Bitumous Roof Membranes, What are bitumous roof membranes

Bitumous Roof Membranes speaks to what are bitumous roof membranes and explain bitumous roof membranes.

They consist of two types built up or modified bitumen.

Built Up Membrane

This type of roof is assembled in place from multiple layers of asphalt impregnated roofing felt bedded in additional layers of bitumen. The felt made from cellulose, glass or synthetic fibers is saturated with asphalt at the factory and delivered to the site in rolls.

The bitumen is usually asphalt derived from the distillation of petroleum, but for dead level or very low slope roofs, coal tar pitch is used instead because of its greater resistance to standing water.

Polymer modified asphalts as described for modified bitumen roofs may also be used. Both asphalt and coal tar pitch are applied hot in order to merge with the saturant bitumens in the felt and form a unified multi ply membrane. The felt is laminated in plies which are overlapping layers, to form a membrane that is two to four plies thick. The more plies used the more durable the roof.

To protect the membrane from sunlight and physical wear, a layer of crushed stone or other mineral granule aggregate is embedded in the top of the surface. Less common, a built up roof may be made from felt plies bedded in cold applied mastics which are solvent based asphalts. These are compounds of asphalt and other substances applied by spray or brush at ambient temperatures and then cured through solvent evaporation.

Modified Bitumen

This type of roof is made from factory manufactured sheets of polymer modified bitumens. Modified bitumens are asphalt materials to which compounds such as atactic polypropylene or styrene butadiene styrene have been added in order to increase the material’s flexibility, cohesion, toughness and resistance to flow. Modified bitumen roof membrane sheets are also reinforced with plastic or glass fibers or fibrous mats. The sheet thickness typically ranges from 0.040 to 0.160 inches.

Like a built up roof, modified bitumen sheets are assembled in place in overlapping layers to form a multi ply system, usually two or three plies thick. The sheets are bonded to one another in a number of possible ways.

With a torch applied membrane, as a roofing sheet is unrolled, an open flame apparatus to thermally fuse the underside of the sheet to the top surface of the substrate or underlying sheet. A hot mopped membrane relies on the application of hot asphalt to bond the sheets.

A cold process or cold applied adhesive membrane uses liquid adhesives and a self adhered membrane relies on factory applied adhesives.

The top or cap sheet in a modified bitumen roof is surfaced with mineral granules, thin metallic laminates or asphaltic or elastomeric coatings for greater resistance to ultraviolet deterioration, wear, and fire. Cap sheets with reflective white coatings that comply with cool roof standards are also available.

In comparison to built up roofs, modified bitumen roofs combine the toughness and redundancy of multi ply field application with the improved material qualities of factory manufactured sheets. Built up and modified bitumen systems may also be combined with a modified bitumen cap sheet applied over several plies of built up roofing to create a hybrid membrane bituminous roof.

Bitumous roofing systems account for approximately 40 percent of the market for low slope roofing membranes, with the larger portion of this share belonging to modified bitumen systems.

Bitumous Roof Membranes

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