What is the difference between built up roofing and modified bitumen?

modified bitumen roof

What does built up roof mean?

Flat roof structures have several choices in what type of roofing material to use. Which one can depend on the climate and environment where the structure is located, and of course, the owner’s budget has a big part of that decision too.  Many structures around the country choose to go with a built up roof system. 

A built up roof consists of layers called, is also referred to as BUR, is a flat roofing system that consists of layers, or membranes, like reinforcing fabric, tar paper, and other materials in a sequence to ensure adherence to the structure roof. Those multiple layers ‘build up’, hence the name “built up roof”. 

The built up roof system became popular in the 1970s and was called “tar and gravel” roofing. The result of the alternating layers or piles of these different materials, known as bitumen,  created a finished membrane that protected the structure in various types of climates and elements. Bitumen consist of asphalt, coal tar, and cold-applied adhesive. The gravel part of “tar and gravel” built up roof is the last layer applied and may be a crushed stone or gravel. 

How do you install a built up roof?

One thing for sure, a built up roof construction isn’t a DIY kind of job!  The built up roof is simple looking by the untrained, but rest assure, there is a reason you need a professional roofing contractor install your built up roof system. Still, curiosity is there, so we share these basics of installing a built up roof with you: 

The installation process has four steps. 

  • Base sheets are attached to the subsystem with roofing nails.
  • A layer of either an adhesive, asphalt, concrete, or tar is then applied.
  • A layer of cap sheets or gravel are applied next.
  • Then a final layer of asphalt is poured and spread over the surface.

Doesn’t sound that complicated or hard, but there is an exact to this process that is learned with experience.  Knowing how much to apply for each layer and how many layers is key to a successful built up roof installation. Another keynote to understand about a built up roof installation is the asphalt is hot and must be spread over the roof quickly and evenly. 

How much does built up roofing weight?          

The following weight values are only for a general purpose of built up roof systems. For exact dead load weight, your roofing contractor should have that information available from the manufacturer. These are important numbers that roofing contractors must consider because of the engineering calculations for what the structure can withstand is critical.

3 ply Built up Roofing with Smooth-Surface2.33 pounds per square foot
4 ply Built up Roofing Membrane with Gravel-Surface6.3 pounds per square foot
4 ply Built up Roofing Membrane with Smooth-Surface2.58 pounds per square foot

How do you repair a built up roof?         

For an asphalt based built up roof with gravel, repairs can be made using the following steps: 

  • Clean the surface needing repair. This is critical in order for the next steps to adhere and provide the protection you need. This includes removing the gravel which can be done with the flat side of a pry bar to scrape the gravel away, going in one direction, for a six inch diameter around the area to be repaired. 
  • Once the gravel is thoroughly removed, brush the area clean of any dirt and dust. 
  • Prime the surface with a liquid asphalt primer using a brush or roller. Allow the primer to flash off so there isn’t any excess on the surface.
  • Cut 2 patches of fabric larger than the area being repaired. One patch should be at least 4 inches larger all the way around and the second patch 6 inches larger all the way around. 
  • With a trowel, spread the asphalt flashing cement to the area needing repair, keeping it uniform in thickness and spreading 4 to 6 inches out around the area being repaired. 
  • Now place the four inch piece of fabric and imbed it into the cement with the trowel.
  • Using the trowel, apply an additional layer of the asphalt flashing cement over that patch, extending past the 4-inch patch you just installed and spread out past the are the 6″ patch will take to ensure the 4-inch patch is well covered in all directions. 
  • Apply your finishing coat of asphalt mastic over that last patch and then reset the gravel your cleaned off earlier so the repair area is concealed, and you have a uniform appearance with the entire roof. 

A flat roof is typical of most commercial buildings because of the many benefits it offers a large size building. They are often a built up roof that is created by alternating a layers or stacks of bitumen as we have discussed earlier. Another option for a built up roof system is the modified-bitumen roof, which is comprised of asphalt and plastic, or sometimes rubber polymers. This method is a safer application with some benefits that the traditional built up roof system doesn’t offer like flexibility, redundancy, and UV resistance.

flat roof with bitumen

What are the built up roof pros and cons?

For a large commercial or industrial structure, built-up roofing and waterproofing are necessary. A flat built up roof is cost-efficient in construction, and post-construction it provides a larger area for HVAC system and more.  The pros of a built up roof are: 

  • Multiple layers of protection create the needed watertight barrier that will last for years.
  • Inexpensive to build and maintain. 
  • Tolerates the harsh elements of the weather year round.
  • Exceptionally resistant to the UV rays, transferring that to lower cooling and heating expense.
  • A built up roof system provides a high ROI for up to 40 years.

Need help with your built up roof in Houston, TX? Call 281-300-4363 today!