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.
What a shingled roof is made of
- Ridge cap — the last course laid and the first the weather touches, bought by the linear foot of ridge plus every hip Roof Ridge Cap Calculator
- Field shingles — sold by the square against true roof area, not building footprint — a cut-up roof consumes noticeably more than a simple gable Roofing Shingle Calculator
- Eaves membrane — self-adhering ice and water shield, run from the eave up past the interior wall line and nowhere else Ice & Water Shield Calculator
- Underlayment — the secondary plane, lapped downhill, sized in rolls against the same true area as the shingles Roofing Underlayment Calculator
- Structural deck — the sheathing everything above is fastened to; its thickness sets the nail length and its span sets the panel grade Roof Sheathing Nailing Pattern Nail Count Calculator
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.
Cap footage is ridges plus every hip, converted to pieces at the exposure the cap manufacturer states — run it before the truck is loaded, because caps are the item crews run short on.
SettingsSettings for this calculation
The standard allowance most suppliers and estimating guides assume for ordinary work.
The total length of ridge and hip lines needing cap shingles.
Extra cap for the pieces cut where hips meet the ridge and at the end of each run.
Ridge cap bundles needed
2 bundles
- Ridge/hip length (with waste)
- 42.9 linear ft
They open the calculator with your figures already in it
Roof Ridge Cap Calculator: 2 bundles — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
What this calculation does not cover
- Every bundle is counted as 33 linear feet (10 m) laid at the exposure the maker intends, the single coverage figure the source gives; there is no field for the rating printed on your own wrapper, and no way to tell the arithmetic you plan to lay the caps tighter or wider than that.
- Hip lines have to be measured along the slope and added into the one total yourself — no pitch is asked for anywhere, so a hip scaled off a plan-view drawing goes in short and the bundle count follows it down.
- The allowance is one percentage of whatever total you type, so at the same setting three short hips adding up to sixty feet and one unbroken sixty-foot ridge return exactly the same figure, even though each extra run adds two more cut ends; raise it for a roof of many hips.
- Bundles are rounded up to whole packs, so a 35-foot run and a 59-foot one both come back as two, with the first leaving well over half of the second bundle unused and no remainder reported anywhere in the output.
- Fixings and sealant sit outside the result entirely: the calculation returns a bundle count and nothing else, so nails for the caps still have to be worked out from the same ridge length.
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.
Square count comes off true roof area, not building footprint, and cut-up roofs with multiple valleys and hips consume noticeably more than simple gables — size the order from the planes you actually measured.
SettingsSettings for this calculation
Waste is set to 12% by hand. Pick a tier above to replace it, or keep your own figure.
The horizontal (plan-view) length of the roof, not the sloped surface.
The horizontal (plan-view) width of the roof.
Rise over a 12-unit run (works the same in feet or meters) — steeper roofs have more actual surface area than their footprint.
Extra field shingles for cutting at hips, valleys, rakes and penetrations, and for starter rows and caps cut from field shingles.
Estimated roofing shingle needed
13.64 squares
- Roof footprint area
- 1,089 sq ft
- Actual roof surface area
- 1,217.54 sq ft
- Roof surface area
- 1,217.54 sq ft
- Bundles needed
- 41 bundles
They open the calculator with your figures already in it
Roofing Shingle Calculator: 13.64 squares — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
This changes the drawing and no number above it. A gable and a hip over the same footprint at the same pitch have the same surface area, so this calculator never asks which you have — and its own limitations say a gable and a hip are charged the same. Pick the one you are building so the picture matches the job.
What this calculation does not cover
- The slope multiplier is applied to exactly the rectangle you enter, and nothing is added for the eaves and rakes, so the two dimensions have to be the roof's plan to the drip edge rather than the building's footprint at the walls. Taken at the wall faces they leave the overhangs out: on a 10 by 10 m building with 500 mm of overhang all round the roof plan is really 11 by 11 m, and the 6/12 job that reads 41 bundles at the wall dimensions needs 49.
- One length times one width times one multiplier is the entire geometry model, so anything that is not a single rectangle under a single pitch has to be split up by hand: an L-plan, a cross gable, a lower porch or garage roof at a shallower pitch, and a dormer cheek are all outside what two footprint figures can describe. Run each rectangle at its own pitch and add the squares. No perimeter is ever derived either, so the code has no idea how long your ridge, hips, valleys, rakes or eaves are.
- Pitch is a six-item list — flat, 4, 6, 8, 10 and 12 in 12 — not a number you can type, so a 7/12 or 9/12 roof has to be forced onto the nearest rung; choosing 6/12 rather than 8/12 on the default job moves the order from 41 bundles to 44, with the true 7/12 answer sitting between the two. The flat entry sets the multiplier to exactly 1 and still returns a bundle count, because this is area arithmetic and makes no judgement about whether shingles belong on a slope that shallow.
- Three bundles to the square is a fixed constant in the code and nothing on the form asks which shingle you are buying. If the coverage printed on the wrapper is not one third of a square, the bundle line is wrong in direct proportion while the squares figure above it stays right, so divide the squares yourself by the coverage stated on the pallet. Rounding up happens only at the bundle, so the answer never rounds to a whole square, a full pallet or a supplier's minimum drop.
- The waste percentage is a flat multiplier on the area, which turns the allowance into more of the same field shingle and nothing else — at the 12 per cent default that is about 1.4 extra squares, roughly four more bundles of the identical product. That works if you are cutting starters and caps from field shingles; dedicated starter strip and hip-and-ridge cartons are sold by the linear metre and would sit outside this figure entirely. Because no edge length is known, the percentage also cannot respond to how much cut edge you have, so a plain gable and a hip roof with four valleys are charged the same 12 per cent unless you raise it.
- Nothing is deducted for what sits inside the roof plane: chimneys, skylights, roof windows and open metal valleys are all shingled straight over, since there is no field in which to enter them. Order from the squares figure at the top rather than from the surface-area rows in the breakdown, because those rows are the bare roof before waste — the 1,203 sq ft (112 m²) line on the default job is about 145 sq ft (13 m²) short of the 13.5 squares the headline is telling you to buy.
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.
- Set the kickout at the eave end of the wall first, leg tall enough to discharge past the drip edge.
- Carry the underlayment up the wall behind where the flashing legs will sit.
- Lay the course, then set a step flashing over it, fastening high on the roof leg only.
- Lay the next course over that leg, then the next flashing — one piece per course, all the way up.
- Have siding or counterflashing lap the vertical legs, holding the siding clear of the roof surface.
- 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.
Count stacks, hoods and flues off the plumbing and mechanical drawings while you are still standing on the ground, since boots are the small line item that halts a crew when the truck arrives one short.
SettingsSettings for this calculation
The standard allowance most suppliers and estimating guides assume for ordinary work.
The total number of plumbing or exhaust vent pipes penetrating the roof.
Extra boots to allow for installation damage or sizing mismatches.
Vent pipe boots needed
9 boots
They open the calculator with your figures already in it
Roof Vent Pipe Boot Count Calculator: 9 boots — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
What this calculation does not cover
- Counts penetrations, not the flashing they take. What a "boot" is depends on the roof covering and the slope: a shingle roof takes a base flashing woven into the courses, a single-ply membrane takes a heat-welded or adhered pipe flashing over a target sheet, a standing-seam roof takes a formed collar bedded in sealant. Same count, three different products at three different prices, none of them interchangeable.
- Vent pipes only. Flue and chimney penetrations, exhaust duct terminations, conduit and cable, lightning protection, satellite and solar mounts and equipment supports all pass through the same deck and each takes a detail of its own. None are in this number, and on most roofs they outnumber the plumbing vents.
- Assumes every pipe can be flashed on its own. A boot needs clear roof around it for its flange or target sheet, so two pipes closer together than that flange — or a pipe sitting in a valley, tight against a wall, or just under a ridge — cannot each take a separate boot. Those get a shared curb or a pitch pan instead, which lowers the boot count and adds a fabricated detail nobody priced.
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.
Once the jurisdiction has settled how far up the slope the barrier must reach, that eave run becomes rolls, and rolls are the unit the supplier actually sells.
The total length of eave needing protection.
How far up the roof slope the membrane needs to extend from the eave.
Ice & water shield rolls needed
1 roll
Required coverage width and whether ice & water shield is code-mandated in your area both depend on local climate zone rules — check your local building code before finalizing coverage width.
- Coverage area needed
- 161.7 sq ft
They open the calculator with your figures already in it
Ice & Water Shield Calculator: 1 roll — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
What this calculation does not cover
- The 10% uplift is spent on laps before it ever reaches waste. Self-adhered membrane laps at the manufacturer's figure — commonly 3 to 4 in (75 to 100 mm) on a side lap and 6 in (150 mm) on an end lap — so a 36 in (915 mm) roll nets about 33 in of covered deck, roughly 9% gone before a knife touches it, and a long eave loses more again at each end joint. Everything cut off at a hip, a rake or a corner comes out of what is left.
- Only an eave strip is ever returned. Where a roof falls below its covering's minimum slope — many asphalt shingle systems stop at 2:12 — the remedy is usually not a wider eave course but membrane or doubled underlayment over the entire deck, and low-pitched porch roofs, shed extensions and dormer tops are exactly where that comes up. A whole-deck area is a different sum from an eave length times a width.
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.
Soffit and ridge products are rated in net free area, so put the attic footprint through that ratio before deciding whether the intake genuinely answers the exhaust you are about to cap.
The length of the attic floor, measured along the ridge; the page takes each eave and the ridge to be this long.
The width of the attic floor, wall plate to wall plate across the building.
The rule the vent area comes from: the IRC's ratio, Canada's, or the UK's and Australia's openings along the eaves and ridge.
The intake product, so the page can count it; the slot is a real product rated by its maker.
The exhaust product high on the roof: a ridge vent, or individual roof vents.
Net free vent area needed
6.5 sq ft (net free area)
R806.2 of the 2021 IRC: 1/150 of the attic floor. The code sets no split at this ratio; the page divides it evenly between intake low on the roof and exhaust high up, and keeps the intake at least equal to the exhaust bought, as GAF's balanced-ventilation guidance asks.
- Attic floor area
- 975 sq ft
- Net free area needed
- 936 in²
- Intake, low on the roof (eaves or soffit)
- 475 in²
- Exhaust, high on the roof (ridge or upper vents)
- 468 in²
- Intake as a continuous gap along both eaves
- 0.51 in
- Exhaust as a continuous gap along the ridge
- 1 in
- Intake vent to fit
- 6 vents (8 ft)
- Exhaust vent to fit
- 38 ft
- High-level share of the total
- 50 %
They open the calculator with your figures already in it
Attic and Loft Ventilation Calculator (Intake, Exhaust and Vent Count): 6.5 sq ft (net free area) — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
What this calculation does not cover
- Net free area is a rating, not a hole size. The counts divide by the rating printed on the product after its louvres, mesh and screens, and assume every vent is open: intake vents painted over, blocked by insulation slumped into the eaves with no baffle, or fitted over solid sheathing count for nothing.
- The shape is a simple gable roof with the ridge along the length. A hip roof has longer eaves and a shorter ridge, a roof with dormers, valleys or a firewall is several vented spaces, and each compartment is worked on its own; the fit checks along the eaves and ridge then need doing by hand.
- The US exception's vapor-retarder condition in climate zones 6, 7 and 8 is taken on trust: nothing here knows the climate zone or whether a Class I or II retarder is in place and continuous. Where either is missing, the 1/300 figure is not available and the 1/150 one applies.
- UK openings are BS 5250's for a cold roof with a high-resistance underlay, as makers restate them, taking the stricter maker at the 35° and 10 m (32.8 ft) boundaries; a breathable underlay, a warm roof, a lean-to or mono-pitch roof, and roofs of buildings other than dwellings each have their own provisions, and the underlay's certificate governs a breathable one.
- Australian openings are Table 10.8.3's, which applies in climate zones 6, 7 and 8 only, applied along both eaves of a dual-pitched roof. The roof space's 20 mm (0.8 in) minimum height, the cathedral ceiling's extra eaves opening, the unsarked tiled roof that needs no openings, and the exemptions for concrete roofs, structural insulated panels and BAL-FZ are the reader's to check against the clause itself.
- This restates a ratio or an opening size; it is not a code check, a moisture assessment or a performance calculation. Air leaking up from the house is usually the bigger moisture source, ember-resistant vents are required in some wildfire areas, and the authority having jurisdiction or building control decides what applies to a given roof.
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.
| Where the drop leaves | Mechanism | What the call sounds like |
|---|---|---|
| Ridge slot | Short cap nails, vent run out to the rake, unsealed final cap | Drips along the peak during driven rain only |
| Field | High nails, stacked joints, unbonded tabs in cold weather | Shingles missing after a wind event; stains mid-slope |
| Valley | Fasteners near the centreline, blade cut through the liner | Ceiling stain directly below the valley, worst in thaw |
| Sidewall | Continuous flashing instead of steps, no kickout | Rot found inside a wall years later, not a visible drip |
| Penetration | Face-nailed downslope flange, split boot collar | Stain around a bath fan or plumbing stack |
| Eave | Drip edge over underlayment, no ice barrier, no gutter apron | Soffit staining, peeling fascia, water above a window |
| Attic underside | Unbalanced or short-circuited ventilation | Seasonal stains with no leak found in any flashing |
Ordering off the water path
Quantify in the sequence the drop travels, so the details that get forgotten — cap, starter, ice barrier — are counted before the field, not after the truck leaves.
- Field shingles, by squares of measured roof area — Plane-by-plane off true roof area; cut-up roofs with several valleys and hips absorb noticeably more waste than a plain gable.
- Hip and ridge cap, by linear foot — Every ridge plus every hip, converted to pieces at the cap manufacturer's exposure; vented ridge changes the fastener length, not the count.
- Starter, eaves and rakes — Adhesive band toward the edge; rake starter is what stops the peel that begins a blow-off.
- Ice barrier and underlayment — Barrier extent at eaves and valleys is set by the adopted code and local amendment — confirm before ordering rolls, not on the day.
- Fasteners and flashings — Length must bite the deck through cap and vent; count kickouts, aprons and step pieces per course, one per course, no continuous runs.
Opens the calculators above on one screen with the dimensions from this article already filled in. Quantities only — this site publishes no price list, because local prices vary too much to publish honestly.