How to Put a Metal Roof on a Shed

To put a metal roof on a shed, you screw formed steel panels through rubber washers into a sheathed deck or into purlins, over underlayment, then close the edges with drip edge, ridge cap and trim. The panel maker’s installation sheet sets the screw pattern, the laps and the minimum pitch, and the model code tells you to follow it.

This guide covers what goes under the panels, how many metal shed roof panels and screws a shed needs, how to attach them, and the one place a lean-to shed metal roof can go wrong: a 2/12 pitch. The counts come from the shed designer, which has a metal roof option. The deck and rafters come first, and the how to build a shed roof guide covers that sequence up to a dried-in deck.

Before you buy metal shed roof panels

Three decisions come before the first panel: what the panels sit on, what goes under them, and whether your pitch is steep enough. A metal roof is not one product. Every maker prints its own instructions, and when they differ, the instructions for the panel you buy win.

Plywood or purlins under metal

The 2021 model code (IRC, R905.10.1) says metal roof panels go on solid or spaced sheathing, unless the panel is specifically designed for spaced supports. So the code allows both a full deck and purlins.

The makers lean toward a deck on a home or a shed you want to keep dry. 1st Coast Metal Roofing Supply says solid decking is highly recommended for all residential applications, and that purlins, when used, should be spaced 24 inches on center at most. Fabral shows 2x4 purlins at 24 inches on center. Metal Sales says its panels can go over open framing or a solid substrate, and recommends 5/8 inch plywood with a 30 pound felt moisture barrier.

The designer draws a solid deck of 7/16 inch OSB, in 4x8 sheets, whatever the roofing. That is the size the model code table gives for a 24 inch span (2021 IRC, Table R503.2.1.1(1)), and the designer’s 16 inch rafters are well inside it. It is thinner than the 5/8 inch Metal Sales recommends, and Fabral’s wind uplift table has a row for 7/16 inch OSB. So the deck is a question for your maker, not a rule: if your panel calls for 5/8 inch, use thicker sheathing or add purlins over the OSB. The shed rafters guide covers the framing under the deck.

Underlayment under metal roofing

Metal panels get underlayment too, and the model code sends you to the maker: its table for metal panels says “manufacturer’s instructions” (2021 IRC, Table R905.1.1(1)). What the makers we read ask for:

• Fabral: cover the whole deck with 30-pound felt, Typar or the equivalent, courses lapped at least 4 inches.
• Metal Sales: 30 pound felt (or ice and water shield where local code asks) over a wood deck.
• 1st Coast: synthetic underlayment or 30-pound felt over solid decking.

The designer lists synthetic underlayment for both shingle and metal roofs, in a 10-square roll: 1.7 squares on the 10x12 either way. Drip edge is also the same for both, 6 pieces of 10 ft on the 10x12. The order of underlayment and drip edge on a dried-in deck is in the how to build a shed roof guide, and it applies here too.

The minimum pitch for metal

This is where metal differs most from shingles. The 2021 model code (IRC, R905.10.2) sets three minimums:

The maker can ask for more. Metal Sales lists 3:12 as its minimum for Classic Rib. Fabral says its Grandrib 3 must go on 2-1/2:12 or steeper. 1st Coast prints 1/2 inch per foot, and says anything under 3 inches per foot needs side lap sealant. McElroy Metal says sealant at exposed-fastener side laps is customary below 3:12. The shed roof pitch guide covers pitch in general; for metal, these are the numbers to keep.

The designer’s gable sheds run 4/12 to 6/12, so they clear every line above. Its lean-to sheds run 2/12 to 4/12, and that range crosses the 3:12 line.

Metal shed roof panels: how many you need

The designer’s metal option keeps the 7/16 inch OSB deck, the synthetic underlayment and the drip edge. It swaps the shingles for 36 inch coverage panels cut to length, ridge cap in 10 ft pieces (gable roofs only), and metal roofing screws at about 80 per 100 sq ft of roof, in bags of 250. The ridge cap line says to buy closure strips to match.

Panels, ridge cap and screws for 16 gable sheds

This is the designer’s result for the 16 gable presets at the default 6/12 pitch with the roofing set to metal. The first number in the shed size is the width across the door wall. The ridge runs along the depth, so the panels are counted along the depth on both slopes.

Screws are the designer’s rounded estimate. One bag of 250 covers every shed up to the 12x20; the 12x24 and the 16x20 need a second bag. The ridge cap pieces are 10 ft long with 6 inch laps, so a 12 ft ridge takes 2 pieces. Screws for the trim, sealant tape and the closure strips are not in the count: add them from your maker’s list.

Metal beside shingles on the 10x12

Here is the same 10x12 gable shed with each roofing choice. The framing, the 8 sheets of OSB, the underlayment and the drip edge do not change.

The shingle roof, its 3 bundles per square and its 10 percent waste are in the how to shingle a shed roof guide. The 10x12 gable plan has the framing cut list under both.

Counts for the 4 lean-to sheds

The lean-to presets default to 3/12. They have no ridge, so the designer lists no ridge cap.

Each lean-to takes one bag of screws. The lean-to shed guide covers how the framing and the single slope work.

How to put a metal roof on a shed, step by step

The steps below are general practice. Your panel maker’s sheet overrides them, and it says so at the top of every guide we read. Fabral’s own guide calls itself a guide only, and says it does not replace local or state building codes.

Get the deck ready

1. Check the deck. Fabral says metal roofing needs a relatively smooth and flat substrate, that high and low spots show up as waves, and that no nails may protrude to puncture the underlayment. Metal Sales tells you to check the roof deck for square: measure diagonally across the slope from matching points at the ridge and the eave, and get the same number both ways.
2. Roll out the underlayment. Start at the eave and run each course parallel to it, lapping the courses as your maker says. Fabral asks for at least 4 inches between courses.
3. Set the drip edge and any eave trim first. The eave trim goes on before the panels, and the gable trim goes over or under them, as your maker shows. Butyl tape or sealant sits in the trim laps in the guides we read.
4. Find the first panel line. Fabral has you mark a line 1/2 inch in from the gable edge, square to the eave, and check that the first panel overhangs the eave by 1 inch. 1st Coast says an eave overhang of 1 to 2 inches gives you the drip edge.

Hang the panels

5. Start away from the weather. 1st Coast says the side laps should face away from prevailing rain, so start at the downwind end, farthest from the wind’s source.
6. Fasten the first panel, then the next. Lay the second panel’s overlap edge on the first and fasten in the maker’s pattern. Repeat across the roof.
7. Do the ridge last. Cap the ridge with the ridge cap and closure strips. 1st Coast says closures may be omitted only if the owner, designer and installer agree, and adds that the roof will then not be protected from wind-driven rain.
8. Cut where you have to. See the cutting notes below.

Cutting metal panels

Metal Sales recommends tin snips or a nibbler for field cutting, and says to flip the panel over so the slivers can be brushed off the back. Metal chips left on the paint damage the finish. Fabral discourages abrasive blades, because the saw cut is jagged, burns the paint and galvanizing at the edge, and leads to edge rust. 1st Coast says the same about self-consuming abrasive blades, and allows a carbide-tipped blade made for light-gauge metal if a saw cut cannot be avoided. Snips or a nibbler are the safe common ground.

How to attach a metal roof to a shed

Screws hold the roof on, and the screw pattern is the part of an installation where the makers write the most. The model code (2021 IRC, R905.10.4) says metal panels are secured to their supports in accordance with the manufacturer’s installation instructions. It gives no screw spacing of its own.

Screws and washers

Use the fastener your maker names. The two you meet most are a #14 x 1 inch screw for solid decking and a #10 x 1 inch wood-grip screw for 2x4 purlins, which are the ones in Fabral’s estimator. With the model code as a fallback where there are no instructions, the screws for a steel roof must be galvanized, and stainless steel is accepted on any metal panel (2021 IRC, R905.10.3 and R905.10.4). 1st Coast says nails are not recommended.

The designer’s screw count, about 80 per 100 sq ft, matches what Metal Sales says is typical for 3 ft wide panels: 80 screws per square. Another maker’s pattern can use more or fewer. Buy by your panel’s pattern, and buy a spare handful.

The washer is the seal, so how far you drive matters. Metal Sales shows a correct screw with sealing material slightly visible at the edge of the washer, a loose one where it is not visible, and a too tight one where the washer is deformed. It says to use a screw gun with a depth-locating nose or an adjustable clutch, and not to use impact tools. Metal Sales also says an overdriven screw causes stress and oil canning across the face of the panel near the screw.

Screws in the flat or in the rib

Makers do not agree here. 1st Coast says to drive its wood screws in the flat area of the panel, next to the ribs, tight enough to compress the washer. Metal Sales lists installing fasteners in the ribs rather than in the flat as one way to handle thermal movement in a long panel run. Each panel’s fastening diagram shows which one it wants, so read your diagram and do not carry a pattern from a different profile.

Laps, closures and sealant

Side laps on a low slope are where leaks start, which is why the pitch table above has a sealant line. On the makers’ sheets, ends get sealant every time: 1st Coast says all end laps require sealant, and Metal Sales lays the ridge-side panel over the eave-side panel on tape sealant with a 6 inch end lap. On a shed the panels usually run the whole slope, so you seldom have an end lap at all. Order the length the designer gives you, or the one you measure, and skip the joint.

Lean-to shed metal roof, and the 2/12 trap

A lean-to is the shed where the pitch minimum bites. The designer’s lean-to range is 2/12, 3/12 or 4/12, and the 3/12 default sits exactly on the model code’s minimum for lapped panels with no sealant.

At 2/12 there is no code-clean way to use plain lapped panels. Under the model code (2021 IRC, R905.10.2) a 2/12 roof needs one of these: lap sealant applied in the laps as the maker directs, a standing-seam panel, or a steeper pitch. And the maker can ask for more than the code: Fabral’s guide starts at 2-1/2:12, and Metal Sales recommends 3:12, so neither guide covers a 2/12 lean-to.

The designer checks the code minimum. Set its 8x6 lean-to to 2/12 with metal roofing and it stops with an error that asks for 3/12 or steeper, or shingles, and the builder’s plan cannot be bought until the design passes. It lists plain lapped panels, so it has no setting for lap sealant or a standing-seam panel; if you go that way, the panel maker’s instructions decide. For shingles at 2/12 it prints a warning instead, about a second layer of underlayment. To stay on the safe side of every source in this guide, keep a lean-to at 3/12 or 4/12. You can try the 8x6 at 4/12 in the designer.

The deck edge matters too. Under the model code the roof sheathing may not cantilever more than 9 inches past a gable end wall without gable overhang framing (2021 IRC, R803.2.3), and that limit applies to a metal roof deck as much as a shingle one.

Mistakes to avoid on a metal shed roof

These come straight from the sheets we read, so they are the ones worth checking before the last panel.

• Overdriving or underdriving screws. Metal Sales and Fabral both print a correct, a too loose and a too tight picture. McElroy Metal says that when exposed-fastener roofs fail, the underlying error is most often credited to improper fastener use.
• Walking on the panels too soon. Fabral says not to walk on panels until every fastener is in, and that a metal roof is slippery when wet, dusty, frosty or oily. Metal Sales says to walk only in the flats, since walking on the ribs can damage them. 1st Coast agrees.
• Cutting with an abrasive blade. Covered above: the edge burns and rusts.
• Skipping lap sealant on a low slope. Below 3:12, a lap without sealant is a code problem on lapped panels, and McElroy calls sealant customary.
• Wrong screw for the substrate. A wood-grip screw for purlins and a different one for a plywood deck: Fabral lists a different screw for each.
• Metal against the wrong wood or metal. 1st Coast warns that some treated lumber and dissimilar metals corrode panels, and separates them with felt, synthetic underlayment or a similar barrier.

Where to go next

Set the designer to metal roofing with your own sizes and the panel, ridge cap and screw list updates with them. If you have not built the frame yet, start with the how to build a shed guide. Permits and roof rules vary by town, so check them on the shed permit basics page before you order panels.