A built-up roofing system consists of multiple plies of roof felts that are laminated together with bitumen. Built-up roof material can consist of bitumen-saturated felt, coated felt, polyester felt or other fabrics. A surfacing is generally applied and can be asphalt, aggregate (gravel or slag), emulsion or a granule-surfaced cap sheet.
The purpose of this section is to you an idea of what a Built-Up Roof (B.U.R. for short) is and how it’s put together. The materials and information listed here are not comprehensive nor do they represent all types of built-up roof materials available.
Built-Up Roofing is one of the oldest and most reliable ways of installing a new roof. It was first known as composition roofing and started in the 1840’s. B.U.R.’s come in two basic types, asphalt and coal tar, and three basic components – (1) the waterproofing component, (2) the reinforcing component, and (3) the surfacing component which is used to protect the other components from the elements.
There are many different types of materials used in Built-Up Roofing. Some of these are:
- Asphalt-Coated Organic Base Sheet – Perforated and Non-Perforated
- Asphalt-Coated Glass Fiber Base Sheet
- Asphalt-Coated Glass Fiber Venting Base Sheet, Types I and II
FELTS AND FABRICS
- Asphalt-Saturated Organic Felt – Types I (A.K.A. No. 15) and II (A.K.A. No. 30)
- Coal Tar-Saturated Organic Felt
- Smooth-Surfaced Asphalt Roll Roofing – Types I, II, III, and IV
- Asphalt- or Coal Tar-Saturated Cotton Fabrics
- Asphalt- or Coal Tar-Saturated Woven B.U.R.lap Fabrics
- Asphalt-Impregnated Glass Felt – Types III, IV, and VI
- Coal Tar-Impregnated Glass Felt – Type I
- Thermoplastic Fabrics for Built-Up Roofing – Types I, II, III, and IV
- Asphalt – Types I, II, III, and IV
- Coal Tar – Types I, II, and III
- Lap Cement – A.K.A. Cold-Applied Liquid Adhesive, A.K.A. Solvent Based (Cutback) Asphalt – Type I (Grades 1 and 2), and Types II and III
- Mineral Aggregate – Gravel or Slag
- Asphalt Roof Coatings – Asbestos and Non-Asbestos
- Aluminum-Pigmented Asphalt Roof Coatings – Non-Fibered, Asbestos Fibered, and Fibered without Asbestos
- Emulsified Asphalt – Fibered and Non-Fibered
- Mineral-Surfaced Asphalt Roll Roofing (Organic)
- Mineral-Surfaced Asphalt Roll Roofing (Glass Felt)
Other items not listed are flashing materials, mastics, caulking material, fasteners, and roof insulation, to name a few.
Differences Between Built-Up Roofing System Materials
Now that you see the wide variety of materials available, you are probably wondering what the differences are. Venting base sheets are specially-designed for use with moisture-cured substrates such as light weight insulating concrete or poured gypsum, neither of which homeowners will have to worry about. Regular base sheets are generally used with nailable decks such as plywood. The differences in the felts is determined by the types of reinforcement materials used. For instance, polyester felts are stronger than fiber glass felts which are stronger than organic felts. The fiber glass felts will vary among themselves in quality with Type III being the lowest quality and Type VI being the highest.
Gravel or slag surfacings are probably the most popular. The next most popular surfacings are the granule-surfaced cap sheets, commonly called 90 pound because they used to weigh approximately that much per square. Now they weigh around 72 pounds per square. Then come the emulsions which can be spray, brush, or roller applied.
How Built-Up Roofing Systems Are Assembled
Base sheets are usually the first piece of Built-Up Roofing material installed and are usually mechanically-fastened (nailed) to the deck or substrate. Then come the felts. These can be installed with either hot asphalt or coal tar, or cold-applied liquid adhesive, a.k.a. solvent-based asphalt or “cutback” asphalt. Approximately twenty-five to thirty pounds of hot asphalt or twenty to twenty-five pounds of coal tar per 100 square feet (roof square) is used between each ply. Three to five gallons of the cold-applied adhesive is used per square. Last is the surfacing. With a cap sheet, the same amount of bitumen or lap cement is used to install the cap sheet as is used to install the plies. With gravel or slag, a flood coat of about sixty pounds per square of asphalt or seventy pounds per square of coal tar is applied and 400 – 500 pounds per square of gravel or 300 – 400 pounds per square of slag is embedded. Emulsion surfacings will vary. Usually it’s around three gallons per square that is used. If emulsion is what is preferred, an aluminum reflective coating should be applied after the emulsion cures to help reflect damaging UV rays.
Uses of Built-Up Roofing Systems
Built-up roofs can be installed over just about any type of roof deck as long as the proper substrate is used. BUR’s can’t be mopped to a wood roof deck. In this case a rosin sheet and base sheet are needed first. With steel roof decks, the thickness of the deck should be a minimum of 22 gauge and some type of approved insulation should be mechanically-attached to the deck to provide a substrate. Manufacturers should be consulted before mopping to Polyisocyanurate (“iso” for short) insulation. Many manufacturers will not warrant a roof if the felts are mopped directly to the iso and a coverboard such as wood fiber or perlite is needed. In some cases adhesives are being used to attach the insulation to the deck but be sure the roofing materials manufacturer is consulted before this is done.
With structural concrete roof decks, the roof is often mopped directly to the deck after it’s cleaned. Sometimes a thermal insulation is used to provide a substrate. Thermal insulation can be attached by mechanical means, adhesives, or even hot-mopped. If the insulation is hot-mopped with bitumen, then care needs to be taken so as not to allow the bitumen access to the interior of the building at joints in the roof deck. Also, if Isocyanurate insulation is used, only 4′ X 4′ sheets can be mopped due to its propensity to warp and curl. With lightweight insulating concrete, pre-cast gypsum panels, or poured gypsum, venting base sheets are attached with special fasteners and then insulation may or may not be installed over the base sheet and then the roof membrane installed. In some cases a vapor barrier will be necessary. Built-up roofs cannot be mopped directly to lighweight insulation or gypsum. Cement fiber roof decks should have a base sheet or insulation mechanically-attached to it before installing the roof. Insulation should be installed in two layers with all joints offset and staggered a minimum of twelve inches.
Service Life of Built-Up Roofing Systems
The service life of a roof is dependent on many factors: geographical location & weather conditions, foot traffic, materials used, conditions under which the roof was installed, slope of roof, type of surfacing material, etc. Under ideal conditions, a 3-ply built-up roof should last at least fifteen years, a 4-ply should last at least 20 years, and a 5-ply should last at least 25 years. This author has seen ten year old 4-ply roof that needed to be replaced and twenty year old 3-ply roofs that were still functioning.