Commercial Roof Substrates Overview

A substrate is the surface upon which a roofing system is applied. It can be the roof deck, an insulation system or a complete preexisting roof system. Choice Roof Contractors select particular Conklin roofing systems for roof repairs and restoration projects depending on the substrate that currently exists on the commercial roof.

Built-Up Roofs

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Built-up roofs (BURs) are constructed by alternating roofing felt layers with coats of either coal tar pitch or asphalt. The number of roofing felt layers with coats of either coal tar pitch or asphalt. The number of roofing felt plies varies, as does the weight of the felts themselves. Roofing felts come in weights of fifteen, thirty, forty-five, and ninety pounds – indicating the weight of 100 square feet of one ply of felt. Most often, a roof either has a smooth surface, with a top flood coat of asphalt or coal tar pitch, or a top layer of gravel set into the asphalt while it is still liquid.


The word bitumen refers to either asphalt or coal tar pitch used in creating built-up roofs. Coal tar pitch is a by-product of coal. To successfully install coal tar pitch, a roof’s slope must be ½” per foot or less. This is because pitch tends to liquefy and slide or “sag” off steeper roofs upon exposure to the sun’s heat.

Modified Bitumen

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Membranes are interesting hybrids that incorporate the high tech formulation and prefabrication advantages of single-ply with some of the traditional installation techniques used in built-up roofing. These materials are factory-fabricated layers of asphalt, “modified” using a rubber or plastic ingredient for increased flexibility, and combined with reinforcement for added strength and stability.

There are two primary modifiers used today: APP (atactic polypropylene) and SBS (styrene butadiene styrene). The type of modifier used may determine the method of sheet installation. Some are mopped down using hot asphalt and some use torches to melt the asphalt so that it flows onto the substrate. The seams are sealed by the same technique.


Asphalt is a by-product of petroleum. There are four types of asphalt, which are selected according to the slope of a given roof.

Type I asphalt (also known as dead level asphalt) can be used only when slopes are ¼” per foot or less. These asphalts are said to be self-healing. On warm sunny days, type I asphalt may soften and even become liquefied. It may fill any cracks or voids that have occurred, which is beneficial, but may also slide off sloped roofs, which is disastrous.

Type II asphalt can be used on slopes of ¼” to 1” per square foot.

Type III asphalt is a sag-resistant material suitable for slopes ranging from ¼” to 3” per foot.

Type IV asphalt is known as “steep asphalt” because it softens only at the highest temperature range. Resisting sagging better than any other asphalt type, it can be used on slopes up to 6” per foot.

Note: A roof showing any evidence of sagging is an unstable substrate and must not be coated until the problem has been corrected. This usually calls for removal of the sagging portions and replacement with sag-resistant roofing materials.

CAUTION: The two bitumens, asphalt and coal tar pitch, cause a strong chemical reaction when they come into contact and should never be used on the same surface. Coal tar pitch is also very hazardous to the mucous membranes and requires great caution in handling. Skin and eyes should be well protected and filtering masks worn when working with this substance.

Surfaces of Built-up Roofs

A built-up roof may have any number of different surfaces. A quick overview follows:

A “glaze coat” (flood coat) or asphalt can be spread over the surface of the roof membrane. To be effective, this must be a thin layer – no more than twenty-five pounds per 100 square foot. Thicker, heavier applications are likely to flow and crack over time.

Installation of “aggregate” (a top later of stone or gravel) is a popular strategy for protecting the black flood coat against the sun’s heat and ultraviolet light while permitting light foot traffic on the roof surface. Common aggregate materials include gravel, sag, crushed stone, limestone, marble chips and volcanic rock. A top pour coat of bitumen, at a rate of about sixty to seventy-five pounds per 100 square foot, is then applied. It is immediately followed by about forty pounds per square foot of aggregate. Although this system has its strengths, its drawbacks include the considerable structural load it places on the roof deck. It is for this reason that “stacking” built-up roofs is a risky practice.

NOTE: Aluminized surfaces offer insufficient adhesion for Conklin coatings. They must be removed by power washing or sandblasting before direct application can take place.


Mentioned previously as a roof decking, plywood also can be used as a substrate over an existing roof. A sound, stable substrate, plywood offers excellent adhesion for Conklin coatings. Plywood panels bearing the American Plywood association stamp are recommended and should meet minimum requirements of 15/32” Exterior – CC plugged or better and rated for exterior exposure.

Because plywood panels have a tendency to expand and contract with changes in temperature, a gap of 1/8” must be left at all panel edges to allow for movement. Failure to leave such gaps can lead to buckling of the panels. These gaps must be thoroughly caulked, then reinforced with SpunFlex, Conklin’s 4” reinforcing fabric. To lend extra strength and durability to the system, a 38” SpunFlex should be embedded throughout the roof.

However, when left unreinforced, plywood is apt to develop “grain checking” (a small crack in the wood grain). This phenomenon is inherent in the wood itself, and is not preventable by the plywood mill. Another common problem with plywood is “delamination,” wherein the individual layers of the panel come unglued and begin to pull apart.

Oriented Strand Board (OSB)

Oriented Strand Board (OSB) consists of panels of compressed, strand-like particles arranged in layers oriented at right angles to one another. Some OSB panels bearing the APA stamp are recommended and must meet the 7/16” minimum thickness requirements. Contact your Conklin product specialists for a current approved substrate list. Conklin recommends several panels and must be approved for application.

Contact the Conklin Building Products Department for a current approved substrate list. Conklin recommends several panels and must be approved for application.

Spray-in-Place Polyurethane Foam

Sprayed Polyurethane Foam (SPF) has quickly grown in the past several years due to its insulation characteristics. However, SPF performs several functions on a roof. Aside from its insulation value, it can be applied to provide slope on a previously flat roof. It also functions as a new substrate for the appropriate Conklin Roofing System.

Since Polyurethane foam degrades quickly and becomes chalky upon exposure to ultraviolet light, the protective Conklin coating must be applied within two to twenty-four hours after foam application. Conklin Roofing Systems are designed to be applied directly over foam, offering outstanding protection against ultraviolet light and moisture. Yet the success of these coatings is limited by the quality of the foam application. Optimum performance is achieved only when the foam substrate is properly applied by a professional.

The Spray Polyurethane Foam Alliance was formed to provide information on product development, technology and application techniques to the roofing industry. They are an excellent resource for all Conklin roofing contractors, especially those interested in acquiring the skills needed to apply foam with Conklin jobs. They can be contacted at:

Spray Polyurethane Foam Alliance
4400 Fair Lakes Court, Suite 105
Arlington, VA 22033
Phone: (800) 523-6154
Fax: (703) 222-5816

Metal Roofs

There are two basic classifications of metal roofing; structural and non-structural (also known as architectural). Structural metal roofing attaches directly to purlins or lathe boards and does not require any sort of solid support beneath it. Non-structural metal roofing requires a solid substrate beneath it, typically plywood, oriented strand board, or a metal roof deck.

Structural metal roofing is broken down into low slope and steep slope categories. Low slope products are available for roof pitches from 1/4:12 to 3:12 while steep slope products are designed for roof pitches greater than 3:12. Low slope structural metal roofing consists of interlocking panels, commonly called standing seam roofing, that run vertically on the roof surface. These products can have a painted, mill-finish, or clear acrylic finish. To ensure a watertight seal on roofs of less than 3:12 pitch, some products will require machine seaming during installation. These special machines are rolled along the panels to crimp the panel seams together.

Steep slope structural metal roofing is available in both vertical and horizontal profiles. The vertical panels include standing seam systems that are fastened to underlying purlins with hidden clips or fastening flanges. A wide variety of corrugated or tile facsimile metal roofs that are attached with exposed fasteners directly through the metal roofing panels are also available. These products overlap or interlock on their side and end laps to form a watertight seal. Special seaming machines are typically not required. Most non-structural metal roof panels are designed for roof pitches of 3:12 or greater. Rather than transmit gravity loads through to purlins or lathe boards beneath them, non-structural systems transfer gravity loads to the roof deck beneath them.

Non-structural systems are available in a variety of styles including vertical standing seam, corrugated, and tile profiles as well as a wide variety of horizontal panels. The horizontal panels simulate the look of standard shingles, wood shake, slate, and tile. Most non-structural metal roofing will have a coating for aesthetics and durability. Coatings include various paint finishes, such as Kynar or siliconized polyester. Other coatings or treatments include galvanized finishes, galvalume or stainless steel.

Single-Ply Membranes

Single-Ply Membranes are flexible sheets of compounded synthetic materials that are manufactured in a factory to strict quality control requirements. This is in contrast to other roofing, typically known as BUR (built up roofs), which utilize hot asphalt and other hazardous components as the roof is constructed in place. Single-ply roof systems provide strength, flexibility, and long-lasting durability. The inherent advantages of pre-fabricated sheet are the consistency of the quality of the products that are manufactured, the versatility in their attachment methods, and therefore, their broader applicability. The Single Ply Roofing Industry (SPRI) identifies three major categories of single ply membranes: thermosets, thermoplastics, and modified bitumens.

Thermoset membranes are compounded from rubber polymers. The most commonly used polymer is EPDM (often referred to as “rubber roofing”). Another thermoset material is neoprene, although this particular formulation is no longer widely used for roofing. Thermoset membranes are successful for use as roofing materials because of their proven ability to withstand the potentially damaging effects of sunlight and most common chemicals generally found on roofs. Hypalon is a unique material because it is manufactured as a thermoplastic, but because it cures over time, it becomes a thermoset. Hypalon materials are heat sealed at the seams.

Thermoplastic Membranes are based on plastic polymers. The most common thermoplastic is PVC (polyvinyl chloride) which has been made flexible through the inclusion of certain ingredients called plasticizers. Also available are TPO (thermoplastic polyolefin). A number of different products in this category are available, each having its own unique formula. Thermoplastic membranes are identified by seams that are formed using either heat or chemical welding. These seams are as strong, or stronger, than the membrane itself. Most thermoplastic membranes are manufactured to include a reinforcement layer, usually polyester or fiberglass, which provides increased strength and dimensional stability.

Single Ply membranes offer the flexibility of three means of attachment: Ballasted, mechanically-fastened and fully-adhered. If the structural part of the roof (the deck) can withstand the weight, a ballasted roof may be the best option. But if the slope of the roof is greater than 2” in 12,” then this system may not be appropriate. There may be other limitations to the use of a ballasted system, such as roof height, proximity to shorelines and other high wind zones, and availability of ballast. A deck that accepts fasteners easily, such as steel or wood, makes a good substrate for a mechanically fastened membrane. These systems can be designed to provide the necessary resistance to known wind forces and are not subject to slope limitations. Another alternative is the fully adhered system, in which the membrane is attached to the substrate using a specified adhesive. Depending on the membrane, the adhesive may be solvent or water-based. The finished surface of an adhered roof is smooth. Colored membranes may be used, which may make an attractive aesthetic contribution to the building’s appearance.

For additional information on single-ply membranes, the Singly Ply Roofing Industry (SPRI) is an excellent resource for objective information about various products and systems. SPRI offers many valuable documents and publications to help educate contractors, architects, and building owner and maintenance personnel about roofing and the many options that are available today. SPRI publications range from generic technical guidelines for design and application to general information about roof maintenance and emergency repairs.

Single Ply Roofing Industry
77 Rumford Avenue, Suite 3B
Waltham, MA 02453

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Written by: Ian Myers
Ian Myers
Ian was the very first team member brought on by the original co-founders of the Choice Roof Contractor Group to assist with nationwide growth. Since then he has written 120+ blog posts and provided excellent support to the roofing contractors in the group. Through his multiple roles in the commercial roofing industry, he has gained tremendous knowledge in the sector.