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Flat roofs, explained

What Is the Best Flat Roof Material in Florida?

For most Florida commercial and low-slope roofs, white TPO is the best all-around choice: it reflects sun and its seams are heat-welded. Modified bitumen has a longer track record here and is easier to spot-repair. Built-up roofs can last longest with upkeep, and a coating can restore a sound, dry roof without a tear-off.

By Josh Hildebrand, owner of Coastline Roofing · Florida license CCC1331076 ·

A flat roof rarely gets a second look until a stain shows up on a ceiling tile. Pick the wrong membrane, or coat a roof that's already wet, and you can end up paying for the same roof twice. Here's how the four common systems stand up to Florida sun and rain.

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The options

What are the main flat roof systems used in Florida?

Four: TPO, modified bitumen, built-up roofing, and liquid-applied coatings. The first three are new roofs. A coating restores the roof you already have.

TPO (single-ply membrane)

TPO is a single layer of tough plastic membrane, usually white, rolled out in wide sheets. Johns Manville's installation guide calls for hot-air welding on every field seam, so the sheets are fused together, not glued.

Modified bitumen

An asphalt-based roll roofing that comes in two main types, SBS and APP. Polyglass lists four ways it goes down: torched, set in hot asphalt, bonded with cold adhesive, or self-adhered. It has a longer track record in Florida than TPO and is easier to patch.

Built-up roofing (BUR)

The original flat roof: several plies of reinforcing sheet and asphalt, built up in layers on the roof. InterNACHI's life chart gives it the longest run of the group, about 30 years.

Liquid-applied coatings

Acrylic, silicone or aluminum-reflective coating, applied as a liquid over the existing roof. It isn't a new roof. It restores one that is still sound and dry, and it skips the tear-off and the haul to the landfill.

Side by side

How do TPO, modified bitumen, built-up roofing and coatings compare?

Modified bitumen is the easiest to patch, built-up roofing has the longest listed life, and white TPO pairs welded seams with a reflective surface. For coatings, InterNACHI lists only aluminum coating, at 3 to 7 years; acrylic and silicone systems are sold with warranty terms instead.

The lifespans below are InterNACHI's national estimates. The chart says real life depends on weather, design, materials and maintenance, so treat them as a starting point, not a promise.

Flat roof systems compared
What mattersTPOModified bitumenBuilt-up (BUR)Coating
Expected life7 to 20 yearsAbout 20 yearsAbout 30 yearsAluminum coating: 3 to 7 years. GAF's coating warranties run 10, 15 or 20 years, by system
SeamsHot-air weldedTorched, hot-mopped, cold adhesive or self-adheredSeveral overlapping pliesNone; goes on as a liquid
Solar reflectance, new and weatheredWhite: 0.77 to 0.79, then 0.70Depends on the surface colorDepends on the surface colorWhite acrylic: 0.85, then 0.75
Standing waterNo ponding 48 hours after rain in drying weather (NRCA's drainage test)Gaco's silicone is rated for ponding; its acrylic is not
Spot repairPossible, but harder than mod bitEasiest to patchPatchable; a sound one can take an overlayOnly as good as the roof under it
Best fitNew commercial roofs, flat additionsHome flat sections, roofs you expect to patchOlder commercial roofs worth keepingSound, dry roofs with life left

Solar reflectance is the share of sunlight a surface bounces back, so 0.70 means 70%. The TPO figures are from Carlisle and Johns Manville data sheets (new, then after 3 years outdoors), the acrylic from Gaco's A47 sheet (new, then weathered), and all of them drop as the roof weathers.

Heat and sun

Which flat roof holds up best to Florida heat and sun?

A light, reflective one. White TPO or a white coating keeps the roof surface far cooler than dark asphalt, and that's less heat baking the roof and the building under it.

The U.S. Department of Energy says a conventional roof can reach 150°F or more on a sunny summer afternoon, while a reflective roof could stay more than 50°F cooler. A cool roof guide from the Department of Energy and Lawrence Berkeley National Laboratory explains the gap: light-colored roofing typically reflects 55 to 90% of sunlight, and most dark roofing only 5 to 20%.

Heat also ages asphalt. InterNACHI describes alligatoring, the cracked, alligator-hide look on old flat roofs, as aged asphalt that has oxidized and lost its volatile oils to the sun, the end result of asphalt's limited tolerance for heating and cooling. Modified bitumen and built-up roofs are both asphalt-based, so their top surface matters.

Standing water

What if water ponds on my flat roof?

Fix the drainage before you pick a material. The National Roofing Contractors Association's test for proper drainage is no ponding water 48 hours after rain, in weather that allows drying.

Flat roofs aren't built dead flat. The Florida Building Code, in both its Building (commercial) and Residential volumes, calls for built-up, modified bitumen and single-ply roofs such as TPO to have a design slope of at least a quarter inch per foot, a 2% slope. (That's FRSA's transcription of the 2017 code; the state's list of 2023 residential code changes shows none to those sections.) If a roof lost that fall or never had it, the fix is rebuilding slope, often with tapered insulation, or improving the drains. Our page on why water pools on a flat roof walks through the causes.

If you're coating, the type matters. Gaco says its GacoFlex S20 and GacoRoof GR16 silicone coatings stand up to ponding water. Its GacoFlex A47 acrylic data sheet says that coating isn't intended for surfaces prone to ponding or for slopes under 2:12 (2 inches of rise per foot). A roof built to the quarter-inch minimum is far flatter than that.

Silicone isn't a drainage fix

GAF's coating manual, which covers its silicone coatings too, requires positive drainage and says the roof being coated should not pond water for more than 48 hours after rain stops. Correct the drainage, then coat.

Restore or replace

When does a coating or restoration beat a tear-off?

When the roof is still sound, dry and draining. A coating skips the tear-off and the disposal of the old roof, but GAF's coating manual is blunt: "a coating cannot add life back to a roof already beyond its service life."

GAF's Liquid-Applied Roofing Manual sets the conditions. Three have to hold before a coating makes sense:

  1. The roof isn't worn out

    GAF notes that coatings generally won't find or fix existing leaks, so leaks get traced and repaired first. A roof at the end of its life needs replacing, not coating.

  2. The insulation is dry

    All wet insulation comes out before coating. If a moisture survey (infrared, nuclear or core cuts) finds moisture in 25% or more of the roof, GAF requires a complete tear-off.

  3. Cracks and blisters are fixed

    On built-up and modified bitumen roofs, cracks 1/16 inch or wider get repaired, and blistered or wet membrane gets removed. Alligatored areas have to be treated before the coating goes on.

Skip the moisture check and a coating can seal wet insulation in. If the surface is cracking like dried mud, read what alligatoring means for a flat roof before you price a coating.

Commercial or home

What's the best flat roof for a commercial building vs a home's flat section?

For a new commercial flat roof, white TPO is the standard pick in Florida. For a lanai, porch, carport or addition that's too flat for shingles, modified bitumen and TPO both work. It comes down to easy patching versus a cooler surface.

Commercial buildings. A big, open roof takes sun all day, which is where a reflective membrane with welded seams earns its keep. Modified bitumen is still a solid choice, especially where the owner expects to patch small areas over the years.

A home's flat section. Florida's residential code allows asphalt shingles only on slopes of 2:12 or steeper (Florida Building Code, Residential, Section R905.2.2), so a flatter porch or addition roof needs a membrane. For the sloped main roof, our guide to shingle, metal and tile for Florida homes picks up where this page stops.

Aerial view of a completed commercial TPO flat roof
A completed commercial TPO flat roof, seen from the air. April 2024.
What we install

What does Coastline install on flat roofs?

Sixty-mil heat-welded TPO, modified bitumen in both SBS and APP, and repair or replacement of built-up roofs. We also apply elastomeric acrylic, silicone and aluminum-reflective coatings, matched to the membrane that's already there.

On an older built-up roof with a sound substrate (the base the roof sits on), we can install a modified bitumen or TPO overlay instead of tearing it off.

Commercial work is scheduled to keep disruption low, with early starts, weekends, or a phased install. The crew works from a checklist, Josh reviews every job's photo report, replacements carry our 5-year workmanship warranty, and manufacturer warranties pass through to you in full. See how we handle commercial roofs and the flat roof systems we repair and replace.

It starts with a free, drone-assisted roof inspection and a written photo report you keep, with no trip fee and no diagnostic fee. You can also text photos of the roof to (941) 345-0072.

Coastline Roofing truck and dumpster on a commercial roofing job at sunset
A Coastline Roofing truck and dumpster on a commercial job at sunset.
Reviews

What customers say after a new roof from us

★★★★★
"Coastline Roofing did a great job for us. They replaced all the roof shingles and the flat roof, they did a great job at the best pricing ever."
Oren S.Shingle and flat roof replacement, Florida Gulf Coast
★★★★★
"Josh and his professional crew did an excellent job replacing my roof."
Bob A.Roof replacement, Florida Gulf Coast
★★★★★
"Wonderful to work with this company very good people and professional."
Paul P.Roofing customer, Florida Gulf Coast
Keep reading

More on flat and commercial roofs

Flat Roofing

TPO, modified bitumen, built-up roofs and coatings, and how we repair, restore and replace them on the Gulf Coast.

See our flat roof systems →

Commercial Roofing

Flat roofs for retail, office and warehouse buildings, scheduled around your business hours.

Plan a commercial roof →

Ponding Water on a Flat Roof

The 48-hour rule, what leaves water sitting on a roof, and how drainage gets corrected.

Read about ponding water →

Seeing cracks across the surface? Read about alligatoring on flat roofs, compare materials for a sloped Florida roof, or ask us about your building.

FAQ

More questions about flat roof materials

What is the best flat roof sealant?

Sealant patches a spot. For the whole roof you want a coating system: silicone where any water lingers, since Gaco rates its silicone coatings for ponding, or acrylic on a roof that drains well and has more pitch. Either one needs a sound, dry roof under it.

How long does a flat roof coating last?

InterNACHI's life chart lists aluminum coating at 3 to 7 years and doesn't list acrylic or silicone. GAF offers 10, 15 or 20-year warranties on its coating systems, depending on the system, but a warranty term isn't a lifespan. How long any coating lasts depends on the roof under it being sound and dry.

Can a new flat roof go over the old one?

Sometimes. Under the Florida Building Code, Existing Building, Section 706.3 (2023 edition), the old roof has to come off down to the deck if it's water-soaked or too deteriorated to serve as a base, or if it already has two or more roof coverings. A single sound, dry layer can usually take one recover. On an older built-up roof with a sound substrate, we can install a modified bitumen or TPO overlay.

Should I replace the flat section when I replace the main roof?

If both are near the end of their life, price them together. The joint where the flat section meets the sloped roof gets built once, and you aren't setting up a second job a few years later. If the flat section still has good years in it, a repair or a coating may be the better use of the money.

Sources

  1. InterNACHI: Standard estimated life expectancy chart for homes
  2. InterNACHI: Inspection glossary, alligatoring
  3. Polyglass: What is modified bitumen roofing?
  4. Johns Manville: TPO application guide and installation instructions
  5. Carlisle SynTec: Sure-Weld TPO product data
  6. Johns Manville: JM TPO data sheet (4-22)
  7. Gaco: GacoFlex S20 silicone roof coating
  8. Gaco: GacoRoof GR16 silicone product data sheet (10/2022)
  9. Gaco: GacoFlex A47 acrylic product data sheet (12/2022)
  10. GAF: Liquid-Applied Roofing Manual, version 2.2
  11. Lawrence Berkeley National Laboratory and U.S. DOE: Guidelines for Selecting Cool Roofs (2010)
  12. U.S. Department of Energy, Energy Saver: Cool roofs
  13. Professional Roofing (NRCA): Still water runs deep (July 2012)
  14. FRSA: Tapered insulation considerations (transcription, 2017 code slope text)
  15. Florida Building Commission: Analysis of Changes, 2023 Florida Building Code, Residential
  16. Florida Building Code, Residential (2023), Chapter 9: Section R905.2.2, asphalt shingle slope
  17. Alachua County: Residential reroof requirements (transcription, R905.2.2)
  18. Florida Building Code, Existing Building (2023): Section 706.3, Recovering versus replacement
  19. FRSA: 2026 FBC code modifications (transcription, Section 706.3)

Find out which flat roof system your building needs

Free drone-assisted inspection with a written photo report you keep. No trip fee, no diagnostic fee, and if a repair or a coating will do the job, we'll tell you.

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