Skylight Panels for a Steel Building and What to Buy

A steel building with no natural light means running fixtures every hour you are inside, even at noon in June. Skylight panels fix that for a few hundred dollars per opening, but the wrong panel turns yellow, cracks at the fasteners, or drips onto your workbench within five years.

I write quotes for a living, and translucent panels are one of the most under-specified lines on a typical kit order. The brochure says "light transmitting panels, qty 4" and nothing else. This guide covers what those skylight panels are actually made of, how many you need, where to put them, and what a good one should cost.

What skylight panels on a steel building actually are

On a metal building, a skylight is rarely a framed glass unit like you would see on a house. It is a translucent panel rolled to the same rib profile as your steel roof sheets, so it laps into the run and gets fastened with the same screws. Swap one 3 foot wide steel sheet for one translucent sheet and you have a skylight.

That matching profile is the whole point. If your roof is a 36 inch PBR panel with 1-1/4 inch ribs, the skylight panel must be rolled to that exact PBR profile from the same manufacturer pattern. A panel that is "close enough" leaves gaps at the side laps that no amount of sealant tape will hold for long.

The two common materials

• Fiberglass reinforced panel (FRP). The traditional choice. Glass fibers in a polyester resin, usually 8 oz or 12 oz per square foot. Cheaper, stiffer, and easy to cut.
• Polycarbonate. Clearer, far more impact resistant, and lighter. It moves more with heat, so fastening details matter more.

Fiberglass or polycarbonate panels compared

Here is how the two stack up on the numbers I see most often on quotes for a 12 foot panel in a common rib profile.

Clear is not always better. A white or "milky" panel transmits less light, roughly 40 to 55 percent, but it diffuses it evenly and cuts the hot spots and glare that make a clear skylight miserable over a workbench or a parked truck. For shops, barns, and storage I usually recommend white. Clear makes sense in a greenhouse style space or where you genuinely want direct sun.

How many skylight panels do you need

The working rule most suppliers use is that translucent area should equal 3 to 5 percent of the floor area for a general shop. Go higher, toward 8 to 10 percent, for agricultural buildings where you want animals or crops to see daylight.

Take a 40 x 60 building. That is 2,400 square feet of floor. At 4 percent you want about 96 square feet of translucent panel. One 3 x 12 panel is 36 square feet, so three panels gets you to 108 square feet, which is plenty for daytime work without lights.

1. Multiply length by width for floor area.
2. Multiply by 0.04 for a shop, or 0.08 for a barn.
3. Divide by the area of one panel (usually 36 square feet for 3 x 12).
4. Round up and split the total evenly between both roof slopes.

Spread them out. Three panels in a row on one slope light one strip of floor brilliantly and leave the far wall in shadow. Stagger them across both slopes and keep each one at least one panel width from the ridge and the eave.

Where skylights cause trouble on a steel roof

Most of the skylight complaints I hear come back to placement and fastening, not the panel itself.

Never in a valley or under a roof drain path

Water collects at side laps, and a translucent panel has more side laps exposed to sun than anything else on the roof. Keep panels out of any area where snow drifts pile up or water channels from an upper roof. If you are already chasing drips, our guide to finding a steel building roof leak covers how to tell a skylight lap leak from a fastener leak.

Fasteners and oversized holes

Polycarbonate grows and shrinks a lot with temperature. A 12 foot panel can change length by close to a quarter inch between a January night and a July afternoon. Pre-drill holes 1/8 inch larger than the screw shank and use screws with a wide bonded washer so the panel slides under the washer instead of cracking around it. Do not overdrive. The washer should just compress, not mushroom out.

Condensation under the panel

A single layer translucent panel has almost no insulating value, so it is the coldest surface in the roof on a winter morning. It will drip before the steel does. If you have insulated the rest of the roof, ask for a double skin panel or an insulated liner that matches the profile, and read our notes on stopping condensation drips in a steel building before you finalize the order.

The common advice I disagree with

You will often hear "just add a ridge vent or wall lights instead, roof skylights always leak eventually." I do not buy it as a blanket rule. In my experience the leak rate on properly profiled, properly fastened skylight panels is no worse than on the steel sheets around them for the first 15 years. The leaks come from panels bought separately from a different supplier, profiles that almost match, and installers who use the same tight torque they use on steel.

Wall lights, translucent panels set high in the sidewall, are a fine option, and they never collect standing water. But they only light the first 15 to 20 feet in from each wall. On a building wider than 40 feet the middle of the floor stays dim, and you end up running lights anyway. For wide buildings, roof panels earn their place.

The fair version of that advice is this: if you cannot get panels from the same manufacturer as your roof sheets, or the roof is lower than a 1:12 pitch, put your translucent area in the walls.

What to put on the order before you sign

Write these into the quote so nobody substitutes a cheaper product later:

• Material (FRP or polycarbonate) and weight or thickness.
• Color: clear, white, or a specified light transmission percentage.
• Profile name, matched to the roof sheets, from the same supplier.
• UV coating and the written warranty term against yellowing.
• Closure strips and butyl tape sized for the translucent laps.
• Fasteners with oversized washers for polycarbonate.

Also check coating compatibility. Some polycarbonate warranties are voided by contact with certain sealants, and some steel coatings react badly to water running off FRP. If you are still weighing panel finishes, our breakdown of galvanized versus Galvalume coatings explains why runoff matters.

Frequently asked questions

Do skylight panels make a steel building hotter in summer?

They add some heat, especially clear panels on a south facing slope. White panels at around 4 percent of floor area add very little noticeable heat in most climates. If summer heat is your main worry, choose white and keep the total translucent area at the low end of the range.

How long do fiberglass skylight panels last?

A good 8 oz FRP panel with a UV surface film usually lasts 15 to 20 years before it gets brittle or noticeably yellow. Cheap panels without a UV film can start fading and showing fibers within 5 to 7 years. The warranty term on yellowing is the best clue to which kind you are buying.

Can you walk on translucent roof panels?

No. Neither FRP nor polycarbonate is designed to carry a person, and falls through skylights are a well known cause of serious roof injuries. Treat every translucent panel as a hole in the roof, step only on the steel at purlin lines, and use a fall protection screen if the building will see regular roof work.