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🏘️ Roofing Basics

Understand what keeps a roof watertight. You'll know the layers of a roof, how shingles and flashing shed water, and how to spot and reason about leaks.

8
lessons
~45 min
to learn
🔬 Science
subject
Adults
level
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What you’ll learn

  1. A Roof Sheds. It Does Not Seal.Grasp the central principle: a steep-slope roof is a water-shedding system of overlapping layers, not a sealed barrier — and slope is what makes it work.Your roof is covered in thousands of unsealed gaps and it doesn't leak, because every layer overlaps the one below it and gravity carries water across the joint instead of into it. Slope is the master variable: below roughly 2:12 to 4:12, shedding stops working and the system must change to a sealed membrane.
  2. The Stack, Bottom to TopIdentify every layer of a steep-slope roof assembly and state the single job each one does.A roof is a stack: deck, underlayment, ice-and-water membrane at the vulnerable edges, drip edge, starter course, shingles, and ridge cap. Each layer overlaps the one below and every one is installed working uphill — because that sequence is what makes the lap logic possible.
  3. The Logic of the LapUnderstand exposure, headlap, and the nail zone — and why high nailing is the failure that looks perfect for two years.Shingles are nailed in a narrow band just below the sealant strip so the next course covers every nail — nails live inside the roof, never exposed. Nail above that band and the fastener lands in the headlap, misses the shingle below, and the sealant never bonds: a roof that looks flawless until the first real storm.
  4. Flashing: Where Roofs Actually LeakUnderstand flashing as the continuation of lap logic across interruptions, and why caulk is not a substitute.The field of a roof almost never leaks; the interruptions do — chimneys, vent pipes, skylights, walls, valleys. Flashing is metal that carries the lap logic across those breaks, which is why step flashing is woven one piece per course and why a single continuous strip up a sloped wall must rely on sealant instead. Caulk is a maintenance item; flashing is geometry.
  5. The Roof You Can't See: VentilationUnderstand attic ventilation and air sealing, and the ice dam mechanism as a failure of the shedding logic.A roof can fail without leaking. Warm indoor air leaking into a cold attic condenses on the sheathing and rots it; escaping heat melts roof snow that refreezes at the cold eave and builds a dam, backing water up under the laps. The IRC requires net free vent area of 1/150 of the vented space, or 1/300 when the exhaust is balanced and placed high — but air sealing comes before any of it.
  6. Water Does Not Only Fall DownUnderstand the forces that move water uphill and sideways — wind-driven rain and capillary action — and why they explain 'impossible' leaks.The shedding logic assumes water goes downhill. Wind can drive rain up a slope and under laps; capillary action pulls water sideways and upward between close surfaces with no wind at all. Both explain leaks that seem to defy gravity, and both are why headlap, drip edge, and hemmed metal edges exist.
  7. Reading a Roof From the GroundDiagnose a roof's condition and locate leaks using observations from the ground and the attic, understanding that water travels before it drips.Nearly all useful roof diagnosis happens from the yard with binoculars and from inside the attic with a flashlight. Granule loss, cupping, and a wavy plane are readable from below; the attic reveals stains, daylight, and condensation. The key principle is that water travels — the stain on your ceiling is almost never under the hole.
  8. The Part Where I Tell You Not to Get On ItUnderstand honestly why steep-roof work is the most dangerous job in construction, what asbestos means for older roofs, and where a homeowner's real leverage is.Falls to a lower level are the most common fatal event in construction, and roofers had by far the highest fatality rate of the occupations examined in 2022 — 57.5 per 100,000 FTE, against 3.7 for all workers. Those are trained professionals. Roofing materials made before roughly 1980 may also contain asbestos, making tear-off a regulated disturbance. The homeowner's real leverage is on the ground: diagnosis and reading a proposal.

Questions this course answers

What makes an unsealed shingle roof watertight?

Nothing on the roof is sealed. Water arriving at a gap is already moving downhill onto the top of the next shingle — getting in would require going up. The sealant strip exists to resist wind uplift, not to make a watertight joint.

Why are asphalt shingles prohibited below a 2:12 slope?

Shedding works only while water is moving downhill. Below 2:12 water lingers and pools, so a lapped assembly becomes a decorative surface with holes in it. Low slope requires a genuinely sealed membrane.

What does the code's 2:12 exception — shingles allowed only with doubled underlayment — tell you?

It's an explicit admission. At reduced slope the shedding logic is marginal, so the code demands a second line of defense underneath — which is exactly what an underlayment is.

What is roof underlayment for?

Underlayment is the backup, not the raincoat. It's lapped from the eave upward, so it's a shedding layer too — a roof is shedding layers all the way down. The deck it protects is structure, not waterproofing.

Why is ice-and-water membrane used at the eaves and valleys rather than over the whole roof?

The membrane is the only genuinely sealed layer up there, and where it's required is an admission: at the eave water can be made to sit and travel uphill, and a valley carries two planes' worth of turbulent water. Lapping isn't enough at either.

What does drip edge actually do?

Water doesn't simply fall off an edge; surface tension makes it cling and creep back underneath onto the fascia and into the soffit. Drip edge projects past the fascia and forces it to release.

Grounded in trusted sources

  • International Code Council — International Residential Code (IRC), Chapter 9: Roof Assemblies; §R806 Roof Ventilation; §R905.2 Asphalt Shingles
  • Asphalt Roofing Manufacturers Association (ARMA) — Residential Asphalt Roofing Manual (https://www.asphaltroofing.org/wp-content/uploads/2017/06/ARMA-Residential-Manual-Final-2.14.14.pdf)
  • ARMA — Nail Application of Asphalt Shingles (https://www.asphaltroofing.org/nail-application-of-asphalt-strip-shingles-for-new-and-recover-roofing/)
  • CPWR — Data Bulletin, July 2024: Fatal Injury Trends in the Construction Industry, 2011–2022 (https://www.cpwr.com/wp-content/uploads/DataBulletin-July2024.pdf)
  • U.S. Bureau of Labor Statistics — Census of Fatal Occupational Injuries (CFOI), 2022 (https://www.bls.gov/iif/)
  • IIBEC — Attic Ventilation 101 (https://iibec.org/attic-ventilation-101/)
  • U.S. Consumer Product Safety Commission — Asbestos in the Home (https://www.cpsc.gov/safety-education/safety-guides/home/asbestos-home)
  • OSHA 29 CFR 1926.1101 — Asbestos (construction) (https://www.osha.gov/laws-regs/regulations/standardnumber/1926/1926.1101)

Every Wunder lesson is built from real, reputable sources — never invented.

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