Why do roofs fail at the details rather than in the field?
Because the field of a roof is a repetitive assembly doing one simple thing — shedding water down a slope — while every detail is a one-off, hand-built interruption where water has to change direction, where dissimilar materials meet, and where the result depends on the order the layers went down rather than on which products were bought. The field is also the part a manufacturer tests and warrants; the details are the part your particular contractor either built correctly on your particular house or did not. Add that details are priced per-each and are therefore the easiest place for an estimate to be quietly light, and you get the pattern behind almost every premature roof failure: a perfectly good product, installed across the easy 90 percent, failing at a wall, a valley, a chimney or an eave that took the crew twenty minutes at the end of the day.
The easy part is most of it, and it is not where the trouble is
Look at the middle of a roof plane. It is the same three feet of assembly repeated a few hundred times: a course of shingles, offset, nailed in a zone, covering the joints of the one below. Water lands on it and runs down. There is no decision in it. A crew can build the field of a roof with real speed precisely because nothing about it varies.
Now look at the edges of the same plane. At the bottom it stops, and something has to make water let go rather than curl underneath. At the sides it stops, and something has to keep water from running off sideways into the deck. Where it meets another plane it forms a channel carrying both catchments. Where it meets a wall the water has to be turned rather than shed. Where a pipe comes through, the plane has a hole in it. Each of those is a one-off, built by hand, in a sequence that has to be right.
Your roof is a large simple thing with a small number of difficult places, and the difficult places are the whole game.
We are not going to give you a percentage
Every roofing company on the internet will tell you that some impressive share of leaks — 80, 90, 95 percent — happen at flashing and penetrations. We are not going to, because we cannot source a figure like that, and a number nobody can source is worth less than the reasoning behind it. What we can tell you is the reasoning, and then give you a way to measure your own house instead of trusting anyone's statistic.
The reasoning is simple enough to check against your own experience of anything built: failures concentrate where variation is highest. The field of a roof has almost no variation, so it has almost no failure. The details have nothing but variation — every house is a different shape, every chimney is a different width, every wall junction meets the roof at a different angle — and that is where the failures are. You do not need a percentage to act on that. You need to know how many of them your house has.
Take the inventory of your own roof
This is the single most useful thing on this whole site, and it takes about fifteen minutes with a phone and a satellite view. What you end up with is a list — the actual number of difficult places on your building — and that list is what you compare quotes against.
How to inventory the failure-prone details on your own roof
Write the numbers down. A contractor who can talk through your list item by item is a contractor who has actually looked at your roof.
- Count the valleys On the aerial view, find every line where two roof planes tip toward each other. Count them and note which look long and which look short. Every one is a channel carrying the water of two planes, and every one is a separate piece of work.
- Trace the edges Follow the outline of the roof. The horizontal bottom edges are eaves, the sloped edges up the gables are rakes. Note roughly how many feet of each, including the garage and any porch. Both need edge metal and they need it lapped in opposite directions.
- Find every wall that a roof runs into Dormer cheeks, a garage roof meeting the house wall, an addition, a second storey rising out of a lower roof. Each one needs step flashing woven in course by course, and each one needs a kick-out where it reaches the eave. Count them.
- Count the holes Plumbing vent stacks, bathroom and kitchen exhaust terminations, chimneys, skylights, satellite mounts, solar stanchions, attic fans. Each is a penetration with its own flashing and its own service life. Pipe boots in particular are the shortest-lived component on most roofs.
- Look for anything nearly flat A porch roof, a dormer top, the pocket behind a chimney, a spot where a roof dies into a wall with no way out. Anything shallow or with no drainage is not a shingle detail at all and needs to be identified as its own scope before anyone prices it.
- Now read the quotes against the list Take the stack of quotes and check each one against your inventory. Does it name your number of valleys, both edges, each wall junction, each penetration, and the low-slope area you found? A quote that matches your list is describing your house. A quote that summarises all of it as flashing as needed is describing a generic house.
What you will probably discover: the quotes in your stack differ far more in the detail work than in the shingle. Two companies quoting the same shingle at different prices are very often quoting a different number of details, and until you have your own list there is no way to see it.
A roof is a sequence, not a shopping list
Here is the part that surprises most people. You can buy every single correct product, hand them to a crew, and still get a roof that leaks — because at every detail, the protection comes from the order the pieces went down in, not from the pieces.
Four examples, all of them ordinary, all of them invisible the day after.
The eave
Drip edge on the deck first, ice barrier bonded over the top of it, field underlayment over that, then starter, then shingles. Every fastener in the metal ends up sealed under membrane, and the bottom of the assembly is continuous.
Fails when: Same five products, membrane first and metal last. Now the edge fasteners sit in the water that stands behind an ice dam, and there is a lap facing uphill at the wettest line on the roof. Identical invoice.The rake
Underlayment down first, drip edge over it. Water running on top of the underlayment crosses onto the metal and leaves.
Fails when: Metal first, underlayment over. Water on the underlayment now runs off its edge and straight into the edge of the deck. Identical invoice. See drip edge.The sidewall
Step flashing woven in course by course as the shingles go up the wall, each piece lapped over the one below, each one behind the wall covering above it. It is built with the roof, in sequence.
Fails when: A single long piece of bent metal set against the wall after the shingles are done, and caulked. It looks tidier. It has no laps working with gravity and it relies entirely on the sealant. See flashing.The chimney
Base and apron flashing built in with the courses, counterflashing let into a cut in the masonry joint above it so the top edge is inside the wall, and a cricket behind anything wide.
Fails when: Flashing set on the roof and a bead of sealant run along the top edge against the brick face. The sealant is the only thing keeping water out of a joint, and it has a life measured in seasons.In all four cases the materials list is identical and the outcome is not. That is the single strongest argument for why a roofing decision cannot be made on product brand, and why the useful questions are all about sequence and quantity. It is also why photographs taken during the build are worth more than any warranty document: the sequence is only visible while it is happening.
Where the estimate hides
The field of a roof is priced by area and it is very hard to hide anything in it. Squares are squares, everyone measures them the same way, and a satellite measurement will get within a few percent. So field pricing between contractors is broadly comparable, which is exactly why it is the number every quote leads with.
Details are priced per-each and per-linear-foot, and that is where the variance lives. Six valleys, four wall junctions, two chimneys, nine penetrations and a low-slope porch is a genuinely different job from a plain gable with two vents, and it is a different job in labour hours rather than materials. A quote can be light on all of it and still look complete, because none of those items has an obvious market price a homeowner would recognise.
The cheaper quote is very often the same roof with the difficult parts left out.
Our published floor for architectural asphalt is $600 per square, and a number like that can look high next to a quote that is several thousand dollars less on the same house. Sometimes the cheaper quote is genuinely a leaner operation. Frequently it is the same shingle with the detail work costed at close to nothing: reused flashing, reused drip edge, membrane in the valleys only, one course at the eave, no kick-outs, no cricket, and a line that says "flashing and underlayment as needed" doing the work of a dozen items. Every one of those becomes either a change order after the roof is open or a leak in year four.
You cannot tell those two apart from the total. You can tell them apart instantly from the itemisation, which is the entire reason to insist on one. Ask every contractor in the stack for the same list: valley count and linear feet, eave and rake linear feet, wall junction count, penetration count with boot type, kick-out count, ice barrier courses, and the per-sheet price for any decking found bad at tear-off. The company that gives you that has told you what they are actually building.
Every detail on that list has its own guide
This page is the argument. The rest of the library is the detail, and each guide ends with the phrases that indicate that particular corner is being cut and what to ask instead. None of it is specific to us and all of it works on somebody else's quote.
- The eave — ice-and-water shield and the 24-inch rule, and drip edge.
- The rake and the ridge — starter strip, hip and ridge components.
- The valleys — open, closed-cut and woven construction.
- The walls and the holes — step, kick-out, apron, counterflashing and boots.
- Underneath all of it — underlayment and the deck that holds every nail.
- Above the insulation — attic ventilation, intake and exhaust.
- The product itself — shingle classes and impact ratings.
- The paperwork — what a warranty actually covers.
What we specify, and why
Every detail on your roof appears on the scope as a number
This is the whole discipline in one panel. If a component can be counted or measured before work starts, it is counted or measured before work starts — and if it genuinely cannot be known until the roof is open, it gets a unit price instead of a shrug.
- Valley count and linear feet, with the construction named for each
- Eave and rake linear feet, listed separately, both with new metal
- Ice barrier courses at the eave, calculated from your overhang and pitch
- Wall junction count, with new step flashing and a kick-out at every eave termination
- Penetration count, with the boot type named rather than implied
- Chimney work itemised, counterflashing and cricket where the width calls for one
- Low-slope and dead-valley areas quoted as their own scope, never shingled over
- A per-sheet decking price agreed in advance for what tear-off reveals
- Photographs at each stage, because sequence is only visible while it happens
Questions about how roofs actually fail
Where do most roof leaks come from?
From the places where the roof plane is interrupted rather than from the middle of it: sidewalls, chimneys, valleys, plumbing and exhaust penetrations, skylights, and the eave under an ice dam. We deliberately do not quote a percentage, because the figures circulated for this are not sourceable. The reasoning matters more than a number: the field of a roof is a repetitive assembly with almost no variation, and failures concentrate where variation is highest. Every detail is a one-off, built by hand, on a house shaped differently from every other house.
If two quotes name the same shingle, why are they thousands apart?
Usually because they are not quoting the same amount of work at the details. Field area is easy to measure and hard to hide, so field pricing between contractors is broadly comparable. Details are priced per-each and per-linear-foot, and a quote can be light on reused flashing, reused drip edge, one course of membrane at the eave instead of two, no kick-outs, no cricket and no valley count, while still reading as complete. Inventory your own roof first, then ask every contractor for the same itemised list, and the difference becomes visible immediately.
Can a roof leak even if all the right materials were used?
Yes, and this is the most important thing to understand about roofing. At almost every detail, the protection comes from the order the layers went down in rather than from the products themselves. Drip edge under the ice barrier at the eave and over the underlayment at the rake are opposite sequences with identical materials, and one of them is right at each location. Step flashing woven in course by course with the shingles is a different assembly from the same metal set against the wall afterwards and caulked. Same invoice, different roof. It is why photographs taken during the build are worth more than a brand name.
What should I ask a roofer to prove they are quoting the details properly?
Ask for numbers rather than adjectives. How many valleys and how many linear feet. Linear feet of eave and of rake, listed separately, both with new metal. How many courses of ice barrier at the eave, and what overhang measurement that was calculated from. How many wall junctions, and how many kick-out flashings. How many penetrations, and what type of boot. Whether the chimney needs a cricket. And the per-sheet price for decking found bad at tear-off. Any competent contractor can answer all of that in writing, and the answers make every quote in the stack comparable.
Is a more expensive roof actually better?
Not automatically, and price alone tells you very little. What price can tell you is whether the difficult work has been costed at all. Our published floor for architectural asphalt is 600 dollars per square, and a quote several thousand dollars below that on the same house is either a leaner operation or the same shingle with the detail work priced at close to nothing. You cannot tell which from the total. You can tell instantly from the itemisation, which is why the itemisation is the thing to insist on rather than a discount.
Where to go next
Start anywhere in the library. If you only read one more, read the one covering whichever detail your own inventory turned up most of.
The UD Standard, all guides
Every component, with what fails in each and the phrases that show the corner is being cut.
See all guides →FlashingFlashing and penetrations
The largest single category of details, and the one where sequence matters most.
Read the standard →ValleysValleys: open, closed-cut and woven
One channel carrying the runoff of two roofs, and the six inches no fastener may enter.
Read the standard →ToolInstant satellite roof quote
Measures your actual roof from above and prices it at current DMV rates. No salesperson required.
Price my roof →