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Carport and sunroom roofs

Carport and Sunroom Roof Replacement: TPO Over Old Metal

Carport and sunroom roof replacement can often go right over old metal. Josh Hildebrand fills the metal's ridges with ISO board to make the surface flat and level, then installs a TPO roof on top. The old metal must be sound, dry and well fastened first.

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

Carport and sunroom roofs are too flat for shingles and too ribbed for most flat roofing. Josh explains what Coastline does about that.

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How Josh does it

How does carport and sunroom roof replacement work over old metal?

Josh Hildebrand's method is to fill the ridges of the old metal roof with ISO board, make the roof flat and level, then put a TPO roof on top of it.

Josh filmed this on a job where a mobile home was getting a new shingle roof. He asked the question homeowners ask him: "What do you do over the carport? What do you do over the sunroom?" His answer: "We're going to get what's called an ISO board. We're going to fill the flutes, make it flat. We're going to make the roof level all the way across and that way we can go ahead and put a TPO product on there." Then: "We're going to do a TPO roof on top of this existing metal roof."

Here is what those words mean. Flutes are the ridges and valleys that run along a metal panel. ISO board is a rigid foam insulation board. TPO is a single-layer plastic roofing membrane, usually white. Put together, the foam board turns a ribbed surface into a level one, and the membrane goes over the foam.

Why not simply put shingles on it? Because the roof is too flat. The Florida Building Code, Residential, Section R905.2.2, allows asphalt shingles only on slopes of 2:12 or steeper, which means 2 inches of rise for every 12 inches of run. Carport and sunroom roofs are often flatter than that. They need a membrane made for low slopes. See our flat roofing page for how Coastline handles them.

That video is the source for what Coastline does here. Every job still starts with a look at the roof, because the metal underneath decides whether this approach fits.

Carport or sunroom roof giving you trouble?

Coastline does low-slope roofs, including TPO over an existing metal carport or sunroom roof. Tell us what the roof is doing, and we will look at it and send a written photo report. Josh reviews every job. Coastline Roofing works all over Florida (license CCC1331076).

Call (941) 896-7793 or text your town and a photo.

TPO basics

What is a TPO roof, and why use it over metal?

TPO is a heat-welded plastic membrane. It is made for flat and low-slope roofs, and white TPO also reflects a lot of sunlight.

Carlisle SynTec's data sheet calls its TPO a heat-weldable, single-ply thermoplastic polyolefin sheet for new roofs and re-roofing. Johns Manville's installation guide says hot air welding equipment is required to make every field seam, and the welded seam must be at least 1.5 inches wide. A welded seam means the sheets are fused together, not just glued.

Both sheets are public. Both show TPO's sun-reflecting side. Carlisle's white TPO measures 0.79 solar reflectance new and 0.70 after three years uncleaned, which means it bounces back 79 percent of sunlight at first and 70 percent after weathering. Its sheet also lists a UL 2218 Class 4 hail rating.

Why does that matter on a carport or sunroom? A dark or bare roof takes Florida sun full force. The Department of Energy and Lawrence Berkeley National Laboratory say a conventional roof can reach 150 degrees Fahrenheit or more on a sunny afternoon, while a reflective roof can stay more than 50 degrees cooler. A white membrane over the old metal changes the surface that takes the sun.

Thickness varies. Carlisle lists TPO in 45, 60 and 80 mils. A mil is one thousandth of an inch. Thicker sheets resist punctures better. Coastline's flat roof work uses TPO.

Decision points

When can TPO go over an old metal roof, and when does the metal have to come off?

Four things decide it, and a roofer has to look at each one.

Is the metal sound and dry?

The Florida Building Code, Existing Building, Section 706.3, requires tear-off to the deck where the roof is water soaked or has deteriorated. A metal roof with rust-through, loose panels or wet insulation under it is not a base for anything new.

How many layers are there already?

The same section requires tear-off where the roof has two or more applications of any type of roof covering. A carport that already had a roof-over is a different job than one with a single original layer.

Will the fasteners hold?

JM says to fasten through the top flange on metal decks, with at least 3/4 inch of thread through the metal. It also says that if you are unsure of the deck's condition, pull-out tests of the fasteners must be done to confirm the grip.

Will water drain off?

Most low-slope roof types need a design slope of at least 1/4 inch per foot, per FRSA's reading of the code. A reroof can be exempt from that minimum if it has positive drainage, meaning it clears water within 48 hours of rain.

The process

What does a carport or sunroom TPO roof job involve?

After the inspection, the job runs in a set order: prepare the old metal, fill the flutes, lay the membrane, weld the seams and flash the edges.

  1. Inspect the roof and the structure

    A roofer walks the roof or flies a drone, checks the metal, fasteners, edges and drainage, and takes photos for a written report.

  2. Prepare the metal

    JM's guide says to sweep the roof and make sure the surface is completely smooth. Loose or damaged metal gets fixed first.

  3. Fill the flutes with ISO board

    This is Josh's step. The foam board fills the ridges and makes the roof flat and level all the way across. JM says a membrane over an old roof may need a separator, most often a roof insulation board, and that insulation must be fastened to the deck on its own.

  4. Lay the TPO membrane

    The sheet is unrolled without stretching and allowed to relax. JM says to let it rest at least 15 minutes above 60 degrees Fahrenheit, 30 minutes below that.

  5. Weld the seams

    Hot air welding fuses the seams, with a welded seam at least 1.5 inches wide, per JM.

  6. Flash the edges and penetrations

    JM's guide says base flashing must extend at least 8 inches up vertical walls. Pipes, vents and wall edges are where leaks start, so they get the most careful work.

Permit rules differ by city and county, so see our Manatee County roof permit guide and check with your building department.

Leaks and repairs

My carport or sunroom roof leaks. Do I need a full replacement?

Not always. Many leaks come from a few failed fasteners, seams or edges, and those can be repaired. A roof that is rusted through or has several layers needs more.

Leaks on these roofs often trace to fasteners or to where the roof meets a wall. If that is your problem, read why metal roof screws leak and flashing failure at walls.

While you wait for a repair, our guide on what to do until the roofer arrives covers the safe steps. Coastline also does temporary repairs and tarps. You can read more on the tarping page.

The choice between repair and replacement comes down to the same four points above: how sound the metal is, how many layers there are, how well fasteners hold, and whether water drains. A written photo report shows each one, so you are not guessing.

If you want to see what a roof job costs in general, the flat roof cost guide explains what moves the price. We do not quote a number in an article because every carport and sunroom is different.

Mobile home carports and sunrooms

Josh shot his video on a mobile home. If yours is one, see our mobile home roof leak guide and mobile home roofing page. The main roof and the carport roof are usually different jobs.

Reviews

What homeowners say about working with Coastline

★★★★★
"From beginning to end the roof was done in 2 days. One night the crew stayed until 8:00. Each evening before leaving the crew ensured the yard was cleaned. We are so happy, and our roof is beautiful!"
Sharon T.Roof replacement
★★★★★
"Russell was great to work with. He was prompt in coming out to assess our roof and very knowledgeable. ... The job was complete in 2 days once work began."
Bryan S.Roof replacement
★★★★★
"Josh was especially amazing. He was professional, patient, and took the time to explain everything clearly so I felt comfortable and informed throughout the entire process. You can tell he genuinely cares about his customers and takes pride in his work."
Madison T.Roofing customer
Keep reading

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How Long a Flat Roof Lasts in Florida

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Also see flat roof drains and scuppers and ponding water.

FAQ

More questions

Can you put a new roof over an old metal carport roof?

Often, yes. Josh says Coastline fills the flutes with ISO board, makes the roof level, and installs TPO on top of the existing metal. The Florida Building Code, Existing Building, Section 706.3, requires tear-off if the roof is water soaked, deteriorated, or already has two or more roof layers.

Why are the flutes filled before the TPO goes on?

A metal panel has ridges and valleys. Josh says the ISO board fills the flutes so the roof is flat and level all the way across, which gives the TPO an even base. JM's guide also says a roof insulation board is the most common separator between an old roof and a TPO membrane.

Can shingles go on a sunroom or carport roof?

Only if the slope is steep enough. The Florida Building Code, Residential, Section R905.2.2, allows asphalt shingles only on roof slopes of 2:12 or greater. Carport and sunroom roofs are often flatter than that, so they need a low-slope membrane such as TPO.

Are the TPO seams glued?

No. Johns Manville's installation guide says hot air welding equipment is required for all field seams, and a welded seam must be at least 1.5 inches wide. The heat fuses the sheets together into one piece instead of relying on adhesive.

Does a white TPO roof keep a sunroom cooler?

It can help. Carlisle's data for white TPO shows 0.79 solar reflectance new and 0.70 after three years uncleaned. The Department of Energy and Lawrence Berkeley National Laboratory say reflective roofs can stay more than 50 degrees Fahrenheit cooler than conventional roofs on a sunny afternoon.

Does Coastline do carport and sunroom roofs?

Yes. Josh says Coastline puts TPO over existing metal carport and sunroom roofs, using ISO board to fill the flutes. A roof inspection with a written photo report comes first, so the plan fits what is actually on the roof.

Sources

  1. Johns Manville: TPO application guide and installation instructions
  2. Carlisle SynTec: Sure-Weld TPO reinforced membrane product data
  3. FRSA: 2026 FBC code modifications (transcription of Existing Building Section 706.3)
  4. Alachua County: Residential reroof requirements (transcription of FBC Residential R905.2.2)
  5. FRSA: Tapered insulation considerations
  6. Lawrence Berkeley National Laboratory and U.S. DOE: Guidelines for Selecting Cool Roofs

Get your carport or sunroom roof looked at

Send the address and a few photos. We will inspect the roof and send a written photo report that says whether TPO over the old metal fits, or something else does.

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