If your home has a low to a non-existent sloped roof, you're limited on what type of roofing material you can use for coverage.
BUR - built-up roof - is one of the best choices for low-slope roofs due to its construction, which has multiple layers of fabric and asphalt topped with heavy gravel. You can pick how many layers you want in your tar and gravel roof, with each new layer adding five years of life expectancy.
What is BUR and Its Components?
A BUR roof consists of 3 main components - bitumen material (asphalt), ply sheets (typically 36" wide and referred to as plies), and surfacing materials - one or multiple. These materials go into alternating layers of reinforcing fabric and bitumen.
Each layer of ply sheet - reinforced fiberglass or organic roofing felt materials - goes on top of the bitumen. And then, the ply sheet gets topped with another layer of bitumen. Then another layer of fabric goes on top. Finally, once the material has the desired thickness, the top layer gets topped with aggregate - gravel or stone rocks.
The top layer of stones gives the roof a finished look while protecting the bottom layers from impacts by debris and sunlight. You can even have a reflective layer added to the top to meet cool roof requirements.
The design of BUR roofs has been around for over 100 years and is ideal for flat or low slope roofs due to creating a continuous sealed surface compared to an unsealed shingled roof, which requires a sloping roof for water to shed effectively.
Although the concept of BUR roofs has been around for decades, the installation process and materials have evolved, including using a more rigid insulation layer that gives better energy efficiency.
BUR Types - Tar & Gravel vs. Ballasted
There are two primary types of BUR roofs - the Tar & Gravel system or the Ballasted roof system. Both types consist of multiple layers of material to make the roof classify as a BUR.
So, what are the differences between the two if they have the same construction?
One significant difference between both BUR types is the size of the rocks used for the top layer. A Tar & Gravel system consists of smaller, coarse rocks with a 0.25" to 0.5" diameter. In comparison, a Ballasted system requires larger stones - 1.5" to 2.5" in diameter.
Another significant variation between the two systems is that on Tar & Gravel roofs, a bitumen chemical sealant goes over the top of the rocks to keep them in place. This sealant can be heat or cold applied.
Cold built-up roofing applied bitumen spreads easier due to being a liquid that hardens as it cures. The application can be done through a squeegee or sprayed on in any weather or temperature. Whereas if the contractor uses hot built-up roofing applied bitumen, they have to use a torch to melt the bitumen, which also hardens as it cures.
But on a Ballasted roof, an unsealed waterproof membrane is held in place with piles of rocks. This style of BUR roof can feature rock piles from a few inches high up to a foot high.
Pros of Built-Up Roof
There are many benefits to choosing a BUR roof type for your home. First, BUR roofs have excellent waterproofing. Second, the construction also protects the roof from damage against UV (ultraviolet) rays.
Thanks to the aggregate (rock) top layer, BUR roofs are fire-resistant. BUR roofs are also low-maintenance, meaning minimal upkeep costs throughout your roof's lifespan.
Cons of Built-Up Roof
The biggest downsides of BUR roofs are the slow installation process, which means high labor costs. Plus, there will be hazardous fumes during the install when using hot bitumen, whereas cold bitumen won't cause harmful fumes.
Also, you'll face high installation costs compared to other roofing types. And some versions of BUR roofs can be more vulnerable to water or wind damage. The final concern about BUR roofs is the heavier weight, which may put too much strain on your roofline. A ballasted roof weighs more, with 10 to 25 pounds per square foot.
Why is BUR Best For Flat Roofs?
The design of BUR roofs is perfect for flat or low-sloped roofs, making them the most popular choice for commercial buildings since the 1970s.
BUR roofs are also suitable for residential homes with low-sloping or flat rooflines where traditional asphalt, metal, or wood shingles won't work.
Roofs need a slope to ensure water runoff, as letting water collect on a roof can lead to leaks, cracks, mold, mildew, and wood rot. When roofs don't have a slope, there needs to be some other type of protection.
Because BUR roofs consist of multiple layers of tar, which is water-resistance by itself, topped by a layer of pea-sized ¼" thick gravel, it prevents water from getting beneath the materials to create leaks that can damage your structure.
The gravel also helps protect the asphalt from impacts and UV rays while being Class A fire rated. This flood coat - small gravel (mineral granules, pea gravel, or slag) embedded into the last layer of asphalt - keeps the roof from blistering, cracking, or degrading from the elements.
And the final layer of gravel can also help evaporate water that doesn't drain, preventing pooling - water settling in one place - because gravel absorbs and releases heat, which turns the water into steam - a gas state.
Common Problems or Basic Repairs
No matter what type of roof you pick, there is always the chance of damage. Therefore, it's always best to make repairs as soon as you notice signs of damage. Not making repairs can lead to costly and severe issues like damage to the materials or the roof structure.
Here are a few problems with BUR roofs and how to fix them with simple repairs.
Some BUR roofs may experience a separation of the seams or joints. The first repair method is to add cement under the separated seam, holding the piece down while it dries. You can also add a large portion of felt over the space, nailing it into place and then covering it with cement and gravel.
If your BUR roof experiences blisters, you can use a knife to cut the affected spot. After letting the layer dry completely (if the top layer is damp, cut down until you find a dry layer), add a new layer of felt to the area. Then add asphalt and top it with chipping. If chipping isn't available, you can use liquid-applied coatings.
Waves or Undulations
You can repair undulations by adding new layers over the top to make it leveled out. The substrate does need to be in good condition before using this method.
Cracks on the asphalt layer are an easy fit. After cleaning the area of any debris and gravel, add a layer of asphalt cement on the spot and then top it with a layer of felt. You'll need 4" of overlap or more to ensure the area is fixed. After repeating the process, add a top coat of asphalt cement and top the cement with gravel.
BUR vs. Modified Bitumen Roof
While a built-up roof consists of multiple layers of material piled together, modified bitumen is another option for a low-slope or flat roof.
Modified bitumen can be made of SBS - styrene-butadiene-styrene (rubber asphalt) or APP - Atactic Polypropylene (plastic asphalt).
Modified bitumen consists of two parts - a base layer and a cap layer. The base layer gives the roof waterproofing and provides a surface for the cap layer. This bitumen-coated layer can be non-woven polyester, fiberglass, or a combination of both.
The top layer - cap - goes over the base layer. Although the cap has a bitumen coat, the top layer has granules - the same as asphalt shingles.
Application of modified bitumen can be made through three methods.
One installation method is self-adhesive, which means the layers have sticky backsides covered in plastic. Once the plastic is pulled off, the sticky layers seal when warmed by the sun.
- Welded/heat adhesive
Modified bitumen can also be installed using a propane torch. The heat from the torch melts the bitumen, so the layers seal together.
- Cold adhesive
The final method uses room-temperature liquid bitumen, which reduces toxic fumes and fire risks that can occur using a heated process.
Q: How long does a BUR roof last?
A: A BUR roof can last between 15 and 30 years, which is about the same longevity as basic 3-tab asphalt shingles. If you need a roof with an extended lifespan, consider architectural shingles, another type of asphalt roofing. Metal and stone roofing has the most extended longevity of 50 years or more.
Q: How much does it cost to replace a built-up roof?
A: Installation for a BUR roof using tar & gravel can cost between $3,750 and $6,750. The final cost varies depending on the roof's size, slope, and pitch. For example, one square foot (12" x 12") can cost $2.50 and $4, equaling $250 to $400 a roofing square (100 square feet).
Q: What are other roofing options for low-slope or flat roofs?
A: Other options for roofing besides BUR include a FleeceBack roof, a fleece polyester material under PVC, TPO, or EPDM - Ethylene Propylene Diene Terpolymer - a rubber-based material.
TPO is a thermoplastic polyolefin single-ply flat roof system. PVC - polyvinyl chloride - is a combination of chlorine and ethylene. SPF is spray-on roofing made of layers of silicone or polyurethane. And finally, there's modified bitumen - a 2-part roofing system.
BUR - Built-up roofs - are an excellent and durable roofing solution for homes and buildings with a low to flat roof slope. This style of roof consists of multiple layers of material that can be up to a foot thick. The top layer of rocks gives the roof resistance against storms, falling debris, strong winds, water, and damaging UV rays.