Pros and Cons of Flat Roofs & Roof Decks

 

 

At our firm, we focus on creating high-end modern homes often pushing the boundaries of form and material. One of the signature forms found in many modern projects is the flat roof. In today’s blog, we look at how flat roofs work and what you should know when considering one for your next project.

Flat Roof Basics

A “flat roof” is not actually flat. A more accurate industry term is a “low-sloped roof.” Low-sloped roofs are classified as any roof slope under a 2:12 pitch (2″ of fall over 12″). Low-sloped roofs have one major difference compared to their high-sloping cousins; their finish material. Typically steep roof use asphalt shingles, slate, thatch, and other finishes common on most homes in the U.S.  These materials rely primarily on an overlapping configuration allowing water to shed off and away quickly given the steep angle of the roof. These overlapping materials become useless as the roof pitch is reduced. Low-sloped roof systems must use membrane finishes that offer multi-directional protection to control slower water movement. Low-sloped roofs are made of either rubber, plastics, or polymers that offer greater moisture protection and can even hold water. Think of a low-sloped roof as a large shower or bathtub that contains and directs water to a specific point.

Pros and Cons of a Flat Roof

Like with anything, flat roofs offer a variety of benefits but also have their issues. These pros and cons are important to understand when designing a new home. One of the biggest downsides to a flat roof is cost. Flat roofs are traditionally more expensive if you compare an asphalt shingle roof to a TPO membrane roof. TPO (a polypropylene membrane most common on flat roofs) costs as much as 6 times standard asphalt shingles. When you consider the savings in the amount of lumber on a flat roof versus a high-pitched roof, the scales start to balance out a little more. If you are thinking of building a flat roof with an occupiable roof deck, just know that there are a lot of layers and materials under the hood that all have a cost. Another downside to flat roofs is the lack of attic space and storage. Modern homes with flat roofs are notorious for reduced storage space unless it is designed into the layout of the floor plan. Having an attic space is a must for many homeowners. It is important to consider your lifestyle and storage needs before committing to a flat roof.

One benefit offered by flat roofs is their simplicity of form offers great efficiency. Sloped roofs often have complex pitches and angles sometimes creating unnecessary complexity and making the architecture look messy and random for the sake of simply shedding water. Modern architecture on the other hand is focused on simplicity and intentional forms, which is why flat roofs are so popular in this genre of architecture. Flat roofs are oftentimes much easier to build. Most of the flat roofs we specify are actually built flat and use special sub-systems to create low slopes. Building the structure flat is easier and saves overall construction time. Another benefit of the flat roof system is related to energy efficiency. Most flat roofs are TPO and most TPO uses insulation placed on top of the roof deck to provide the low slope. This tapered insulation also provides high R-values (thermal resistance unit) that keep the home warm in the winter and cool in the summer.

 

pros and cons of a flat roof
Figure 1: Flat Roof with Spray Foam Insulation

Figure 1 Source: https://basc.pnnl.gov/resource-guides/below-deck-spray-foam-insulation-existing-roofs

 

Tapered Insulation and R-Value

One of the most common ways to build a flat roof is to actually build it flat (build the structural members flat) and use a sub-system to create the slope. The most common sub-system is tapered insulation. Tapered insulation as we discussed above not only provides the slope but also provides a higher thermal insulation value making the home more energy efficient. Tapered insulation is a closed-cell polyisocyanurate or otherwise just known as dense foam board. This foam board is shaped and placed over the roof sheathing and under the membrane to direct the water toward the gutter systems.

When it comes to R-value tapered insulation can provide between R-4 to R-6 per inch of thickness. The current energy code here in Nashville requires homes to be built with a min of R-49 for roof insulation in new single-family homes. To meet this requirement approximately 8 inches of tapered insulation would need to be used if you are going to only use tapered insulation to provide all of your thermal resistance. 8 Inches is a lot of tapered insulation. Most homes we design split up the thermal resistance allowing part of the r-value to be provided by the tapered insulation and the other by spray foaming the underside of the roof decking. There are several benefits here. The spray foam used under the roof deck helps seal up the roof, while the tapered insulation reduces the thermal bridging caused by the roof framing. Thermal bridging is when an element like a stud or a roof rafter has a different thermal resistance than the surrounding insulation. This difference in resistance will reduce the overall thermal performance of the roof system and therefore the home. Using tapered insulation minimizes thermal bridging because the whole system is insulated and is placed between the structure and the exterior. See Figure 3 below.

 

pros and cons of flat roof
Figure 2: Thermal Bridging in Stud Walls

 

 

pros and cons of roof deck
Figure 3: Tapered Insulation on Roof

 

Source Figure 2: https://www.buildinggreen.com/primer/thermal-bridging

Source Figure 3: https://www.mulehide.com/Roofing-Products/p/PolyISOTapered

Pros and Cons of Roof Deck

Tapered insulation has a spongy feeling when walking on it. This soft sponginess greatly reduces the noise during heavy rain compared to many other roof finishes. The issue with this softness is that it does not work very well when it comes to carrying a heavy weight like an occupiable roof deck. Roof decks are becoming more and more common these days with cities becoming denser and yards becoming smaller. The roof offers one of the best places to enjoy the outdoors in an urban environment. The issue is that placing a heavy roof deck on top of a soft TPO or EPDM membrane could cause the membrane to fail. This failure occurs when the decking system combined with the soft tapered insulation punches through the membrane due to the increased weight and the point load from a roof deck system like a pedestal. To prevent this, a cover board (aka protection board) must be placed between the insulation and the membrane. A cover board is simply a high-density board that provides a harder surface for the decking system to sit on. See figure 4 below.

 

pros and cons of roof decking
Layers of Roof Deck: Pedestals and Cover Boards

Figure 4 Source: https://www.jlconline.com/how-to/exteriors/decks-over-roofs_o

The image above shows the various layers that go into a roof deck system. The topmost layer in the image indicates a wood decking system that acts as the roof deck finish and allows water to drain down onto the roof membrane. Roof deck finishes don’t have to be wood. There are other popular options like pavers and green roofs. These decking systems are held up by a pedestal system that levels the deck and also allows water to pass freely over the membrane. Below the pedestal, you will find the roof membrane that is providing the primary water protection, then the cover board, tapered insulation, roof sheathing, and finally the roof structure. All these systems work together to keep the water flowing and the home dry.

Flat roofs and roof decks are some of the most complex constructs in a building but offer a variety of benefits. From creating outdoor space to providing a modern aesthetic, flat roofs are becoming more and more popular in residential construction.

Want to know more about how a flat roof could benefit your specific needs? We would love to chat and share our ideas. Stay tuned as we continue to post pragmatic content monthly.

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