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All About the Flat Roof

flat roof

The flat roof is a fairly recent invention in construction only having come into mainstream use in the early 20th-century. While there are examples of very early flat roofs most of the earliest roofs constructed by man were thatched roofs that were made of straw, leaves, branches, or reeds; they were usually set at a slope, or pitch, so that rainfall could drain off them.  

So if humans learned early on that having a slope on your roof is important why did flat roofs come into the picture and why are they still in use today? I know some flat roof homeowners with neverending, leaky tar-and-gravel roofing that are asking the same question. 

But let’s not look too poorly upon the flat roof design quite yet. There are reasons why flat roof construction began and why you may consider it for new home construction. 

Let’s look at what defines a flat roof, its history, the pros and cons,  and both old and new options for roofing materials.

the flat roof

Some folks don’t like to use the term “flat” to describe roof type since all roofs should have some slope in order to drain water off or direct water to a scupper for drainage.  The term “low-slope” is often used.

The National Roofing Contractors Association (NRCA) defines a low-slope roof to have no greater a slope than 25 percent, or 3 inches of vertical rise for every 12 inches horizontal. The most popular roofing material these days is asphalt shingles. These shingles are great for shedding water, but only one way, and that is down a slope. For example, Florida building code says these shingles cannot be used on roof slopes less than 17 percent, or 2 inches of vertical rise for every 12 inches horizontal. This is starting to get into “flat” roof territory now and this requires a different roofing system such as using a membrane to stop water intrusion.

Here’s a comparison of the roof pitches above.

Early flat roof construction is found predominantly in arid climates such as in Persian, Arabian, and Egyptian architecture. The discovery of the oldest flat roof construction dates back between 7100 BC to 5700 BC in the middle east in an ancient settlement called Çatalhöyük in modern-day Turkey.

It is likely that the first flat roofs in America can be dated back to the mid 8th century in Puebloan design. The oldest structure in the U.S.,  Taos Pueblo, was built between 1000 and 1450 AD, has a flat roof and is still occupied today in New Mexico. Before this time period, archaic inhabitants made tipi-like shelters from sticks, brush, and mud.

Flat roofs are found in both residential and commercial applications.  They’re constructed from wood, steel or concrete and have some form of barrier to prevent water intrusion.

While almost all roofs on homes appear to have been sloped in early America except in arid climates such as in New Mexico, in the mid-1800s flat roof construction on commercial buildings began using pine tar and gravel according to a Roofing For Historic Buildings composition. However, it did not really become popular in home-building until the Modern architecture movement beginning in the 1920s. 

With structural advancements brought on by the Industrial Revolution and famed architects such as Ludwig Mies van der Rohe and Frank Lloyd Wright choosing flat roofs in their designs, flat roof homes began popping up in many states, including states where rain and snow are frequent. This architectural trend grew increasingly into the mid-century.

These days, modern architects still prefer flat roofs in home design and the majority of commercial buildings such as strip malls, stand-alone retail stores, and warehousing.

Pros and cons of flat roofing

It’s a “con” if it’s leaking into your home. It’s a “pro” if you love the look, but a “con” again if drainage is not maintained and the roof collapses under the weight of pooling water. Yes, flat roofs are known for leaking, but all roofs can leak. With advancements in structural design in new home building or upgrading your roof to better materials, you can have a reliable flat roof in a sunny, rainy, or snowy climate.

Pros

Flat roofs are the most economical to build compared to other roofing.  Height for height, they allow more air-conditioned living space than pitched roofs. They allow more usable space for solar panels, passive lighting solutions such as skylights and tubes, and air conditioning components. They are great for extra outdoor living spaces, gardening, or even greenhouses. They’re an excellent design for high-wind regions. And maybe you just love that sleek modern look.

cons

Flat roofs leave little to no attic or storage space under the roof. They’re not as effective as pitched roofing at shedding water or snow. They’re more prone to problems with mold. They may not last as long between reroofs depending on materials used and may require more maintenance for water drainage than pitched roofing.

Flat Roof Materials

Many old houses still have tar and gravel roofing and they can be leak-free for many years, but when they do leak it can be a pain finding where to patch up the hole. If there have been many patches completed over the years, it may be time for a reroof. If you are needing a reroof or building a new home with a flat roof there are many options to choose from in roofing systems and materials.

The 3 most common roofing systems are the Built-Up Roof (BUR), Modified Bitumen roofing, and Single-ply Membrane roofing.

Built-Up Roof (BUR)

A BUR system uses multiple layers of natural or synthetic roofing felt alternated with hot-applied layers of tar, or asphalt. The top layer is usually a layer of hot asphalt and topped with gravel. The gravel helps reflect the sun and protects the layers below.

Built-Up Roof (BUR)

Sometimes a reflective coating is applied rather than tar and gravel. This stinky and messy roofing process uses specialized tooling and is not a DIY project. The more layers added the more durable the roof, but also heavier. In general, it is the heaviest solution, so the roofing structure needs to be strong. Done right, a built-up roof is still a reliable solution and is the most cost-effective.

Modified Bitumen Roof

Typical modified bitumen roof systems use overlapping, single or double-ply asphalt-based sheets that are rolled out and are either heat-applied or are self-adhering.

Modified Bitumen Roof

Modified bitumen was created as a replacement for BUR.  No stinky tar here. A modified bitumen roof can also be coated with a reflective elastomeric topcoat to seal any cracks and reflect the sun’s rays.

This is a solution that some DIYers can tackle. It is tear-resistant and durable like BUR, but costs more, landing somewhere between BUR and single-ply membrane systems depending on how many layers are used.

Single-ply Membrane Roof

Single-ply membrane systems use flexible, waterproof sheets made from synthetic materials and is the most expense of these three types of flat roofs. Some of the most popular types of single-ply membrane roofs are below.

  • TPO (thermoplastic olefin)
  • PVC (polyvinyl chloride)
  • EPDM (ethylene propylene diene M-class rubber)

Systems like EPDM have been around as long as modified bitumen and proven to be as durable. If it does get punctured, repairs are easy. It is the most commonly used material of the single-ply membranes. One downside is it is dark and will absorb heat.  You can add a reflective coating but this also adds to the cost.

single-ply membrane flat roof
Single-ply Membrane Roof

If you are looking for a reflective solution, both PVC and TPO membranes are light in color.  Both are more expensive than EDPM, but TPO is cheaper than PVC. However, TPO is a younger product and doesn’t have the best track record over the years for reliability.  Be sure to do your research as there are several brands of TPO membranes available.

All three of these roofing systems can vary in the installation and type of materials used. Consult a professional to see which is the best application for your particular need in your particular climate.

Conclusion

I have dabbled in roofing, including installations, inspections, and little in sales, but most of my experience has come first hand with my own roof. I have a hip roof home with an attached flat roof studio apartment. My hip roof uses shingles and the studio has a modified bitumen roof. Both leaked several years ago and it was time for a reroof. Both were reroofed with the same systems and both leaked again in less than 2 years.

They were the right roofing systems and materials for my home, but the application was not. It is critical to install a roof or hire a roofing contractor that will use best practices and follow the local building code. If you’re not doing the roof yourself, do your homework and look at those roofing reviews thoroughly so you are not swimming in your living room like me. Only use licensed roofing contractors in your state or use reliable roofing material companies such as GAF to find roofing contractors with continued education and certification in roofing applications.

If you really like the benefits of owning a flat roof house, modern roofing systems and better materials make them a reliable and viable roofing solution.

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