Flat roofs are more accurately referred to as “low-slope roofs.” The lowest-sloped roof commonly used today has a slope of ¼-inch vertical “rise” for every 12 inches of horizontal “run.” This allows the movement of water on a roof toward an outlet such as a downspout or other type of roof drain. Roof slope also minimizes “ponding,” the accumulation of water in specific areas of a roof. Standing water can lead to deterioration of the roofing material or ice buildup, which can cause damage through the cycle of freezing and thawing. An appropriately-specified and well-installed low-slope roof can effectively serve a building for years.

Roofs by Frank Lloyd Wright can be especially challenging because it was not uncommon for his roofs to be constructed as close to flat as possible. While this may have been Wright’s design intent, it has the potential to cause a multitude of issues. An important consideration in the installation of a new roof on a low-slope Wright-designed roof is to allow for some slope, while respecting Wright’s design goals. This piece will focus on the options available for specifying roofing materials.

This post provides basic information on flat-roof materials available to building stewards today. Each material option has pros and cons and each restoration/maintenance/repair situation involving a Wright building requires analysis and research to identify the correct approach.

As is always the case when sharing technical information, the Conservancy distributes this data so it may be of use to others as they evaluate their own specific situation, and may consider these and other approaches. We strongly recommend that building stewards consult with design professionals and an experienced contractor to determine which method is best for their specific projects. The provision of this information or mention of specific products does not constitute endorsement, recommendation, preference or approval by the author or Conservancy. Particular hazards of some materials are noted here, but each process should be researched for risks before specifying or installing.

Roofing Material Types

 Built-Up Roof (BUR)

Description: A built-up roof membrane (BUR) is assembled in place from multiple plies (or sheets) of asphalt-impregnated felt bedded in bitumen. The felt may consist of cellulose fibers or glass fibers. It is saturated with asphalt at the factory and delivered to the site in rolls. The bitumen material is usually asphalt derived from the distillation of petroleum, but for dead-level or very low-slope roofs, coal-tar bitumen or coal-tar pitch is used for its self-healing qualities. Coal tar should be used with care though as it contains harmful chemical compounds. New coal-tar installations are rare.

Installation: Both asphalt and coal-tar plies are applied using a torch at high temperatures in order to merge with the bitumen in the felt and form a single-piece membrane. The more plies used, the more durable the roof. To protect the membrane from sunlight and physical wear, a layer of aggregate (crushed stone or other mineral granules) is embedded or applied to the surface; this layer is a ballast.

Cold-applied mastics can be used in lieu of hot bitumen in built-up membranes. A roofing mastic is compounded of asphalt and other substances to bond to felts or to synthetic fabric reinforced mats. The mastic may be sprayed or brushed on and hardens by the evaporation of solvents, typically in four hours, depending on temperature and relative humidity.

Advantages and Disadvantages:

  • Advantages:  BURs are relatively easy to repair. Their coverage of stone aggregate make roofs resistant to UV radiation, and provide good impact resistance and fire resistance. A BUR will last about 5 years per layer giving a 20-30 year life span for a typical installation, depending on the layers installed. There are no seams that need to be heat-welded or glued down. Typically maintenance costs are low (though installation costs can be high compared with other roof types).
  • Disadvantages: BURs require lots of time to install. Although leak repair is fairly simple, finding leaks can be an involved process because there are no seams to check and layers of gravel may need to be removed to locate the leak. If standing water is present, roofing materials can become damp, which can cause blistering. If the roofing material dries out, splits may occur. Ballasted roof systems can be heavy depending on how much rock is used. If you decide on a ballasted roof, you will need to ensure your roof structure can support the weight. Torch applications can significantly increase the risk of surrounding concealed building components, such as wood decking and joists, catching fire. The Conservancy strongly recommends caution when working with any sort of heat-applied building materials.  See the Conservancy’s post on risks during renovation.


Modified Bitumen

 Description: Modified bitumen roofing was developed in the 1960s, as a modification of the common built-up roofing (BUR) system. Modified bitumen flat roofing is a permutation of layers that include an asphalt emulsion base coat, a polyester/fiberglass reinforcing fabric, a base coat and a top coat.

Installation: Modified bitumen roofs may be applied either with or without heat using self-adhesive components.

 Advantages and Disadvantages:

  • Advantages: good leak resistance; UV and heat reflective; easily patched; some materials are recyclable.
  • Disadvantages: life expectancy of 10-20 years is relatively short compared with other roofing materials; standing water may cause modified bitumen to break down faster than other membrane roofs; potential objectionable fumes and smells. Torch applications can significantly increase the risk of surrounding concealed building components, such as wood decking and joists, catching fire. The Conservancy strongly recommends caution when working with any sort of heat-applied building materials.  See the Conservancy’s post on risks during renovation.

A modified bitumen roof installation at the Tonkens House

EPDM (Ethylene Propylene Diene Monomer) Single-Ply Membrane

Description: EPDM is a synthetic rubber roofing membrane. Its two primary ingredients, ethylene and propylene, are derived from oil and natural gas. EPDM is available in both black and white, and is sold in a broad variety of widths, ranging from 7.5 feet to 50 feet. 45 and 60 millimeters are common thicknesses, though 90 and 105 millimeter rolls are also available.


  • Fully adhered: The membrane roofing is rolled on to the roof to acclimatize. The underside is fully coated with a bonding adhesive. When both sides are tacky, the membrane is pressed onto the substrate with a push broom.  Adjoining sheets must overlap at least 3 inches. with laps spliced and cemented.  The membrane is mechanically secured at the perimeter and penetration edges. Flashing protects all edges, openings and penetrations.
  • Loose laid: The membrane is laid loose over substrate, either deck or rigid insulation, and ballasted in place. It is positioned without stretching, allowed to attain natural shape and adjacent sheets are spliced with at least 3” overlap.  Sheets are cemented and rolled together to seal seams. The membrane is mechanically secured at edges and penetrations, and flashing is installed. For ballast, a sufficient amount of gravel is laid over the membrane to provide 10 pounds per square foot.  Alternately a precast paver system can be applied.
  • Mechanically fastened: The membrane is directly attached to the roof deck with mechanical fasteners. The substrate is anchored to the roof deck, and the fasteners either go through both membrane and insulation or only go through the insulation and deck, with the membrane held down by retainer and cap over the base. Sealant protects against moisture infiltration at the fasteners.

Advantages and Disadvantages:

  • Advantages: EPDM roofing exhibits a high degree of resistance to ozone, ultraviolet rays, extreme temperature and other elements, as well as degradation from abrasion. It is resilient, strong, elastic, and less prone to cracking and tearing when compared to other forms of membrane roofing.
  • Disadvantages: Application methods, specific formulas and configurations for adhesives, fasteners, and coatings are unique to each system manufactured. Close supervision and regular inspection by manufacturer are a requirement.  Labor cost and time allotted for installation may vary.


PVC (Polyvinyl Chloride) Single-Ply Membrane

Description: PVC is commonly known as vinyl. As a roofing material, it typically comes in rolls that are 48 inches wide and 60 millimeters thick. It comes in gray, though other colors are available. It is available reinforced with fiberglass or polyester.


  • Loose-laid and ballasted: Not for slopes exceeding 2” rise in 12” run. Membranes are laid loose over loose-laid insulation and secured at perimeter and penetrations. Laps are heat-welded or chemically fused. The membrane is covered with ballast of river-washed stones (typically 10 lb/sf) or appropriate pavers.
  • Fully-adhered: No slope limitations. Secure membranes to substrate with bonding adhesive and by mechanically fastening membrane to perimeter and penetrations. This installation is appropriate for contoured (curved) roof and roofs that cannot withstand the weight of a ballasted system. The membrane can be directly applied to deck surfaces of concrete or wood, or be adhered to compatible insulation that is mechanically fastened to the deck.

Advantages and Disadvantages:

  • Advantages: Excellent weldability for lap joints and attaching to PVC-clad metal flashing. Relatively low cost. Damage to membranes can be repaired by welding a patch to an existing membrane using hot air or a solvent.
  • Disadvantages: Over time, patches can degrade. Weather restrictions apply during installation: 0-120 temperature range. Substrates and welding/bond surfaces must be dry.


TPO (Thermoplastic Polyolefin) Single-Ply Membrane

Description: TPO is actually one of a few different types of rubber, usually a blend of polypropylene and ethylene-propylene rubber.

Installation: TPO roofs may be mechanically or fully adhered using adhesives.

Advantages and Disadvantages:

  • Advantages: Mold growth, punctures and tears, and the accumulation of dirt are rare with TPO roofing membranes. TPO roofs are flexible and allow for shifting, settling, and thermal expansion and contraction. The lightweight sheets of TPO roofing membranes allow for ease during installation.
  • Disadvantages: TPO is a relatively new material so product research is still ongoing. Since TPO roofing’s introduction, seam failures and membrane curing and cracking have occurred in many instances. Different manufacturers produce widely-varying qualities of TPO roofing membranes. Be sure to select manufacturers that have sustained longevity in selling TPO roofs, since these manufacturers are more likely to offer a formula that ensures long-lasting products. Thickness seems to have little to do with the durability of this material. The uppermost part of a TPO roof is laminated, which introduces weak points, spurring shrinkage, cracking, and general deterioration over the course of time. Less costly TPO roofs are known to develop surface cracks rather quickly. TPO compounds have changed in recent years, so it is impossible to say how long current roofs will last, but a general estimate puts a TPO roof lifespan between 10 and 20 years.


Flat-Seam Metal

Description: Galvanized steel and copper (20 oz. rectangular cold-rolled copper) are the most widely used metals, with stainless steel and zinc rarely used. The panels are typically made from 18” x 24” sheets.

Installation: Flat-seamed roofs are used on very low-pitched roofs (or alternatively very high-pitched roofs) and have soldered seams where slopes are 3:12 or less. All four sides are folded over, two sides one way and the other two the opposite way. They are installed with clips that are fastened into the decking. In this way each panel can move independent of the others. After the panels are interlocked and clipped, the seams are laid flat; then all the seams are soldered. To accommodate the accumulation of expansion, roofs over 30 feet in the direction of continuous, longitudinal seams should be divided by expansion battens. These tapered battens should be spaced no more than 30 feet apart. They are covered with 20 oz. copper sheets in 8 to 10 feet lengths, locked and soldered together.

Advantages and Disadvantages:

  •  Advantages: Copper roofs, with proper routine maintenance, have a prolonged life-span, while the galvanized decks may last around 20 years. Shallow curved eyebrow roofs are typically done with galvanized panel, as are other complex areas of a roof.
  • Disadvantages: The soldering process can be difficult and requires an experienced roofing contractor to ensure there is a complete bond across all seams. Galvanic corrosion may occur when dissimilar metals are used adjacent to each other. This is an easy detail to overlook, especially when considering fasteners or flashing

A flat-seam metal roof installation at the Westcott House

Fluid-Applied Membranes

Description: Fluid-applied membranes are used primarily for domes, vaults, and other complex shapes that are difficult to roof by conventional means or with standard dimension materials. Materials applied by this method include neoprene, silicone, polyurethane, butyl rubber and asphalt emulsion.

Installation: Fluid-applied membranes are applied with a roller or spray gun, usually in several coats, and cure to form a rubbery membrane.

Advantages and Disadvantages:

  • Advantages: Systems can be installed over existing roofing, adding insulation and eliminating the need for the existing roofing to go into a landfill. Some systems are water-based and therefore more environmentally-friendly.
  • Disadvantages: Installation is labor-intensive and materials are costly. A skilled contractor is required. Products are very temperature-sensitive and are not compatible with all substrates, which can lead to early failure. The final appearance of the material may be incompatible with the aesthetics of the site.

It is important to keep in mind that a roofing material is only as effective as its details and installation. Leaking occurs more often at joints between materials and surfaces than in the middle of a surface. A future piece will look at the design and installation of flat roofs focusing on such elements as inconspicuous ways to increase slope, the design of flashing, and providing adequate ventilation to a roofing system.

For an interesting article o the use of fluid-applied membranes on a Wright-designed building, take a look at this article on an installation at the Pope-Leighey House.


  • Ballast: Materials that are impregnated in or installed over roofing materials. Ballast can be crushed stone, pavers or other materials. Its has multiple purposes. It can shield the material from UV deterioration and/or weight a non-mechanically-adhered material in order to resist uplift from heavy winds.
  • Low-slope roof: A roof whose slope is less than a 3-inch vertical “rise” for every 12 inches of horizontal “run.”
  • Ponding: Areas of low-slope roofs where water can accumulate. If the accumulation stands for a significant length of time (more than a few hours), moisture may begin to degrade the roofing material.
  • Self-healing: The ability of a material to seal punctures and other types of minor damage itself. For example, modified bitumen has the ability to self-heal, while metal roofs do not.
  • Torch-applied: The use of heating elements to install roofing materials. Torch applications can significantly increase the risk of surrounding concealed building components, such as wood decking and joists, catching fire. The Conservancy strongly recommends caution when working with any sort of heat-applied building materials.  See the Conservancy’s post on risks during renovation.

Posted December 7, 2022