foot, depending on thickness.

Pro Tip: If you like using products made from recycled materials that are 100% recyclable after you've used them, then TPO roofing could be your key to doing your part to save our planet.

EPDM - Cured Ethylene Propylene Diene Monomer

EPDM is a type of rubber roof that's a suitable replacement for using BUR roofs. EPDM has been used for roofing membranes since 1965. 

EPDM roofing is available in black or white, with options for reinforced or non-reinforced. It comes in sheets with widths of 10 to 50 feet and up to 200 feet in length, with thicknesses from .045 to 0.9.

Around 50% to 60% of EPDM materials consist of equal portions of carbon black (UV protection) and polymer (for flexibility). 

The other ingredients allow EPDM to be fire-resistant and stable. Installation can be done with mechanical fasteners, an adhesive, or by a ballast system - using pavers or natural stone to hold the material in place. 

Pricing

EPDM can cost you between $6.50 and $12.50 a square foot for a standard flat roof. 

SPF - Spray Polyurethane Foam

SPF - spray polyurethane foam roofing is an excellent Liquid-Applied Systems solution for flat roofs due to its ability to get into every nook and crack to seal the top completely. 

SPF is also easy and quick to apply and can go over the top of existing roofing materials, like shingles, metal, the roof deck, or a suitable substrate. Polyol and isocyanate chemicals combine and come out as sprayable liquid foam. 

After cleaning the roof thoroughly to remove any contaminants, the top gets a coat of foam pumped out of a sprayer. As it contacts the roof, the foam expands to 1 to 1.5" in height to create a waterproof seal. 

Once the foam dries, one or more elastomeric or silicone coatings go over the top to protect the roof from fire, UV damage, and weather elements. First, there is typically the SPF layer, a base coat, then a reflective topcoat. Some contractors may even top the foam layer with granules. 

Benefits of an SPF roof include improved energy efficiency (has 6.25 R-value per inch) - many of which can qualify as cool roofs - seamless construction for waterproofing, low maintenance and repairs, and a long lifespan of 20 years with additional years possible by recoating. 

These roofs do need to receive professional inspections twice a year. And you'll need to give it a good examination after severe storms.

Pricing

SPF can cost $3 to $4.50 a square foot for installation, including materials and labor fees. While it is possible to DIY an SPF roof, most experts recommend leaving the job to a professional to make sure your roof gets appropriately installed and has a warranty.

BUR vs. MB vs. Single-Ply vs. SPF

When you line all of these materials up side-by-side, you'll notice some similarities and some differences. 

While BUR is one of the oldest and most-tried roofing methods, the installation process is the worst. However, modified bitumen gives you all the benefits of BUR without the harmful chemicals, and it's the easiest to DIY install. 

Single-ply material TPO offers the most budget-friendly choice out of all three types. Both EPDM and PVC offer high solar reflectivity. SPF gives the most waterproofing, but it does require a top coat to protect the foam from UV rays, impacts, and other damage.