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Comparing Hot Mop Roofs vs. Torch Down Roofs

Structures with flat roofs, such as apartment buildings and commercial properties, use a range of roofing materials unique to their construction. Unlike traditional roofing materials, like tiles or shingles, flat roofs are built from asphalt or plastic membranes.

In California, hot mop and torch down are two of the most popular flat roofing methods. If you own or are planning to build a flat-roofed structure in California, it’s important to understand the key features of each method and the main differences between them.

What is Hot Mop Roofing?

Hot mop flat roof

Hot mop roofing is a flat roofing technique that involves applying multiple layers of materials to create a durable, weatherproof surface that can resist rainwater, wind, and the elements.

To build a hot mop roof, roofing contractors prepare the substrate of a flat-roofed structure by cleaning and priming it. Then, they must apply multiple layers of materials in a specific order:

  • Base layer. The base layer of hot mop roofing is made from asphalt and heated to 400°F for even application. Roofing contractors use a special high-temperature-resistant mop to spread the liquid asphalt. Once solidified, this layer provides basic protection against the elements and a foundation for additional roofing materials.
  • Roofing felt. Once this base layer has been applied, roofers apply a second layer made of a different material called roofing felt. This material is constructed from organic or fiberglass felt, providing additional resistance to moisture and structural support. Once applied, the roofing felt layer helps make a flat roof a safe and walkable surface.
  • Additives and surface treatments. In California, it is common for the roofing felt layer to include additives to increase its solar reflectance and resistance to ultraviolet (UV) rays. These additives help ensure the roof doesn’t degrade when exposed to summer sun and can help a building adhere to California’s Cool Roof regulations. Additional surface treatments, such as mineral granules and sand, can further increase the roof’s resistance to the elements.

Pros and Cons of Hot Mop Roofs

Hot mop roofing offers a flat-roofed building with many benefits but may not be suitable for all building types. Here are the pros and cons of considering a hot mop roof for your flat-roof property:

Pros Cons
Highly durable. Can last around 20 years with proper maintenance.
Labor-intensive. The process of hot mopping is complex, requiring skilled professional application.
Weather resistant. Resistant to extreme weather conditions, including heavy rain, high winds, and intense weather fluctuations.
Odor. The process can produce a strong, unpleasant odor.
Versatile. Suitable for a wide range of flat and low-slope residential, commercial, and industrial buildings.
Potential health hazards. Involves hot asphalt, which can pose health risks if not handled properly.
Over existing roofing. Can be applied over existing roofing systems, reducing the need for complete roof tear-offs.
Messy installation. The process can be messy and requires careful cleanup.
Cost-effective. While labor-intensive, the materials used are relatively inexpensive, keeping the cost of a hot mop roofing project low. The average cost per square foot of a hot mop roof ranges from $2.75 to $5.75.
Temperature sensitive. Requires specific weather conditions, such as dry weather and temperatures higher than 45°F, for proper application.

What is Torch Down Roofing?

Torch down flat roof

Torch down roofing is another common flat roofing technique. Instead of applying multiple layers of materials to a flat roof’s substrate, torch down involves the application of protective membranes.

In torch down roofing, a membrane is primarily made from a mixture of asphalt and a rubberized or plastic polymer material, such as Styrene-Butadiene-Styrene (SBS) or Atactic Polypropylene (APP). These types of membranes are similar to big rolls of plastic sheeting or bubble wrap.

Two membranes are applied in a torch down roof: a base sheet and a cap sheet. Roofing contractors may supplement them with protective coatings to enhance the roof’s durability.

  • Base sheet. The base sheet is installed directly on a primed substrate. It provides the roof with a waterproof layer and creates a stable surface for the cap sheet and potential extra layers.
  • Cap sheet. The cap sheet is applied over the base sheet and “torched down” onto it using a propane-powered, open-flame torch. As the heat partially melts the top membrane, it sticks to the base sheet, adapting to the roof’s shape. When it cools, it remains secured in place, forming a strong waterproof seal that provides moderate UV protection.
  • Protective coatings. Many torch down roofs feature additional coatings for extra protection. For example, acrylic coatings can provide a reflective surface, improving the roof’s resistance against UV rays. Polyurethane coatings are often used on roofs intended to receive frequent roof traffic due to their high resistance against abrasion and impacts.

Pros and Cons of Torch Down Roofs

Pros Cons
Fast installation. Torch down roof installation processes require fewer steps and can be applied more quickly than hot mop roofs.
More expensive. The materials required to install a torch down roof are typically higher than hot mop roofs. The average price of a torch down roof is $3.50 to $7 per square foot.
Long life. Torch down roofs are very durable, with an average life expectancy of about 15 to 20 years with regular maintenance.
Requires specialized skills. Installing a torch down roof requires professional roofers trained in the use of propane torches to avoid fire hazards.
Many coatings available. Torch down membranes are available with many protective coatings, including resistance to fire, chemicals, impacts or UV rays.
Less environmentally friendly. The modified bitumen and plastic materials in their composition are harder to recycle and cost more to transport and incinerate.
Easy to repair. Maintaining and repairing a torch-down roof is relatively easy, typically taking just a few hours.
Less suited for complex roofs. Torch down roofing is less flexible or adaptable to complex roof shapes than hot mop roofs, requiring more time and labor to achieve a proper seal.

Find the Best Flat Roofing Materials

When it comes to selecting a hot mop vs. torch down for your flat roof, the right choice depends on your budget, the size of your property, and whether your roof needs additional protective properties. If you need guidance, an experienced roofing contractor in your area can assess your building, review your needs, and help you make the right choice.

FAQs

A hot mop roof typically costs $2.75 to $5.75 per square foot, whereas a torch down roof usually costs $3.50 to $7 per square foot. For a 1,500-square-foot flat roof, a hot mop roofing project should cost approximately $4,125 to $8,625, and a torch down project can be expected to cost $5,250 to $10,500.

Hot mop projects typically take 2 to 4 days to complete, depending on the roof’s size. Torch down projects can be completed within 1 to 2 days.

Torch down is a better technique for making your flat roof walkable. The materials used to install a torch down roof are more resistant to impacts and the wear and tear of frequent foot traffic.

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