Conventional built-up roof systems have been in use as an industry mainstay for over 100-years. These applications typically use coal tar pitch or asphalt as the waterproofing material which is installed in successive layers and reinforced with ply felts. With multiple plies, a contractor is essentially “building up” the roof, hence the terminology. This post covers the asphalt side of BUR systems.
Built-up asphalt roofs are field fabricated using hot applied asphalt in alternating layers with Type IV, Type VI or organic ply sheets. The entire assembly is then surfaced with protective coverings such as gravel, fibered aluminum or other coatings to avoid UV degradation of the asphalt in the assembly. A built-up roof is greatly enhanced with additional plies and redundancy is the key to its performance.
Layers of a Build-Up Roof
A typical BUR assembly might consist of an anchor or base sheet installed over a lightweight concrete deck or multiple layers of tapered insulation. The next step is the alternating felts and asphalt mopped in place at an application temperature within 25 degrees F of the EVT for the particular type of asphalt. Most contractors today use Type 4 asphalt with an EVT of around 472 degrees so the temperature at the mop should be within the 450-500 degree range for the optimum roofing. The asphalt is spread to allow approximately 23-25 lbs. of asphalt per 100 square feet per ply. Too much asphalt and the roof can “slip”, too little and the system will fail prematurely. The fiberglass ply sheets are porous and allow the asphalt to saturate at this temperature and this combines to form a “ply”. Four of these layers constitute a “4-ply BUR”. Finally an additional layer of asphalt is spread and gravel is broadcast while still hot at the rate of 400-600 lbs. per 100 square feet. A minimum standard is 50 per cent embedment of the surfacing gravel. This system then becomes a “4-ply BUR/G”.
Other alternatives for surfacing include a clay/asphalt emulsified coating, fibered aluminum coatings or acrylic/elastomeric coatings. All of these are acceptable, however, each is subject to deterioration and will require “recoats” at 5-7 year intervals and none have the protective properties or excellent fire ratings of gravel.
Build Up Roof Challenges
As is easily noticed from the installation guidelines, built-up roofs are extremely labor intensive. Kettles at ground level to heat the bitumen allowing it to be pumped onto the roof must be constantly monitored. Moving hot asphalt safely around a roof surface requires additional personnel and specialized equipment and to physically install ply sheets and surfacing requires a substantial team. A typical built-up crew can easily involve 10-12 men or more.
Combining the high labor costs with volatility in the petroleum market, increasing awareness of open flame and fume hazards and building department concerns over gravel surfacing becoming airborne projectiles has led to a decrease in market share for this roof type. Additional concerns include the aesthetics of a “hot kettle” at ground level and the stringent safety procedures that must be followed when dealing with open flames and super-heated asphalt with its’ resultant fire potential. Lastly, when the cost of tapered insulation (asphalt fares poorly in ponding water) are combined with high labor and increasing bulk asphalt prices it is east to have a 15-year BUR system at double to triple the cost of other systems with the same warranty period. In today’s economy, many Owners and Managers are increasingly in search of alternative systems for their properties.
For further information concerning your built-up asphalt roof, we suggest the Asphalt Roofing Manufacturers’ Association . Or please contact All Area Roofing and we will be glad to visit your site, evaluate your BUR and provide maintenance or design/build insight as needed..
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