Yes, a flat roof can have a metal roof, but only the right kind. A metal roof on a flat roof means standing seam panels on a low slope, because no roof is truly level and water has to be able to run off. Screw-down panels need more slope than most flat roofs have.
The short version
- Standing seam metal can go on a slope as low as 1/4 inch per foot.
- Screw-down metal needs more slope, and its screws are a leak risk.
- Shingles and metal shingles are out on a flat roof.
- Metal costs the most of the flat roof options.
- Pipes, vents and skylights are where a low-slope metal roof leaks.
What slope does a metal roof need on a flat roof?
Standing seam panels can go on a slope as low as 1/4 inch of rise per 12 inches, which is a 2% slope. The Insurance Institute for Business & Home Safety (IBHS) says a standing seam roof may be installed down to 1/4 in. on 12 (opens in a new tab). The Pacific Northwest National Laboratory guide gives the same figure and calls it one-quarter unit vertical in 12 (opens in a new tab).
That is almost flat to the eye. It is also the reason a “flat” roof is never dead level. The slope is worth measuring before anyone prices metal.
Can you use screw-down metal panels on a flat roof?
Rarely. Screw-down panels, also called through-fastened or exposed fastener panels, need more slope than standing seam. The PNNL guide says lapped metal roofs with sealant need one-half unit vertical in 12 (opens in a new tab), and those without sealant need three in 12. A true flat roof is below all of those numbers.
Even where the slope allows it, every screw is a hole with a rubber washer, and washers wear out. We cover that in why metal roof screws leak. On a roof where water can sit, a roof held down by hundreds of washers is not where we would put money.
Which roofs can’t take shingles or metal shingles?
Anything flatter than 2/12 can’t legally take shingles, and metal shingles are stricter still. PNNL says the residential code does not allow metal roof shingles on slopes below three units vertical in 12 (opens in a new tab). So on a low porch roof, an addition or a garage, the choices come down to modified bitumen, TPO or standing seam metal.
IBHS notes that some codes call a roof low slope at a pitch of 2:12 or less (opens in a new tab). We see this on low sections of otherwise pitched roofs all the time, and the work is often subcontracted at a minimum of $2,500 to $5,000, which is a typical range for the area, not a quote.
How does a metal roof compare with other flat roof options?
Metal lasts longest and costs most. The table sets it beside the two membranes we hear about most.
| Standing seam metal | TPO | Modified bitumen | |
|---|---|---|---|
| What it is | Metal panels with raised, joined seams | One white sheet, seams welded with heat | Layers of asphalt roofing with polymers |
| Cost | Highest | Lower | Lower |
| Weak spot | Anything that cuts through a seam | The welds | Scuffs and tears |
This Old House says metal roofs average 40 to 70 years (opens in a new tab). That is an average for metal roofs in general, and a low-slope roof with lots of pipes will not always reach it.
Standing seam metal runs about 2 to 2.5 times the price of asphalt shingles, our rule of thumb for the area. Our guide to the best material for a flat roof compares all five types side by side.
Where does a metal roof on a flat roof leak?
At anything that cuts through the panels. IBHS says penetrations that cut through the seam (opens in a new tab) of a panel are a potential source of water getting in, and that pipes and vents should be placed to avoid the seams. Where one has to go through, the fix is a custom curb, a raised frame built around it.
On a steep roof a bad detail drips and drains. On a low slope, water lingers, so the detail has to be right the first time. We also treat caulk as a scheduled leak. Sealant lasts about 5 to 10 years, so a flat metal roof that depends on a tube of it is on a countdown.
Does a metal roof on a flat roof need underlayment?
Yes, and it needs to be the right one. Metal roofs can collect moisture on their underside in cold weather. The PNNL guide says condensation can occur (opens in a new tab) under metal roofing and calls for a continuous layer of underlayment to protect the sheathing.
Standing seam panels also need a solid deck. PNNL says they must be installed on roof decks with sheathing (opens in a new tab), not over strapping. If the deck underneath is soft, it gets fixed first.
Common questions
Can I put metal over my old flat roof?
Sometimes, but we want to see what is under it first. A buried layer that is wet or rotted would trap moisture under new metal. A walk on the roof tells us whether it is a cover-over or a tear-off.
Is metal a good choice for a flat porch roof?
For a small porch with few pipes, it can be. Fewer penetrations mean fewer weak spots. The higher price is harder to justify on a roof that only covers a few squares.
Do I need an engineer?
Not for the roofing itself. If we find sagging or rotted framing once the old roof is off, an engineer’s letter can be needed. That typically runs $600 to $1,500.
What to do next
If you are weighing metal for a flat section, start with a look at what is there. Our free roof inspection includes a written report with the measured pitch. Our flat roof installation and metal roofing installation pages explain what we put on and when each makes sense.
