Shingling a Roof

Following One Drop

Rain does not fall on a roof so much as it is handed downhill. A drop lands on a cap shingle at the ridge, spreads into a film across the granules, finds the seam between two courses, and then travels thirty or forty feet across laps, a valley, a wall line, a pipe flange and a drip edge before it clears the fascia and stops being your problem. Nothing along that route is waterproof by itself.

Asphalt shingles shed water; they do not seal it out. Each piece works only because the piece above overlaps it, and the plane works only because gravity moves faster than capillary action. Reverse a lap anywhere — a nail driven above the common bond, a step flashing installed backwards, an eave drip edge laid over the underlayment instead of under it — and the drop stops travelling down and starts travelling in.

Organise the day around that route. Each heading below marks a place the drop can leave the assembly, taken in the order it arrives: the ridge slot, the open field, the valley, the wall, the penetration, the eave, the gutter. Estimate in the same order too, because a takeoff that forgets cap footage forgets the last course you install and the first surface the weather touches.

The Slot Cut Into the Highest Point

A ridge carries almost no runoff and still fails more often than the field below it. Wind-driven rain arrives horizontally up there, snow sits on it, and on a vented roof the ridge is a deliberate hole cut through the deck — a narrow opening running the length of the building, protected by nothing but a baffle and three courses of cap.

Cut the slot to the vent manufacturer's written dimension, measured off the centre of the ridge board, and set blade depth so the cut takes sheathing and not the rafter tops or truss chords. Stop the slot short of the gable ends by the distance the vent instructions state; running it out to the rake leaves an opening facing straight into the prevailing weather. On a hipped roof the vent stops short of the hip intersections for the same reason.

Cap shingles run into the wind, not with it, so the laps face away from the direction storms usually arrive. Hips are capped from the bottom up so each piece laps the one below. Exposure comes from the cap manufacturer, and holding it consistently matters more here than anywhere else, because these are the courses visible from the street. Over a vent the fastener has to pass through the vent body and the cap and still bite the deck — the vent maker specifies that length, and it is longer than what your field gun is loaded with.

Short cap nails back out first. Cold caps split when bent over the ridge, so warm the bundle or score the back on a cut board rather than forcing a crack that opens at the first freeze. The final cap in a run takes face nails, and those nails get covered with sealant; leaving them proud is a leak you built on purpose.

Thirty Feet of Nothing But Laps

The field is the longest leg of the trip and the one the drop spends most of its life on. Water crossing an undamaged plane of laminates is doing exactly what the product was tested to handle, so failures here are almost always geometry: courses that drifted, joints that stacked, or fasteners set where the shingle above no longer covers them.

Snap control lines off the eave and check them against the rake, not against the last course you laid — error compounds one course at a time and only announces itself at the ridge as a two-inch sliver. Measure to the ridge before starting and adjust exposure fractionally across the run if the roof is out of square, which most are. Verify with a tape every four or five courses; a gauge on a hatchet drifts.

Offsets follow the shingle maker's printed pattern. Stacked joints on adjacent courses hand the drop a continuous vertical channel down to the underlayment, and racked straight-up installation on laminates voids more warranties than any other single habit. Watch sealant strip alignment as you climb: if a course sits high or low, the strip contacts the wrong part of the shingle beneath and the tabs never bond.

Nailing ruins more roofs than weather does. Fasteners belong in the strip the manufacturer marks, which on a laminate means through the common bond where both layers overlap — high nails miss that layer entirely and hold one thin sheet by its edge. Overdriven heads cut the mat; underdriven heads telegraph through the course above and lift it off the plane. Check gun pressure on the first bundle each morning and again when the day warms, then spot-check by lifting a tab. Four fasteners is the standard pattern and six the high-wind pattern; which applies comes from the wind provisions of the adopted code together with the manufacturer's instructions, not from preference.

Cold installations need help. Below the temperature the manufacturer names, factory sealant will not activate before the next wind event, so hand-seal with the specified quantity of asphalt cement under each tab — a spot the size of a coin, not a smear, since excess bleeds through and blisters the shingle.

Two Planes, One Channel

A valley takes the runoff of two roof planes and forces it into a line a few inches wide. Velocity is high there, volume is high, and wind pushes water sideways across the channel and up under laps that would never be tested out on open field. Treat a valley as a gutter that happens to be built from shingles.

Line it before anything else touches it, using a self-adhering membrane meeting ASTM D1970 Standard Specification for Self-Adhering Polymer Modified Bituminous Sheet Materials Used as Steep Roofing Underlayment for Ice Dam Protection, centred and rolled bottom to top so its own laps shed. Open metal valleys are fastened at the outer edges only, with clips or nails outside the water line; a nail through the middle of valley metal is a slow, certain leak.

In a closed-cut valley, run the plane carrying the lower flow through the intersection and lap it well past centre, then cut the second plane back to a line offset from that centre. Clip the upper corner of every cut shingle so water crossing the cut cannot ride a square edge upslope into the lap, and bed each cut course in cement. Hold every fastener back from the centreline by the distance the manufacturer specifies — a nail near the channel sits in moving water for the life of the roof.

Cut on a board. A hook blade dragged along the surface finds the membrane underneath, and the slice you cannot see becomes a ceiling stain under the valley two seasons on. Debris matters as much: valleys under trees dam, ice builds behind the dam, and standing water on a shedding system is outside what the assembly was ever built to survive.

Walls Standing in the Path

Where a roof plane runs into a wall, the drop stops going straight downhill and starts running along the intersection, collecting everything the wall above sheds into the same line. Sidewalls concentrate water the way valleys do, with a vertical surface adding to the volume.

Step flashing is one piece per course, woven in as you climb — never a continuous L of metal run up behind the siding, a detail that survives exactly as long as the caulk holding it. Each piece laps the one below by enough to keep that lap above the water line, and each is fastened high on the roof leg so the next shingle covers the fastener. Counterflashing or the wall's own drainage plane laps over the vertical legs, and the siding never lands on the roof surface.

Kickout flashing at the foot of a sidewall is the cheapest part on the roof and the most expensive omission. Without it, the whole collected flow of that wall line runs behind the siding at the eave, and the damage surfaces as sheathing rot inside a wall cavity, found years later during a window replacement. Fit it before the first step flashing and make the leg tall enough to discharge past the drip edge.

Headwalls take apron flashing with the vertical leg lapped by the wall's drainage plane and the horizontal leg over the shingles below. Chimneys need base, step and counter flashing, plus a cricket on the upslope side once the chimney exceeds the width the adopted code sets — commonly measured perpendicular to the slope, with local amendments governing. A wide chimney without a saddle collects snow and debris behind it, and no quantity of sealant substitutes for a sloped surface.

1. Set the kickout at the eave end of the wall first, leg tall enough to discharge past the drip edge.
2. Carry the underlayment up the wall behind where the flashing legs will sit.
3. Lay the course, then set a step flashing over it, fastening high on the roof leg only.
4. Lay the next course over that leg, then the next flashing — one piece per course, all the way up.
5. Have siding or counterflashing lap the vertical legs, holding the siding clear of the roof surface.
6. Photograph the flashing run before anything covers it.

Obstacles in the Line

Every penetration is a small dam. Water arriving from above splits around it and rejoins below, and the rejoining point takes concentrated flow — which is why the downslope side of a vent hood or a boot is the side that fails.

Flanges lap in the same order as everything else: shingles under the lower flange edge, over the sides and the top. Fasten the upper flange only, let the courses above cover those fasteners, and leave the exposed downslope apron unnailed. A flange bedded in cement and face-nailed through the visible portion is a repair somebody will make twice.

Pipe boots fail at the collar, not the flange. Thermoplastic collars harden and split under ultraviolet long before the shingles wear out, so pick collar material with the exposure in mind and treat a metal-collared or two-piece boot as the default on a south-facing plane. Slip-over replacement collars are a legitimate repair; a bead of sealant around a cracked collar is a callback with a date on it.

Sealant earns its place as secondary defence at cut edges, exposed fasteners and counterflashing reglets. Nowhere on a shingle roof does it serve as the primary water barrier, and using it that way converts a maintenance-free detail into an annual inspection item.

The Last Three Feet

Everything the roof collected arrives at the eave, and the eave is the one place water may also try to move uphill. Ice dams form when melt refreezes over the cold overhang, and the pond behind the dam stands above the lap line, so the shedding logic that held for forty feet stops applying in the final stretch.

Ice barrier membrane answers that, and its required extent is set by the adopted code — measured from the eave edge to a point inside the exterior wall line, with distance and trigger conditions varying by jurisdiction and by local amendment. On low-slope planes and in valleys the same membrane does duty against wind-driven water. Underlayment above it follows the products the code names, such as ASTM D4869 Standard Specification for Asphalt-Saturated Organic Felt Underlayment Used in Steep Slope Roofing, or a synthetic with an equivalent listing.

Sequence at the eave decides whether membrane drains onto the metal or behind it. Drip edge at the eave goes on the deck first, with ice barrier or underlayment lapping over it; at the rake, drip edge goes over the underlayment. Reversing those two is common and invisible once shingles are down.

Starter strip runs with its adhesive band at the eave edge rather than the top, so the first course bonds exactly where wind wants to lift it. Overhang past the drip edge stays inside the manufacturer's stated range — far enough that water clears the metal, short enough that tabs neither curl nor snap off. Rake starter deserves the same attention on an exposed site, because edge peeling is how nearly every blow-off begins.

Into the Gutter, or Behind It

The final hand-off is a short one and it fails constantly. Drip edge has to kick water clear of the fascia and land it inside the back of the gutter; if the gutter was hung before the roof and its back edge sits above the kick, the drop runs behind it, down the fascia, into the soffit.

Gutter apron — the wider, longer-legged profile — solves it where the gutter sits low or the fascia is out of plane. Check that relationship with a straightedge before the first course goes down, because after the roof is finished the correction means pulling starter and eave courses.

Symptoms of this failure read exactly like a roof leak: stained soffit, peeling fascia paint, water at the head of a window below. Nobody looks at the last inch of the water path. Photograph the eave assembly during install so the diagnosis takes minutes instead of a tear-off.

The Water That Never Fell

Condensation on the underside of the deck produces the same ceiling stain as a leak, on the same schedule, in the same season. Before chasing flashings on a callback, check whether the attic ventilates in balance — intake at the soffits matched to exhaust at the ridge, at the ratio the adopted code's attic ventilation provisions require.

Mixing exhaust types short-circuits the system. A ridge vent paired with open gable louvres or a powered fan draws air in through the ridge instead of the soffits, and the field of the roof simply stops ventilating. Insulation stuffed over the soffit without baffles does the same thing more quietly.

Fastening the deck belongs to the water path indirectly: an uplift event that lifts sheathing takes the shingles with it and turns a weather day into a structural one. Sheathing nail patterns come from the code's fastening schedule and tighten in high-wind regions, particularly at panel edges and in the roof's edge and corner zones.

Run the Path Backwards Before You Leave

Final inspection reverses this guide. Begin at the gutter, walk the eave line checking overhang and starter bond, follow the rakes, then the valleys, the wall lines and every penetration, and finish at the ridge you capped last. Reversing direction forces you to view laps from the side the water sees them.

Wind classifications printed on the bundle wrapper mean nothing if the installation did not match them. Those ratings come from ASTM D3161 Standard Test Method for Wind-Resistance of Steep Slope Roofing Products (Fan-Induced Method) and ASTM D7158 Standard Test Method for Wind Resistance of Asphalt Shingles (Uplift Force/Uplift Resistance Method), and both assume the fastener count, placement and sealing the manufacturer specifies. Conformance to ASTM D3462 Standard Specification for Asphalt Shingles Made from Glass Felt and Surfaced with Mineral Granules describes the shingle, not your roof.

Mapped below are the exits, in the order the drop meets them, against what a homeowner will describe on the phone.

Tie off while doing any of it. OSHA 29 CFR 1926 Subpart M covers fall protection on this work, and a perfectly followed water path is worth nothing if you take the fast way to the ground.