How to Build a Shed Roof: Sheathing to Drip Edge

To build a shed roof, set the roof type first, then work in order: frame the rafters, ridge and ties, hang the fascia and rake boards, lay the OSB deck, add drip edge and underlayment, and only then shingle or roll on metal. The order matters, because each layer laps over the one before it.

This guide covers the sequence and the deck, from the last rafter to the last sheet of underlayment. The rafter maths, the pitch chart and the two coverings each have their own guide, and this one points to them. The numbers below are what the shed designer computes for its 20 presets, so they match the cut lists on the free plans.

Shed roof types: gable, lean-to and the rest

Pick the roof type before you buy any lumber, because it sets the walls, the framing and the covering. The designer plans two types. Everything else is a different build.

Gable roofs

A gable roof has two slopes that meet at a ridge, with a triangle of wall (the gable) at each end. It is the roof most people picture on a backyard shed. On the designer it comes in 4/12, 5/12 or 6/12, with 6/12 the default. The frame is rafters in pairs, a ridge board at the top and a 2x4 tie across each pair. A gable roof sheds water to both long sides and gives headroom in the middle.

Lean-to (single-slope) roofs

A lean-to roof is one slope, high at the front and low at the back. The designer offers 2/12, 3/12 or 4/12 with 3/12 as the default. Its rafters span the full depth of the shed and rest on a high front wall and a low back wall. There is no ridge board and no rafter ties. Our lean-to shed guide covers the wall heights and the single-slope details.

Gambrel, hip, saltbox and flat roofs

The designer does not draw these. As general practice, a gambrel is the barn shape with two slopes on each side, a hip slopes on all four sides, a saltbox is a gable with one long slope and one short slope, and a flat roof is a very low single slope. Each one needs its own framing plan and, for a hip or gambrel, angled cuts that a plain rafter table does not cover. If you want one of them, work from a plan made for it and check the panel and covering rules below against your plan.

Which one is easiest to build

The lean-to has fewer parts. The designer’s 10x8 lean-to lists 9 rafters, and its 10x12 gable lists 20 rafters, a ridge and 10 ties. The lean-to costs you in other ways: a taller front wall, and a shallow pitch that limits which covering you can use. See the shed roof pitch guide for the minimum pitch by covering.

The order of work on a shed roof

This is the sequence the rest of the guide follows. Each step depends on the one before it.

1. Frame the roof. Rafters, ridge board and ties on a gable, rafters only on a lean-to. Add the hurricane ties.
2. Hang the fascia and rake boards. These set the edge the deck and the drip edge finish against.
3. Lay the sheathing. 7/16 in OSB with the long side across the rafters, 1/8 in gaps, staggered end joints and nails at the schedule.
4. Add the H-clips on the unsupported edges as you go.
5. Fit the eave drip edge.
6. Roll out the underlayment, eave first, then work up.
7. Fit the rake drip edge over the underlayment.
8. Cover it. Shingles or metal panels, following their own guide.

Steps 5 to 7 are in that order for a reason the 2021 IRC spells out, covered in the drip edge section below.

Roof framing in one page

The full rafter job is in the shed rafters guide: sizing from a span table, laying out the plumb and seat cuts, setting the ridge, and the tie rules. Here is only what the deck depends on.

Rafters, ridge and ties

The designer tries 2x6, 2x8, 2x10 and 2x12 in that order and keeps the first that passes. It never picks a 2x4, and all 20 presets end at 2x6. The ridge board is one size deeper than the rafter, so 2x8 over 2x6, which matches the 2021 IRC rule that a ridge board is at least 1 in nominal and no shallower than the cut end of the rafter (R802.3 and R802.5.2). A 2x4 tie goes across every pair of rafters. The IRC asks for continuous ties to stop the roof pushing the walls apart, and lets the ridge go without ties only if it is a designed beam (R802.3 and R802.5.2). Tie nail counts come from IRC Table R802.5.2(1).

Hurricane ties

The designer lists one rafter-to-plate connector, a hurricane tie, at every rafter bearing, fixed with the nails the tie’s label calls for. On a gable that is one per rafter: 20 on the 10x12. The 2021 IRC lets you nail rafters to the plate with the standard toenail schedule, 3-16d box or 4 nails 3 x 0.131 in, only where uplift per rafter is 200 lb or less, or in a narrow case of 115 mph wind, exposure B, pitch 5:12 or more, span 32 ft or less and rafters at 24 in or less (Table R602.3(1) and R802.11). Otherwise a connector is needed. Simpson Strong-Tie describes its hurricane ties as a positive connection between rafter and wall against wind and seismic forces, says to use all specified fasteners and says they do not replace solid blocking. The H2.5A in its 2019 catalog lists 5 nails of 0.131 x 2-1/2 in into the rafter and 5 into the plates. Your local wind rules and the tie’s label decide.

16 in or 24 in rafter spacing

Spacing changes the count, not the deck. On the designer’s 10x12 gable, 16 in on center gives 20 rafters, 10 ties and 20 hurricane ties. Set the spacing to 24 in and it gives 14 rafters, 7 ties (with 4 nails per heel joint instead of 3) and 14 hurricane ties. The rafter is still a 2x6 and the OSB count stays at 8 sheets. At 24 in the 2x6 has less spare capacity: 97 percent against 142 percent at 16 in, on the same 4 ft 11-1/4 in span. A 2x6 at 24 in on center is allowed 9 ft 8-13/16 in by the designer’s check, and 11 ft 11-1/8 in at 16 in. Try 24 in in the designer and watch the list change.

Trusses instead of rafters

Trusses are a different route: engineered, delivered, lifted into place. The 2021 IRC wants truss design drawings approved before installation. Read the trusses vs rafters guide before you order them. The designer does not plan trusses.

Rafter, ridge and tie counts for every gable size

This is the designer’s framing result for the 16 gable presets, all at 6/12, a 6 in eave overhang, rafters 16 in on center and 30 psf snow. The first number in the shed size is the width across the door wall. Ridge boards are 2x8 and as long as the shed is deep; ties are 2x4 as long as the shed is wide.

The 16x20 ties take 5 nails per heel joint where the others take 3. The four lean-to presets have no ridge and no ties. Each plan page has the full cut list with every rafter length.

Fascia and rake boards

Fascia is the board on the ends of the rafter tails along the eaves. Rake boards run up the sloped edge of the roof at each gable end. They finish the edge, give the drip edge something to hang over and hide the end grain of the deck.

Fascia along the eaves

The designer lists a 1x6 fascia for each eave, 144 in long on the 10x12, two boards. It cuts the tails of every rafter to a plumb line so the board sits flat against them. Common practice is to nail the fascia to the tails before you sheathe, top edge even with the top of the rafter, so the deck ends on it. That is practice, not a code rule, and your plan or the covering maker’s instructions decide.

Rake boards up the gable ends

Rake trim is 1x4 on the designer, four boards of 73-13/16 in on the 10x12, two per gable end. On the 10x8 lean-to it is two boards of 111-5/16 in, and the 1x6 fascia is two boards of 120 in. The designer draws the rafters flush with the gable walls. If you add a gable overhang, the 2021 IRC limits structural panel roof sheathing to 9 in past the gable end wall unless it is supported by overhang framing (R803.2).

How much overhang

The designer’s default eave overhang is 6 in and it accepts 0 to 12 in. On the 10x12 the tail along the rafter is 0 in at no overhang, 6-11/16 in at 6 in and 13-7/16 in at 12 in. The deck grows with it: 6 sheets of OSB at 0 in, 8 at 6 in and 8 at 12 in, and 6 drip edge pieces at 6 in and at 12 in against 5 at 0 in. The 2021 IRC allows a notch on a cantilevered rafter tail if at least 3-1/2 in of rafter remains and the cantilever is 24 in or less (R802.7.1.1). A 2x4 is 3-1/2 in deep, so it cannot pass that test with a notch. More overhang keeps water off the siding, and it also gives wind more roof to lift, so keep it modest.

Roof sheathing: OSB or plywood

The roof deck is the layer everything else nails to. The designer plans 7/16 in OSB in 4x8 sheets.

OSB or plywood for a shed roof

Either works, and neither wins on paper. APA calls plywood and OSB wood structural panels, and says they provide a strong foundation for a variety of roof assemblies. The 2021 IRC asks for grade-marked structural panels with spans taken from its table or from APA E30 (R803.2). So buy by the grade stamp and the span rating, not by the name. A 24/16 panel with a nominal 7/16 in thickness spans 24 in on a roof, with or without edge support, at a 40 psf live load (2021 IRC, Table R503.2.1.1(1)). That covers rafters at 16 in and at 24 in on center.

Sheet layout on the roof

Lay the sheets with the long dimension across the rafters. LP’s installation sheet says to install with the long dimension of the panel across supports and with the panel continuous over two or more spans, and to make end joints meet over framing and stagger them in each succeeding row. Start at the eave with a full sheet flush with the fascia line, run the row to the far end, then start the second row with a half sheet so the end joints do not line up. Rafters at 16 in or 24 in on center divide 96 in evenly, so an 8 ft sheet ends on a rafter at either spacing if the first rafter sits at the wall end. APA adds: if necessary, trim panel ends to center on framing.

The designer counts each slope as full 4 ft courses plus a ripped strip. On the 10x12 a slope is about 74 in wide, so it is one full course and a strip of about 26 in, and the designer plans 4 sheets a slope, 8 in all. It rounds up and does not credit offcuts, so treat the count as a purchase quantity, not roof area divided by 32 sq ft.

Gaps, nails and H-clips

Leave a gap. APA says to always space wood panel sheathing 1/8 inch, because OSB and plywood swell and shrink with moisture and tightly butted panels can buckle. A 10d box nail makes a good spacer. APA’s roof sheathing tip also calls for a minimum of 8d common nails. The designer lists 8d galvanized nails, 2-1/2 in, at 6 in on the edges and 12 in in the field, about 500 of them and 6 lb on the 10x12. Your panel maker’s sheet and local code decide the nail and spacing.

H-clips are small metal clips that slide onto the joint edge between rafters. The 2021 IRC lists panel edge clips, one midway between supports, among the accepted kinds of edge support (Table R503.2.1.1(1)). At 24 in on center the table does not require them for 7/16 in 24/16 panels, but LP’s sheet calls for clips midspan on all unsupported edges, and APA notes spacer-type clips are sold by some makers. The seam that runs along the roof between the two courses is the unsupported edge. On the 10x12 that seam is 144 in long on each slope, so 9 clips a slope at 16 in and 6 at 24 in, 18 or 12 clips for the roof. That count is our arithmetic, the designer prints no clip quantity.

Deck numbers for every gable size

This is the designer’s roof-deck result for the same 16 gable presets, at 6/12 with a 6 in eave overhang and 16 in on center. Roof area is the two slopes over the shed depth. Nails are 8d, in pounds.

Drip edge is in 10 ft pieces and includes 5 percent extra. Sheet counts jump in steps because of the strip rule above, so a 10x14 and a 10x16 both list 8 sheets. Every gable preset lists one 10-square roll of synthetic underlayment.

Lean-to roof decks

The four lean-to presets run at 3/12 with the same 6 in overhang. They have one slope, so the deck is a single plane.

Nails are 2, 2, 3 and 3 lb, and drip edge is 3, 4, 5 and 5 pieces. A lean-to rafter bears on two walls, so it needs a hurricane tie at each end. The 10x8 rafter is a 2x6 of 112-11/16 in with a horizontal run of 108 in.

Drip edge and underlayment

Drip edge is the bent metal strip on the roof edge that sends water off the deck into the gutter or onto the ground, not behind the fascia. Underlayment is the water-shedding layer over the deck. Their order matters.

Drip edge

For asphalt shingle roofs, the 2021 IRC requires a drip edge at the eaves and rake edges. Pieces overlap at least 2 in, extend at least 1/4 in below the sheathing and at least 2 in back onto the deck, and are nailed no more than 12 in on center (R905.2.8.5). The designer lists it in 10 ft pieces for the eaves and rakes: 6 pieces on the 10x12 and 5 on the 10x8 lean-to. For metal roofs the panel maker’s flashings decide.

Underlayment

On a shingle roof at 4:12 or steeper, the 2021 IRC table calls for one layer of underlayment, applied like shingles from the eave up, each course lapped 2 in and end laps 4 in, offset by 6 ft. Between 2:12 and 4:12 it calls for two layers: a 19 in starter strip at the eave, then 36 in sheets each lapped 19 in (Tables R905.1.1(1) and R905.1.1(2)). This is the column for areas where wind design is not required. Asphalt shingles are only allowed on slopes of 2:12 or more, and only on a solidly sheathed deck (R905.2.1 and R905.2.2). So a 6/12 gable needs one layer. A lean-to at 2/12 or 3/12 with shingles needs two, and the designer’s 10x8 warns about this and counts the second layer: 2.1 squares for one 10-square roll. For metal panels the table sends you to the manufacturer’s instructions (Tables R905.1.1(1) and R905.1.1(2)).

The order at the edges

The 2021 IRC puts the underlayment over the drip edge along the eaves and under the drip edge along the rake edges (R905.2.8.5). So the sequence is: eave drip edge, then underlayment, then rake drip edge on top of it. The usual reasoning is that water running down the underlayment at the eave drops over the metal, while at the rake the metal laps the deck edge and the underlayment tucks under it.

Ice barrier

Where the local climate table flags ice at the eaves, the code asks for an ice barrier that runs 24 in inside the wall line, with an exception for detached accessory structures without conditioned floor area (R905.1.2). A detached shed usually falls under the exception. A lean-to built against a house is not detached, so it may not. Your building department decides.

Hand over to the covering

The deck is finished when the sheathing is nailed, the clips are in, the drip edge is fitted and the underlayment is down. From here the two coverings split.

Shingles

Asphalt shingles need a solid deck, a pitch of at least 2:12 and the underlayment above. On the 10x12 the designer plans 5 bundles of shingles, 1 of starter strip and 1 of ridge cap. The IRC calls for at least 4 fasteners per strip shingle, and the maker’s instructions may ask for more (R905.2.6). Nail lengths for 7/16 in OSB have to go through the panel. The shingle a shed roof guide has the courses, starter, and ridge cap step by step.

Metal panels

Metal roofing goes on solid or spaced sheathing. Its minimum slope depends on the seam: 3:12 for lapped panels without sealant, 1/2:12 with lap sealant, 1/4:12 for standing seam (R905.10.2). The designer’s metal option is 36 in coverage panels cut to length, a ridge cap and about 80 screws per 100 sq ft. The metal roof on a shed guide covers panel layout, screws and the ridge.

What the pitch decides

The pitch limits both. A 2/12 lean-to with shingles needs two layers of underlayment, and with lapped metal panels needs sealant on the laps (R905.2.2 and R905.10.2). The shed roof pitch guide has the chart and calculator for the minimum by covering.

Before you build: permits and local rules

The numbers in this guide come from the 2021 International Residential Code, which is a model code. Your city or county adopts a specific edition and often amends it, so the local building department has the last word on rafter sizes, snow load, wind connectors and roof coverings. Many small sheds are exempt from a permit, but not everywhere, and the exemption does not cancel other rules. See the shed permits guide and call before you order lumber. The designer is a planning aid, not an engineered design.

The whole build, from the base up to the roof, is laid out in the how to build a shed guide. When you are ready, open the 10x12 gable plan for the full cut list, or set your own size in the designer.