What’s the difference between a Traditional Built-up roofing system and a Modified Bitumen system? (Part 2 of 2)
Modified Bitumen Systems
Modified Bitumen Systems (mod-bit), as their name suggests, generally use the traditional waterproofing medium – asphalt (aka bitumen) modified with the addition of various polymers to change the properties.
There are basically two types of modified system on the market today. One is known as APP (attatic polypropylene), the other SBS (styrene -butadien-styrene). SBS can be installed using hot or cold asphalt; however, modified membranes provide a third alternative. Heat can be applied to the underside of either type of modified membrane and the asphalt on that surface will become molten and adhere to the surface beneath it as it cools. This then becomes the waterproofing element of the assembly and the “glue” which holds it together. In the roofing lexicon, this is referred to as “torching”.
APP cannot be applied using hot asphalt. It is either torched or installed using cold asphalt.
The newest modified bitumen systems provide yet another alternative – self adhering plies. In this instance, the underside surface has a factory applied adhesive protected with a removable paper or plastic backing. The more popular name for this application is “peel and stick.” On “nailable decks” such as wood, metal or even lightweight concrete, the base ply is applied using a mechanical fastening system and the layers above are installed using peel and stick membranes, including the granulated “cap sheet” top surface ply. If insulation or other boards are mechanically fastened directly to the deck, the peel and stick can be adhered directly to that substrate.
- SBS (Styrene-Butadiene-Styrene) modifies the asphalt by forming a polymer network within the bitumen. SBS gives the bitumen rubber-like characteristics and improved resistance to aging and weathering. Most SBS-modified bitumen sheets are either set in hot mopping asphalt, torch-applied or adhered with cold-process adhesives. SBS-modified bitumen sheets that do not have factory applied granule surfacing need some form of field-applied ultraviolet protective coating.
- APP (Atactic Polypropylene) is a thermoplastic polymer which forms a uniform matrix within the asphalt. This enhances the bitumen’s performance by increasing its UV resistance, increasing its flexibility at low temperatures and improving its flow resistance at high temperatures. APP-modified bitumen sheets are generally applied using a propane-fueled torch.
Some brands of SBS Modified Bitumen membranes have a metal finish on the top surface. These are usually either copper or aluminum. These sheets are usually embossed with a waffle-like pattern used as decorative finishes and /or adding solar reflectance which can increase the life of the membrane and save energy costs.
The benefits of modified membranes include greater flexibility when handling the sheets during installation, enhanced tensile strength, puncture resistance, and dimensional stability which all contributes to an extended service life.
Copper finished modified membrane
BEST Contracting Services is a large specialty contractor that provides construction services for the entire Building Envelope. We serve commercial, institutional and industrial building owners in both the private and public market sectors. We also enjoy a large clientele in the General Contracting community.
Broadly, BEST’s services include Roofing, Waterproofing, Glazing, Metal Panels, Sheet Metal, Concrete Restoration, HVAC and “Green” Construction (including Solar PV, Cool Roofs, Daylighting and Green Roofs). Licenses
Established in 1982, BEST has ranked in the top 10 nationally for a decade in the roofing category by Engineering News Record.