Built up roof is a generic term for a roof system that is literally built by adding layers.
This is by far the oldest of the “modern methods” of roofing dating back to the early 1800’s. Formerly, the built up roof membrane was the waterproofing agent and the bitumen was the adhesive. With the advent of synthetic fibers, the rolls of the two principle elements have been reversed. Now, the bitumen is the waterproofing agent and the membrane is used to hold the bitumen in place. Build up roofs can be installed using different waterproofing materials including coal tar pitch, asphalt, cold application adhesive, and modified asphalt. All systems will utilize either base sheet, ply sheets (felts), or cap sheets in some combination. The roof has a variety of surfacing options ranging from gravel, asphalt, coatings, to mineral surfaced.
Felts
Glass-fiber Mats
The introduction of the glass-fiber mats redefined the built up roof system. Glass-fiber has almost completely replaced the organic felt across the United States. The glass-fiber has been able to reduce the formation of blistering and ridging. Glass-fiber does not absorb water like its counterpart organic felts reducing the tendency of water becoming trapped in a new roof system.
Organic Felts
Organic felts were the material used before the introduction of glass-fiber. Organic felts contain cellulose fibers, shredded wood, and felted papers. Organic felts are susceptible to water absorption which leads to increased blistering and ridging.
Saturated Felts
Organic, polyester, or glass-fiber is saturated with bitumen. Saturated felts generally serve as base sheets.
Coated Felts
Saturated felts that have been subjected to additional manufacturing and stabilizing with finely ground minerals to help improve their durability and resist deterioration in cold weather.
Mineral-surfaced Sheets
Coated felts that are taken another step in the manufacturing process. Mineral granules are embedded in the topside surface (also known as mineral cap sheet).
Shingling of felts (inter-ply mopping)
Shingle of felts is a common installation of built up roofs. Overlapping the felts to develop an achieved minimal thickness characterized by 2-ply, 3-ply, and 4-ply. It is said that each ply represents 5 years of service, so a 3-ply would indicate a 15-year roof system and a 4-ply would signify a 20-year roof system. This is only if the roof system is properly installed and maintained over its life span.
The most common built up roofs
- Bitumen Asphalt Bitumen’s are heavy and black or very dark brown. Asphalt is the dense residue left behind from petroleum distillation. There are several versions of bitumen asphalt ranging from type I (dead level) to type IV (special steep). Generally a type II is used on the field of the roof system and type III is used on the penetration and base flashings.
- Coal Tar Pitch Coal tar is derived from the distillation of high temperature coke oven tar, which is produced during refining of bituminous coal. It has a natural resistance to temperature changes, water, moisture, and vapor transmission. Its low melt point makes it naturally “self-healing.” Coal tar does have an odor associated with its heating and can cause some discomfort to the nose, eyes, and skin. Coal tar roofs must be surfaced with gravel in order to keep the bitumen in place. It is not recommended for roofs with slopes over 1/2″ in 12″.
- Cold Process An inexpensive tar/asphalt based semi-liquid material commonly called cold process. Used mainly for smaller jobs that by cost, or other restriction, does not require a more substantial roof (this material/application is being changed to create a better roofing system). Application similar to hot, but applied with a long handled brush or roller.