Which roofs work for solar? Flat, metal, slate and tile

Solar panels work on flat roofs, and on metal, slate and tile roofs too. The roof type changes how the panels attach and which checks apply. A PVWatts model run for Washington, DC in October 2026 found a panel lying flat made 15.5 percent less than one tilted 30 degrees.¹

Flat roofs work, but the rack does the heavy lifting

A flat roof can carry solar, usually on racks that tilt the panels toward the south. Panels set flat lose output, and tilt wins most of it back. The racks have to resist wind and the roof has to carry the weight, so the roof comes first.^2,1

Flat roofs are rarely flat. The District’s Historic Preservation Office says they usually slope toward the rear of the building to drain water.³ That slope matters for ponding, and a roofer checks drainage before any rack goes down.

The numbers below come from PVWatts, a calculator from the National Laboratory of the Rockies (formerly the National Renewable Energy Laboratory).⁴ They show what tilt does to yearly output.

Four rows ran on October 17, 2026, and the 20-degree row is a stored run from October 16. All of them assume restricted air flow behind a roof-mounted panel, which raises the assumed operating temperature compared with an open rack.⁵ PVWatts’ manual describes self-shading estimates only for one-axis trackers, so these runs leave out row-to-row shading on tilted flat-roof racks. Your installer adds that check in the design.⁵

More tilt isn’t free. The Metal Construction Association, a trade group, writes that a tilted system intensifies wind loads on the roof and can cause undue snow accumulation due to drifting.⁶ An engineer weighs that against the extra output.

How are panels held down on a flat roof?

Ballasted racks use weight, usually concrete blocks, and need few or no roof penetrations. Attached racks use roof-penetrating hardware. Hybrid racks combine ballast with anchors. The roof’s load capacity and the local wind load decide which one fits, and your design engineer makes that call.²

The U.S. Department of Energy (DOE) wrote that comparison in 2015 for commercial building roofs. It describes the methods, not a rowhouse design. A separate DOE Building America guide adds two flat-roof options: elevated racks set the panel angle and can sit on curbs or standoffs, and on a synthetic rubber (EPDM) membrane, one system heat-welds plastic flanges to the membrane to avoid penetrations.⁷

What should rowhouse owners in DC know?

The District’s preservation guide says most rowhouses have flat roofs, so the rowhouse is the case worth walking through.⁸ Structure, flashing, neighbor notice and historic review all come into play. A District Department of Buildings (DOB) plan examiner can tell you how its codes apply to your roof before you commit to a design.^9,10

DC permits solar under its 2017 construction codes, which DOB describes as the 2015 International Code Council model codes and the 2014 National Electrical Code, as amended by the District. DOB is developing an update, with final codes planned for late 2027. Check DOB’s code page for the edition in force when you apply.^9,11

The District and Montgomery County sort residential solar projects differently, so check which path your roof lands on. The table sets the two side by side.

How do the DC and Montgomery County permit paths compare?

A garage or other outbuilding is a separate question, because Montgomery’s automated path covers dwelling main structures only.¹⁴ DOB’s solar page also links to Pepco’s My Green Power Connection, the utility’s application portal.¹³ Other counties publish their own solar permit rules, so check with the local permit office.

What changes if your DC rowhouse is historic?

In a DC historic district, preservation review is an extra step inside the normal building permit process, and the instant permit is off the table. There’s no separate preservation permit to file, but the review can shape where panels go and how high a rack stands.^10,16

For flat roofs, the Historic Preservation Office (HPO) guideline says to set solar back from the front edge of the roof, and from the exposed side edge on corner properties, to minimize visibility from the street. It also says solar shouldn’t cause a perceptible change in the building’s massing, height or roofline as seen from public street view.⁸ The Historic Preservation Review Board approved the guide on May 1, 2025.⁸

Here is how that plays out in a made-up case. A homeowner on a historic rowhouse wants a tilted rack on a flat roof that sits behind a front parapet. Pulled back from the front edge and hidden by the parapet, the layout matches what the guideline asks for. A rack tall enough to rise above the parapet line where passers-by can see it gets tested against the roofline principle, and that’s the conversation to have with HPO before anyone draws a final layout.

The Review Board has authorized HPO staff to approve solar that fits those principles. Proposals that don’t fit can go to the Board for review.⁸ The Office of Planning says more than 95 percent of preservation-review permit applications go through HPO’s expedited process.¹⁶ That figure covers all projects, not solar alone, so ask HPO for review times on a solar project. In Georgetown, the U.S. Commission of Fine Arts also reviews building projects.¹⁶

Should you fix the roof first or put panels on it?

Replace or repair the roof first if it will wear out before the solar system does, or if it leaks or drains poorly.^17,2 The District’s historic-building guide says to confirm the roof is watertight, with adequate slope and drainage.¹⁷ DOE’s commercial-roof FAQ says owners are encouraged to replace a roof that will need it during the system’s life.²

A leak is not something solar can wait out. DOE’s FAQ warns that faulty roof penetrations can reduce a roof’s weather-tight integrity, and a roof that already leaks has less margin.² Our guide on replacing your roof before solar goes deeper, and the one on roof leaks under solar panels explains who to call if one starts.

Before work starts, get these in writing:

1. A roofer’s report on remaining life, leaks, ponding and drainage. Stay off the roof yourself.
2. A structural engineer’s or architect’s view of the roof’s capacity for the added load.¹⁷
3. Who will flash and seal each penetration, and how the roof warranty is handled.^2,7
4. The rack, ballast and penetration layout.¹⁵

Whether a coating is enough or the membrane needs replacing is a roofer’s call.

How do panels attach to metal, slate and tile roofs?

Metal roofs often take clamps, while slate and tile need extra care. Standing seam metal can carry clips that grip the seam without a roof penetration. Most other roof types need penetrations that are flashed and sealed. Foot traffic can crack slate, and tile adds weight, so these roofs call for a specialist and a structural check.^7,18

Metal

A standing seam roof attaches through concealed clips in its seams, which avoids penetrating the weathering membrane. A face-fastened roof screws panels to the structure with visible fasteners.⁶ The Metal Construction Association says mounting must not compromise the roof or jeopardize its warranty.⁶ Ask the installer how each attachment is fastened and sealed.

Slate and tile

The National Park Service’s Preservation Brief 29 (Spring 1993) says slate roofs last 60 to 125 years or longer when installed properly, advises keeping foot traffic off them, and says a damaged slate can usually be repaired or selectively replaced.¹⁸ DOE’s guide says masonry roofs may need extra structural review because of the weight of the tiles, and tells installers to coordinate with roofers so the roof warranty stays intact.⁷ Neither document covers mounting solar on slate, so settle the attachment method with a roofer who works on slate, and find out who replaces a slate or tile that breaks. In a historic district, ask HPO whether work on a slate or tile roof needs review.

Pitched roofs also come with layout rules. DOE’s guide says the panel layout has to meet the International Residential Code’s solar section, which calls for firefighter access: a 36-inch-wide clear path along the ridge and two paths from the eave to the ridge.⁷ Your local code edition governs the details.

Five facts that decide your roof’s path

Five facts move your roof from a quick permit to a standard review, or from solar-ready to repair-first:

• Historic status. A designated historic property can’t use DC’s instant permit and goes through preservation review.^10,16
• Ballast. In Montgomery County, a ballasted system uses the standard application, not eSolar.¹⁴
• Building type. A flat apartment or commercial roof isn’t a one- or two-family home, so the instant permit doesn’t apply. Ask DOB which path fits.¹⁰
• Roof life and leaks. A roof that will need replacing during the system’s life, or already leaks, gets fixed first.^2,17
• Load capacity. An older rowhouse roof may need an engineer’s review before ballast or tilted racks.¹⁷

How do you get your roof checked?

Soleil Energy’s residential page describes its free consultation as a 30-minute call that tells you whether solar is worth doing, and its assessment step covers your roof’s layout, age, condition and shading, plus your electrical panel and electric bill. Tell the team what kind of roof you have (flat, metal, slate or tile) and how old it is.¹⁹

For home solar installation in Maryland, start with a free consultation. You can also call (301) 953-4140. The service area page names the counties covered, and our guide on what happens at a solar consultation and site survey shows what to expect.

This guide is general information, not legal, tax, or financial advice.

Sources (19)

1. Illustrative calculation, run October 2026, with the inputs sourced at ⁴. ↩ ↩² ↩³

2. U.S. Department of Energy, Better Buildings Alliance, Commercial Rooftop Solar, Frequently Asked Questions, August 2015, https://web.archive.org/web/20251013211236/https://betterbuildingssolutioncenter.energy.gov/sites/default/files/attachments/BBA_Commercial_PV_Roofing_Guide_071715_508.pdf (Internet Archive copy of October 13, 2025; the original site address no longer loads in browsers); read October 25, 2026. Written for commercial roofs. ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶ ↩⁷ ↩⁸ ↩⁹ ↩¹⁰ ↩¹¹

3. District of Columbia Office of Planning, Historic Preservation Office, Roofs on Historic Buildings, undated (file created June 2015); read October 25, 2026. ↩

4. National Laboratory of the Rockies, PVWatts Calculator, model version 8.5.0; runs for a Washington, DC point on October 17, 2026, with the 20-degree case a stored run from October 16, 2026. ↩ ↩²

5. National Renewable Energy Laboratory (now the National Laboratory of the Rockies), PVWatts Version 5 Manual, September 2014; read October 25, 2026. ↩ ↩²

6. Metal Construction Association, Metal Roofing and Solar PV Systems, Part 2: Mounting System Methods, white paper, September 2021 (Internet Archive copy, read October 25, 2026; the publisher’s own address returned an error to automated requests that day). Trade-group source; context only. ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶

7. U.S. Department of Energy, Building America Solution Center, Roof Anchor System for Solar Panels, last updated September 15, 2020; read October 25, 2026. ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶ ↩⁷ ↩⁸ ↩⁹

8. District of Columbia Office of Planning, Sustainability Guide for Older and Historic Buildings, published 2019, revised 2025, with approval date on the publication page; read October 25, 2026. ↩ ↩² ↩³ ↩⁴ ↩⁵

9. District of Columbia Department of Buildings, DC Construction Codes, undated page; read October 25, 2026. ↩ ↩²

10. District of Columbia Department of Buildings, Solar Panel, page footer 2019-2025; read October 25, 2026. ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶

11. District of Columbia Department of Buildings, Construction Code Development, undated page showing the 2026-2027 schedule; read October 25, 2026. ↩

12. District of Columbia Department of Buildings, Neighbor Notification, undated page; read October 25, 2026. ↩

13. District of Columbia Department of Buildings, Energy and Green Building Solar, page footer 2019-2025; read October 25, 2026. ↩ ↩²

14. Montgomery County Department of Permitting Services, eSolar Residential Rooftop Solar Panels, undated page; read October 25, 2026. ↩ ↩² ↩³

15. Montgomery County Department of Permitting Services, Permit Guidelines for Solar Photovoltaic (PV) Systems, footer date December 9, 2025; read October 25, 2026. ↩ ↩²

16. District of Columbia Office of Planning, Permits and Design Review, undated page; read October 25, 2026. ↩ ↩² ↩³ ↩⁴

17. Same source as ⁸. ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶ ↩⁷ ↩⁸

18. National Park Service, Preservation Brief 29: The Repair, Replacement, and Maintenance of Historic Slate Roofs, Spring 1993 print date; read October 25, 2026. ↩ ↩² ↩³

19. Soleil Energy, Residential Solar, undated service page; read October 25, 2026. ↩