I put a new page on the site today: How a Roof Actually Works. It has 16 drawings, a roof you can pull apart with a slider, and an attic you can watch breathe. Every number on it comes from the shingle maker's instructions or the Ohio and Kentucky residential code. The sources are listed at the bottom of the page.
Here's the short version, layer by layer.
A Roof Is a Stack, Not a Layer.
A roof isn't just shingles. It's a stack of parts, and each one hands water to the next one down. On the pull-apart roof they go on in this order:
- Roof deck
- Drip edge at the eave
- Ice & water shield
- Synthetic underlayment
- Drip edge at the rake
- Starter strip
- Shingles
- Ridge vent
- Hip & ridge cap
Each layer has one job. Drag the slider on the guide page and tap any layer to see what it does.
The Eave: Where Most Water Leaves.
Most of the water on a roof leaves over the bottom edge, the eave. The order of the layers there is the difference between water dripping into the gutter and water running behind the fascia.
Illustration · not a photo of an actual job
The eave: drip edge first, ice barrier over it
- Roof deck: solid plywood or OSB; rotted boards replaced
- Drip edge goes on first: nailed to the deck at the eave and turned down over the fascia
- Ice & water shield: laps over the eave drip edge and runs at least 24 in past the inside of the exterior wall
- Synthetic underlayment: laps over the ice barrier, shingle-style, so water runs out
- Starter strip: a sealant row at the eave, set ¼–¾ in past the edge
- Shingles: the first course flush over the starter
- Exterior wall line: projected up to the deck; the 24 in is measured along the slope from here
Details per GAF Timberline installation instructions, Ohio residential code ch. 9 and Kentucky residential code (815 KAR 7:125).
The drip edge goes on first. It's nailed to the deck and turned down over the fascia. Ice and water shield laps over it and runs at least 24 inches past the inside of the exterior wall. Synthetic underlayment laps over that, shingle-style, so water runs out. Then comes the starter strip, set a quarter to three quarters of an inch past the edge, and the first course of shingles.
Up the sloped sides, the rakes, the order flips. The underlayment goes first and the drip edge goes on top of it.
Four Nails, in the Right Spot.
Shingles hold on by four small nails each, or six for the enhanced pattern. Where those nails go matters more than how many there are. On GAF Timberline HDZ and UHDZ the nailing area is about 6⅞ inches up. Too high and the nail misses the shingle below. Too low and the next course won't cover it. TAMKO Heritage uses its own line, 6⅛ inches. So I go by the maker's wrapper, not habit.
What Class 4 Actually Means.
"Class 4" gets thrown around a lot after hail. It's a lab test, UL 2218. A 2 inch steel ball hits the same spot twice, and a Class 4 shingle can't crack. The GAF Timberline UHDZ I install is UL 2218 Class 4.
It's a lab rating, not a promise. Hail bigger or harder than the test can still damage any roof.
Leaks Start Where the Roof Meets Something.
Shingles rarely leak in the middle of a slope. Leaks start at a wall, another slope, a pipe or a chimney. That's where the flashing does the work.
Illustration · not a photo of an actual job
Where a roof meets a wall: step flashing and kick-out
- Step flashing: one L-shaped piece per course: 5 in on the roof, 5 in up the wall (code minimum is 4 by 4)
- Each piece laps: the one below it by 2 in
- Kick-out flashing: required by code where a roof edge meets a wall: it turns water out into the gutter instead of down behind the siding
- Housewrap: laps over the flashing legs
- Siding: over the wrap, held up off the shingles
- Ice & water shield: turned up the wall at least 5 in
- Gutter: catches what the kick-out sends it
Details per GAF Timberline installation instructions, Ohio residential code ch. 9 and Kentucky residential code (815 KAR 7:125).
Where a roof runs into a wall, there's one L-shaped step flashing per course: 5 inches on the roof and 5 inches up the wall. Each piece laps the one below it by 2 inches. At the bottom, a kick-out flashing turns water into the gutter instead of down behind the siding. Code requires it where a roof edge meets a wall.
Valleys get ice and water shield under the whole valley, then either an open metal liner or a closed-cut valley. Woven valleys aren't one of the methods GAF's or TAMKO's instructions show for these laminated shingles, so I don't weave them. Pipe boots follow one rule: every lap points downhill.
The Attic Has to Breathe.
Fresh air comes in low at the soffits. Warm, damp air goes out high at the ridge. On the guide page you can watch the airflow arrows move, and pause them.
The code minimum in Ohio and Kentucky is 1 square foot of net free vent area per 150 square feet of attic floor. It drops to 1 per 300 when 40 to 50 percent of it is high exhaust near the ridge and the rest is low intake. So a 1,500 square foot attic at 1 per 300 needs 5 square feet of vent area.
Illustration · not a photo of an actual job
Ice dams start in the attic
- Heat leaks into the attic: and warms the roof deck
- Snow melts up high: and runs down the roof
- It refreezes on the cold eave: into a ridge of ice
- Water backs up: under the shingles, toward the wall
- An insulated, air-sealed floor: keeps house heat down
- Soffit-to-ridge airflow: keeps the deck cold
- Ice & water shield: the backup: at least 24 in past the wall line
Ice and water shield doesn’t stop an ice dam. It keeps the water that backs up behind one out of the house.
Details per Ohio residential code ch. 9, Ohio residential code R806 and Kentucky residential code (815 KAR 7:125).
In a Cincinnati winter, a warm attic and a cold overhang are how ice dams start. Heat leaks into the attic and warms the deck. Snow melts up high, runs down and refreezes on the cold eave. Then water backs up under the shingles.
Ice and water shield doesn't stop an ice dam. It keeps the water that backs up behind one out of the house. The real fix starts in the attic: an insulated, air-sealed floor and soffit-to-ridge airflow.
Why I Drew It All Out.
When I'm on your roof, these are the details I'm checking. Now you can see what I'm looking for before I get there. You can also hold any roofer's work up to the same drawings, mine included.
Want to Know What's on Your Roof?
I'll get on the roof, check these details and show you the photos. The inspection is free and takes 45 to 60 minutes. A written photo report is $150 if you want one.
Book a Free Inspection →Disclosure: I'm Joe Deal, owner of No Big Deal Home Solutions. I'm a GAF Certified™ Contractor and a TAMKO Pro Gold™ certified contractor. The drawings are illustrations, not photos of actual jobs. Every number in them comes from the manufacturer's installation instructions or the residential code as Ohio and Kentucky adopted it, and the full sources list is on the guide page. If a drawing ever disagrees with the manufacturer, the manufacturer is right.