A number of potential problems can stem from the design, application, aging and environmental factors that affect built-up roofs. The following information will give you a very basic background on this roofing method that may help you to understand what you’re looking at during inspections and allow you to explain more clearly to your clients the limitations of inspecting built-up roofing.

Much of this information includes discussion of materials that will not be visible during the inspection, but understanding the materials and methods involved may help inspectors to differentiate natural aging from premature failure or the use of improper construction materials or methods.

Remember that you are NOT REQUIRED to identify the cause of problems that you identify. You need to accurately:

  1. Identify defects and deficiencies;
  2. Explain how serious they are; and
  3. Recommend a course of action.


About Manufacturing Standards

Although ASTM standards exist for many (but not all) materials commonly used in built-up membrane roofing, inspectors will not be able to tell by visual examination whether the materials they see comply with any standards. If an inspector sees what appears to be premature failure, the use of sub-standard materials is a possibility.


Built-up Roof Membrane Components

The main components used in constructing built-up membranes are:

  • Bitumens;
  • Reinforcement layers (felts and ply sheets);
  • Membrane flashings
  • Accessories

A built-up roof is a single membrane consisting of multiple plies, each of which acts as reinforcement. Plies may be saturated felts, coated felts, fabrics, or mats. Plies are installed shingle fashion, overlapping both at their edges and along their lengths.

Built-up membranes may consist of several types of material layers:

  2. PLY SHEETS; and



Base sheets may function to:

  • Separate (bond break) the membrane from the roof deck;
  • Provide support for the membrane over a slightly rough or uneven roof deck;
  • Hold fasteners securely to anchor the membrane to the roof deck;
  • Serve as a base layer for adhering rigid insulation board on nailable roof decks;
  • Resist wrinkling;
  • Provide good tensile strength and dimensional stability; and
  • Provide fire resistance.


Common base sheets are:

  • Asphalt-coated fiberglass mat base sheet

These are manufactured using non-polymer modified asphalt to coat fiberglass mat.


  • Asphalt-coated fiberglass venting base sheet

Venting base sheets are manufactured the same as asphalt-coated fiberglass mat base sheet but have mineral granules embedded on the underside of the sheet to vent certain types of roof decks (substrates). They may be adhered to the roof deck through pre-punched holes or with spot-mopping. Solid mopping would prevent the venting they are designed to provide.


Slip Sheets

Base sheets installed over a wood roof deck are separated from the deck by a slip sheet that prevents the base sheets from adhering to the deck. This is done to avoid having moisture-related movement in the wood deck cause bucking or splitting in the built-up roof membrane.



Ply sheets are installed over base sheets or over rigid board insulation as “interply” sheets in built-up roof membranes. Since at least about 2006, mat reinforcement is fiberglass-based. In the past, cellulose-based “organic” mats were also used. But this material has been phased out, at least in North America. The weight and design of the fiberglass mat and the amount and properties of the asphalt used in coating the mat will both affect the durability and lifespan of the roof membrane.


ASTM provides standards for the properties of the mat reinforcement and the asphalt coating for some but not all products. The performance characteristics of products for which no standards exist may vary widely.


The following are modern ply sheet materials:

  1. Asphalt Fiberglass Ply Sheets:

Asphalt fiberglass ply sheets are the most commonly used ply sheets in North America.


  1. Asphalt-coated Fiberglass-mat Base Sheets:

Products used as base sheets in built-up roof membranes may also be used as membrane plies, particularly in construction of cold-applied membranes.


  1. Asphalt-coated and Polyester Fiberglass-mat Sheets

Combining polyester and fiberglass into the reinforcement mat creates a composite mat with enhanced strength and flexibility. Asphalt-coated and polyester fiberglass-mat sheets may be used as base or ply sheets, or as flashing.


Plies are installed shingle-fashion, so that each layer overlaps adjacent layers both at the ends and along their length.



Plies may be adhered to each other using one or more of a variety of bituminous materials;


About Bitumens:

Bitumens are engineered asphaltic materials produced by the distillation of crude oil during the petroleum refining process. They are non-volatile (won’t boil or rapidly evaporate crucial chemical components) at ambient (indoor/outdoor) temperatures. At the refinery, raw asphalt- called “flux”- is heated to about 500°F. In a process called “air entrainment”, air is blown through the liquid mix. The amount of time entrainment takes place produces different properties in the asphalt, such as viscosity (flow rate) and hardness.

Proper viscosity and temperature during application are crucial and are specific to different types of project conditions and for effective long-term performance.

Polymer-modified bitumens were developed in Europe in the 1960s and have been used in North America since about 1975. They generally fall into two categories:

  1. Atactic polypropylene (APP)-modified gives bitumen plasticized properties.
  2. Styrene butadiene styrene (SBS)-modified gives bitumen rubberized properties.


The main bitumens used to adhere reinforcement plies together are:

  1. Cold-applied asphalt adhesives;
  2. Cold-applied polymer-modified asphalt adhesives;
  3. Hot-applied asphalt; and
  4. Hot-applied polymer-modified asphalt.

Polymers are molecular compounds added to materials to improve certain properties. In roofing materials, they may:

  • Improve durability;
  • Improve flexibility;
  • Improve low temperature performance
  • Improve fire resistance;
  • Improve resistance to ultra violet (UV) damage from sunlight;
  • Increase lifespan;
  • Lower cost;
  • Reduce weight,


  1. Cold-applied asphalt adhesives

Cold-applied asphalt adhesives are thinned (“cut back”) with solvents, which allow them to be more easily spread between reinforcement plies. A variety of other additives are available. These may include stone powder or fibrous fillers, stabilizers, surfactants (compounds that lower surface tension to improve bonding), or viscosity modifiers.

Application may be by spray, brush, or squeegee, depending on product, manufacturer, and project/environmental considerations. Cold applied asphalt adhesives must be specified by designers and installed by applicators according to the manufacturer’s recommendations. Although inspectors may see the results of failures due to improper design or installation, they probably will not be able to accurately identify the cause, since a large variety of causes are possible, and identification lies outside the scope of the General Home Inspection.


  1. Cold-applied polymer-modified asphalt adhesives

Polymer-modified asphalt adhesives use asphalt that has been enhanced by the addition of polymers to improve one or more performance characteristics.

Polymer-modified asphalt is available in a wide variety of formulas that may include from high to low percentages of solvents and asphalt.

Like with cold-applied asphalt adhesive, application of cold-applied polymer-modified asphalt adhesives may be by spray, brush, or squeegee. Cold-applied polymer-modified asphalt adhesives must be correctly specified by designers and installed by applicators according to the manufacturer’s recommendations. Again, inspectors are not responsible for identifying the causes of defective conditions they find.

Cold-applied polymer-modified asphalt adhesives may be used to bond plies and cap sheet, or as a top–pour application to be exposed to weather.

Polymers commonly used to modify cold-applied asphalt are:

  • Styrene ethylene butadiene styrene (SEBS)
  • Polyurethane; and
  • Polyether


  1. Hot-applied asphalt

Using this method, asphalt is brought to the jobsite as solid rolls that are melted in a kettle. The liquid asphalt may be spread across plies using mops or with mechanical spreaders. Proper temperature is crucial to achieving a viscosity that will allow proper spreading and to providing an effective bond between plies.


  1. Hot-applied polymer-modified asphalt

Hot-applied polymer-modified asphalt roof membrane systems are available that may be used to bond plies or as a top–pour application to be exposed to weather. These systems use conventional roofing asphalt that has been modified by styrene ethylene butadiene styrene (SEBS).


Benefits can include:

  • Low temperature flexibility;
  • UV resistance;
  • Softening point;
  • Flash (ignition) point;
  • Penetration resistance; and
  • Damage from thermal cycling.
  • Fewer plies needed




Asphalt-Coated Coated Cap Sheet Surfaced With Mineral Granules

The weather surface of built-up roofs is often a mineral surfaced cap sheet that consists of a fiberglass mat, embedded in- and coated on both sides with- roofing asphalt. The weather surface is covered with mineral granules to help protect the asphalt from UV damage from sunlight. Cap sheets should meet or exceed the manufacturing standards for roll roofing.


Flood Coat

A flood-coat of bitumen is sometimes used instead of a cap sheet. Typically, it is polymer-modified to help resist weathering and is often protected from UV damage from sunlight by installation of a layer of gravel.



Bitumen-based Sheets:

Several types of bitumen-based membrane flashings are used to protect areas where roof membranes end, like at the roof edges, parapet walls, and roof penetrations. Flashings typically consist of a base or backer layer(s) and a cap sheet. Additional components may also be used. Membrane type flashings are all are polymer-modified.


Liquid-applied Flashings:

Liquid-applied flashings are one- and two- component moisture-cured resin with low- or no- solvent content. Curing may require a catalyst. They typically consist of a base coat, polyester fabric reinforcement, and a topcoat. Surfacing like granules or a coating may be applied with some systems.



In addition to the components mentioned already, a number of accessory components may be used:



“Sizing” is the process of coating paper with a material that increases its resistance to absorbing moisture. Reddish colored rosin-sized building paper has been used in building construction since about 1850. No asphalt is used in its manufacture, but instead it is coated with rosin.


In modern built-up membrane roofing rosin-sized felt paper is used as a slip sheet to prevent the base sheet from adhering to wood roof decks. A base sheet adhered to a wood deck may telegraph expansion or contraction of the deck as the wood reacts to changes in moisture content. Adhesion could result in buckling or splitting of the membrane above joints between deck panels.

Rosin-sized building paper also prevents bitumen from dripping through the roof deck.



Polyester mats and fabrics of different weights are used as reinforcement in some manufacture’s built-up roof membrane systems. Polyester fabrics are used in both hot- and cold-applied systems.



Bituminous roof cements are typically roofing asphalts that have been thinned (cut back) with solvents. You’ll hear them called roof mastic, roofer’s cement, or plastic cement. Only asphalt-based roof cement should be used with built-up roof membrane systems.

Roofing cements are available as horizontal- and vertical-grade:




Horizontal-grade cements may contain less filler, more solvent, and have a lower softening point than vertical grades. Typical uses are embedding flashing, sealing roof penetrations, and making roof repairs.

Vertical-grade cements can also be used for repairs but have a higher softening point and are more resistant to flow when exposed to high roof temperatures.



Most manufacturers offer winter and summer version of their roofing cement that have had solvent content adjusted to improve application characteristics.


NOTE: Some manufacturers recommend specific types of asphalt or modified bitumen cements be used with their materials, so inspectors may see defects/deficiencies resulting from failure to use proper cements.



Polymer-modified bitumen adhesives are typically asphalt that has been solvent-thinned (cut back) and then blended with polymer modifiers and fillers.

Polymer-modified bitumen adhesives are available in high viscosity typically used to bond flashings to vertical or steep-slope surfaces, and low viscosity typically used for adhering polymer-modified bitumen sheets.

In certain situations, polymer-modified bitumen adhesives can be used to adhere roof insulation to substrates like base sheets or primed concrete roof decks.