70 Flat Roof Facts
Are you thinking about installing a flat roof on your home or commercial property?
Flat and low slope roofs look attractive on a wide range of architectural styles, especially on modern homes and buildings. Many traditional style homes also have sections of flat roofing, such as on porches/balconies, garages and over shed dormers.
In addition to aesthetics, a roof should serve its main function of protecting your property against the elements and remain intact for years to come.
Most flat roofs are typically not as long lasting or durable as many sloped ones, because the latter shed snow and rain a lot quicker. However, over the last few decades new and advanced flat roofing membranes have been developed that are highly durable, energy efficient and can compete with sloped roofs in longevity.
Here are 70 facts about flat roofs to help you choose the best material for your needs and budget.
Flat Roof Materials
1. There are three general types: 1. Single – Ply 2. Built-up 3. Spray-on/paint-on
Single-Ply Roofing
2. Single-ply means just one layer of membrane as a waterproofing and weather surface. EPDM (Ethylene Propylene Diene Monomer), PVC (Polyvinyl Chloride), and TPO (Thermoplastic Olefin) are the major types of single-ply membranes.
3. Today, single ply membranes are the most popular flat roof materials used in both commercial and residential construction.
4. Single ply roofing is not all equal in quality and longevity. EPDM, TPO and PVC all have very different formulations, durability specs and cost.
5. Membranes range in width from 6-18 feet. The ones most commonly used are around 10 feet.
6. The average thickness ranges from 45-90 mil. It is typically recommended to install a membrane that is at least 50 mil thick.
Built-up Roofs
7. Built-up roofs are manufactured using built-up layers of either tar-saturated paper and liquid tar with gravel, asphalt, rolled asphalt, or modified bitumen. As the roof is installed, more and more layers are added to increase durability and longevity, hence the name “built-up”.
8. Among the built-up roofs are: 1. Tar & Gravel 2. Modified Bitumen 3. Rolled Asphalt
9. Built-up roofs are considered to be outdated by modern construction standards. These materials are almost never installed on large commercial properties, but continue to be installed on residential homes, due to their low cost.
10. Any type of built-up roofing requires a positive roof slope ( at least 1/12 inches).
Spray-on/Paint-on Roofs
11. There are two main types of spray-on roofs:
- Spray Foam Insulation (sprayed directly onto the roof deck and then coated with acrylic or urethane coating, as well as a layer of crushed stones/sand.
- Roof coatings (manufactured for existing roofs and used to extend their service life by 10-15 years). They are typically installed on top of single ply, modified or low slope metal roofs.
Cost of Materials
12. A flat roof costs less than a pitched/sloped one.
13. EPDM cost of material is about $1.50 per sq foot (excluding insulation), making the cheapest single ply membrane.
14. TPO is about $1.70 per sq foot (excluding insulation).
15. PVC is about $1.90 per sq. foot (excluding insulation).
16. Modified bitumen and rolled asphalt roofs cost between $1.30-2.20 per sq foot, depending on the number of plies (excluding insulation).
17. Tar roofs are installed extremely rarely, have to be special ordered and therefore no material prices are readily available.
Flat Roof Installation
18. Single ply roofing is either fully adhered or mechanically attached.
19. Fully adhered method of installation consists of gluing the membrane directly to insulation, or to the substrate.
20. Mechanically attached method consists of fastening each row of membrane to the roof deck with roofing screws and barbed plates. Fasteners are typically spaced 12 inches O.C.
21. The most common problem with the fully adhered installation (typically EPDM rubber) is that the seams are poorly glued, there are bubbles in the membrane, and leaks inevitably result.
22. Torching is used in the installation of Modified Bitumen, and therefore can be a fire hazard to your home or commercial property.
23. EPDM is the most commonly installed single-ply membrane both for residential and commercial applications.
24. EPDM rubber requires no special equipment for installation.
25. TPO is the second most installed flat roof material in US.
26. It is critical to find a contractor who specializes in single ply roofing to ensure that there will be no leaks after installation. Many customers make the mistake of hiring contractors specializing in sloped roofing (asphalt shingles), who have little to no experience with complicated flat roofs.
27. It is best to do a full tear-off before installing a flat roof, to eliminate any moisture and replace any rotted substrate.
Cost of Installation
28. Installation prices quoted below are for very simple roofs, over 1,000 sq. feet. Your total cost will vary depending on many variables, such as roof size and complexity, geographic region, etc. Any additional insulation and details will increase the cost.
29. PVC roofing cost 6.50 – 7.50 + per sq foot installed.
30. EPDM rubber cost $5.50 – 6.50 + per sq foot installed.
31. TPO roofing cost $6.0 – 7.0 per sq foot installed.
32. Modified bitumen and rolled asphalt roofs cost $ 5.50 – 6.50 + per sq foot installed.
Longevity
33. PVC material is considered to be the longest lasting among flat roofs. Because of heat-welded seams, PVC remains flexible throughout its lifetime and can also withstand thermal movement.
34. PVC can last well over 30 years. Manufactures offer a 25 year warranty for commercial installations and some offer LIFETIME warranty for residential (IB, Flex, Fibertite). Here is a good article on real world performance of PVC roofs: http://www.coolflatroof.com/flat-roof.php
35. EPDM lasts about 10-15 years. Even though manufacturers offer warranties that are up to 40 years for commercial roofing (none for residential), its important to be aware that these warranties do not cover failing seams or ponding water. Both of these are the primary causes of failure for EPDM rubber.
36. Longevity of TPO roofing varies greatly due to issues with membrane’s formulation, ranging from 7-20 years. Manufacturer warranty is up to 20 years for commercial TPO installs and none for residential.
37. Manufacturer warranties offered on different types of flat membranes vary greatly depending on membrane’s thickness, size, method of installation, and environmental conditions.
38. Tar & Gravel and Modified Bitumen roofing systems have a lifecycle of 10-20 years, but are very costly to remove at the end of their service lives.
Flat Roof Durability
Leaks / Moisture
39. The biggest problem for most flat roofs is the presence of seams and flashing, because that is precisely where leaks occur.
40. PVC and TPO are the only two membranes that have hot-welded seams that will never come apart, and therefore do not allow any moisture to penetrate.
41. Tar and Gravel, Modified Bitumen and Rolled Asphalt offer extremely poor protection against leaks and ponding water.
42. EPDM rubber typically fails at roof penetrations, flashing and seams, allowing moisture to penetrate.
43. One of the biggest issues with durability of spray-on flat roofs is that the insulation can be eaten by birds, therefore resulting in leaks.
Puncture Resistance
44. Built-up roofs are typically 0.5 + inches in thickness, are made from hard materials and are therefore very difficult to puncture.
45. Single plies and spray foam roofs are generally easy to puncture by direct contact with a sharp object.
46. Some PVC materials come with a fiberglass reinforcing scrim, which makes it very difficult to puncture.
Other Important Durability Considerations
47. PVC membrane is considered to be the most durable flat roofing material. It is especially formulated to remain intact in a wide variety of adverse weather conditions (rain, snow, wind, sun, hot and cold temperature fluctuations).
48. Another reason why PVC is very durable is because the top and bottom ply of the membrane have almost equal thickness.
49. TPO has questionable durability, as there is no consistent formula amongst manufacturers, who are experimenting to make it as durable, but also cheaper than its direct competitor, PVC.
50. One factor that compromises TPO’s durability is the fact that the top ply (weathering surface) of the membrane (which ensures durability) is actually thinner than the bottom ply. In most TPO membranes the ratio is 40/60.
51. TPO roofs offer weak resistance to heat and solar overload.
52. Increasing the width of a single ply membrane DOES NOT improve its ability to protect against leaks, but can extend its overall life, and makes it more puncture resistant.
Maintenance and repair
53. PVC membranes are easy to maintain and repair. Their heat-welded seams remain weldable throughout their service life, which makes any repair, maintenance or modifications to install new equipment fast and simple.
54. Typically repair of a heat – welded membrane, such as TPO or PVC consists of welding a patch on the membrane over the hole that has been discovered as the source of a leak, or re-welding a void in the seam.
55. If no leaks occur, its still recommended to re-seam (apply 6 inch cover tape over all seams) an EPDM rubber every 7-10 years, when adhesive starts to naturally fail. Repairs are done using EPDM peel & stick flashing + EPDM primer (recommended method) or using older black glue. Penetrations are repaired using uncured flashing that can be stretched to form around angled surfaces.
56. Built-up roofs are very difficult to repair, or even to find a leak.
Unique Applications/Benefits
57. A flat roof is the best material on a very large size building. Installing a pitched roof on such a building is not recommended, because it will be a lot more difficult, and costly. It will also require special engineering to accommodate the additional structural weight, as well as rooftop HVAC equipment and drainage.
58. A flat roof can be utilized as a deck, green roof (rooftop garden), patio or penthouse.
59. Green building enthusiasts prefer flat roofs, because they can be turned into rooftop gardens.
60. In green roofing applications, it is best to use 80 mil or thicker PVC or TPO membrane, because hot-air welded seams ensure that the roof will not leak under the soil and plants.
61. A flat roof can offer great convenience in cases when large HVAC equipment needs to be installed. It can go straight on the rooftop, instead of being installed on the ground.
62. Flat roofs offer great potential for large-scale solar PV installations, as they are typically not shaded and get great sun exposure throughout the day.
Energy Efficiency
63. TPO and PVC are both considered to be highly reflective, reflecting up to 90% of heat back into the atmosphere. This means that one of these membranes can help you save hundreds of dollars on your energy bills. Both are Energy Star Rated for their cool properties.
64. White roofs collect a lot of dirt and dust over their lifetime, thereby decreasing their energy efficiency. However, they still remain highly reflective.
65. Darker color PVC and TPO membranes are still formulated to have light-reflective properties and offer energy savings.
66. Modified Bitumen and EPDM offer the lowest energy efficiency. In fact, because they are so dark, they can heat up to 170 degrees in the summer, making your HVAC system run over-time and rake up huge energy bills.
Environmental Impact
67. Both TPO and PVC are 100% recyclable. At the end of their service lives, old membranes get recycled back into making new ones.
68. It is possible to install PVC over an old roof, thus eliminating tear off, which cuts your costs and saves landfill space.
69. EPDM, Tar & Gravel, Modified Bitumen are not eco-friendly and cannot be recycled, because they are made from petroleum based by-products. Therefore, they have to be landfilled at the end of their service lives.
70. A rooftop garden installed on a flat roof improves air quality and reduces the heat island effect in large urban areas.
