Flat and Low Slope MCM Roof Options
This Streng Neighborhood in Shelfield Oaks, Carmichael, has many houses with flat and low-slope roofs. Most have TPO materials. Photo: Bob Wick
Facts About The MCM Roofs
Why is it that modern homes with flat or nearly flat roofs don’t have shingle or shake roofs? What’s up with the white roofs on all these houses? Are there actually different kinds of flat roofs currently available for replacing your existing roof? We will answer these questions and more about how to keep your beautiful flat-roofed home dry, cool and gorgeous.
Why Can’t Shingles Be Placed on a Flat Roof?
If a roof is too flat, it tends to have water pool (or pond) after a rain. If the water sits too long, it can work its way between shingles, shakes or tiles. To guard against this, it needs to be covered with a seamless surface.
A roof is considered low-pitch when it rises less than two inches in height for every 12 inches in horizontal travel. This pitch is referred to a 2:12. Flat roofs or roofs with a pitch of less than 2:12 cannot use shingles. Asphalt shingles are designed for higher pitched roofs so that water and snow run off the roof rather than stand. In standing water, the water can work its way under the shingles.
Low pitch or flat roof homes built in the 40s, 50s and 60s often employed a tar and gravel roof. A substrate of plywood was put on the roof and then coated with tar and then gravel on top. Some later houses have sheets of asphalt covered with small rock bits. This is similar to asphalt shingles, but in sheets.
Tar and gravel roofs are still used and are sometimes referred to as Bitumen roofs. Sheets of fiberglass are integrated. But most flat roof homes do not use this process. They can be prone to ponding water (think small puddles on your roof) due to their minimal slope, can deteriorate quickly under UV exposure, and may require more frequent maintenance compared to other modern flat roof materials like TPO or PVC membranes.
Many Mid Century Modern homes have flat roofs. Careful planning and maintenance are required.
What’s with All The White Roofs on Flat Roofed Houses?
Over the last twenty or so years, most replacements on flat roofs have been done using TPO or PVC. TPO (thermoplastic polyolefin) is a single-ply white membrane used in both commercial and residential roofing and is the most common. TPO roofs have been around for about 30 years. PVC (polyvinyl chloride) membrane roofing came into use in the early 70s. PVC is also a single-ply white membrane used in commercial and residential roofing. Most flat and low-slope roofs these days are TPO. TPO can come in grey, white or tan, but most people pick white. This does the best job of reflecting sun and keeping the house cool.
Completely Replacing A Flat Roof
Typically the old roof is removed and the underlying structure is repaired if necessary. The old roof is removed down to the substrate, which is usually plywood. Plywood is replaced or repaired where needed. A fire retardant slip sheet is installed on top on the plywood. An insulating material is placed between the slipsheet and the membrane. The membrane sheets are heat sealed. A TPO drip edge may be installed as well.
A typically used insulating material for a TPO roof is a fanfold poly insulation varying in thickness from half an inch to an inch. Foam board insulation which is more difficult to work with and can be up to 4.5 inches thick can also be employed. In either case, the thicker the insulation the greater the R-Value or resistance to heat flow.
Of course many homes have substandard downspouts. Make sure these are examined and repaired or replaced as warranted. I have seen failed downspouts where folks have installed gutters on the outside of their flat roof. This is not a great look, but is better than sheets of water spilling over the side of the roof during an atmospheric river.
If you have skylights, have them inspected and replaced if they are nearing the end of their life. We have an article about bubble skylight replacements here.
Cost to replace a typical roof on a single story home of around 2000 square feet is often $40,000 or more according to homeowners we have spoken to. Generally, guarantees are about 20 years on materials and sometimes longer on workmanship, but most roofs, if well maintained can last longer.
This Carter Sparks custom home features an undulating, low slope roof.
Flat Roofing and Fire Resistance
A note on fire danger. A TPO roof is considered fire resistant and typically receives a Class A fire rating, which is the highest rating for roofing materials, meaning it does not readily spread fire when exposed to direct flames; making it a good option for areas prone to wildfires. See our article about fire proofing you home.
Silicon Coating Your Roof Can Save Money
Recently lots of folks have been taking advantage of the the new Silicon coating product that just rolls on to a flat roof. As long as the underlying roof is sound, this product is just rolled on top of the existing roof. Different options I have heard mentioned are GE Enduris, Mule Hide and Henry Dure Brite. These can come in various colors, but why not get white to reflect the sun and keep the house cool? Generally these require two to three coatings for ideal coverage.
Many people have applied this themselves to save money. Of course you want to make sure that the underlying roof structure is solid and does not have dry rot or other structural problems as do many older roofs.
We spoke with homeowner Michael Balderrama whose flat-roofed Streng home is in Davis’ Sycamore estates. His model has a split roof with the center section higher than the other parts of the roof. His original roof was put on in 1971 and was rolled asphalt sheets.
After talking with three different companies, he went with the GE Enduris Silicon Roof system and used California Flat Roofs as his vendor. Michael stressed that this approach can only be used on a roof in good repair. His roof was working fine but he wanted to install solar panels and have the roof underneath be in good repair with no structural rot or damage under the existing roof. His roof was at least 20 years old but had no apparent leaks or damage.
To start with the roof is power washed. Any wood that needs replaced on the edges is replaced. Then a thicker GE coating that is grey is placed on the edges where the roof meets the metal. Any low spots in the roof are leveled up using a product called pond busters. This is applied in liquid form and fills in the low spot before hardening. The silicone product is rolled on using several coats. Michael’s total cost from California Flat Roofs was $21,000.
The new roof has a 20 year warranty. It needs to be cleaned periodically, at least once per year. Remove any organic material and make sure mold does not form. Michael’s home has drain holes in the higher roof section and the lower section has downspouts and gutters on edges. These need to be cleaned and kept clear as well.
Michael used Sun Metrics for his Solar. They recommended the Silicon coating roof as being very compatible with the solar. Michael raved about California Flat Roofs. He said they are on time, always came through and did what the said they would do.
Some other roofers we have heard good things about in Sacramento include Rouse Roofing and Cal-Vintage Roofing.
This Streng Bros atrium home has a flat roof and a huge skylight. Pay special attention to the skylight when re-roofing.
Conclusions for Flat Roofs
If you have a flat or low sloped roof, you have unique challenges in finding roofing options. If your underlying roof is in great shape you can extend its life using a Silicone coating. If you need to replace your roof, TPO is still the best option.
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