The Lapping Rule: Why Every Roof Layer Goes Over the One Below
Water only runs downhill. If every layer is lapped over the one below it, water is always carried to the outside of the roof.
- Intermediate
- 20 minutes to read
- Updated
The short answer
The lapping rule says every layer on a roof must lie over the layer below it, like fish scales, so water running downhill always passes over the top of each joint and never behind it. Install roofing from the bottom up, and flashing from the bottom up, so each upper piece laps over the lower. A reverse lap, even a small one, is a leak path that sealant only hides temporarily.
Every roof leak comes from water getting somewhere it was not supposed to go, and in nearly every case a lap is involved: a joint where one piece of roofing meets another. The lapping rule is the principle that makes joints safe. Water runs downhill, so the piece higher on the roof has to lie on top of the piece below it, like the scales of a fish. Then water running down the upper piece drops onto the top of the lower one and keeps going.
It sounds obvious, and yet reverse laps are among the most common installation errors. They happen when someone installs flashing before underlayment, tucks a piece under the wrong layer, or cuts corners on a wall. A single reverse lap turns a joint into a funnel.
This guide explains the rule itself, the order in which a roof is built to follow it, and how it applies to step flashing, counter flashing and every other detail. It is worth understanding before any other flashing detail.
Tools
- Chalk line
- Tape measure
- Hook blade utility knife
- Roofing hammer or nailer
- A dry layout of the pieces before fastening
Materials
- Underlayment and ice and water shield
- Drip edge and flashing pieces
- Roofing nails
- Shingles or panels
- Sealant only where the manufacturer specifies
Safety first
- Lapping is a fit-and-check process done on the roof, so set up fall protection before starting. WorkSafeBC requires it at 3 metres (about 10 feet) and wherever a fall could cause serious injury.
- Work from the eave up so you are never kneeling on an unlapped edge of membrane or wet underlayment.
- Loose underlayment and plastic film are slippery. Fasten each piece as you place it.
Step by Step
Step 1: Think in terms of the water path
Before placing any piece, picture water running down the roof. At any joint, ask which side the water arrives from. The piece that water arrives on must be the upper one. If water would have to travel upward or sideways to get under an edge, the edge is lapped correctly.
Wind changes this. Wind-driven rain can be pushed up under edges that only have a shallow lap. That is why laps are sized in inches and not left to a minimum contact.
Step 2: Build from the eave up
Roofs are built bottom to top for this reason. The eave ice and water shield goes first, then the underlayment lapped over it, then the starter course, then the field shingles. See how to install ice and water shield and how to install synthetic underlayment.
Horizontal underlayment laps are commonly 2 to 6 inches for the upper course over the lower, and end laps are commonly 6 inches. Confirm the exact figure on the product's instructions, because synthetic products print lap lines on the sheet.
Step 3: Flip the order at the eave and rake edges
At the eave, water runs down the underlayment and must drop onto the metal, so the metal goes under the underlayment. At the rake, there is no downhill flow toward the edge, but wind can drive water under the membrane edge, so the metal goes over the underlayment and holds it down. See how to install drip edge.
This is the most common case where people think the rule has an exception. It does not. It is the same rule applied to two different water directions.
Step 4: Lap walls upward, then cover with cladding
A wall adds a second surface that water runs down. The wall's water resistive barrier (building paper or housewrap) must lap over the top of the flashing, just like a shingle course. Flashing tucked behind building paper with the paper lapped backwards under it is a reverse lap hidden behind the cladding.
The same idea drives step flashing, headwall flashing and how to flash a roof to a wall by cladding type.
Step 5: Shingle pipe and penetration flanges into the field
A pipe flashing is a flange with a collar. The flange's upper half is slid under the course above and its lower half sits on top of the course below, so water running down the roof sheds over the flange, not into the gap. The same is true of pipe boots and every flanged unit.
Step 6: Check every joint for a reverse lap before covering it
Put a hand on the joint and run it the way water would run. If your fingers catch an edge that would face the water, it is a reverse lap. Fix it now. After the next course covers it, the error is buried.
Take photos of each detail before it is covered. They are useful for warranty paperwork and for diagnosing a leak later.
Tip: Dry fit flashing pieces before fastening. A piece that fits neatly under the upper layer and over the lower one will show it immediately.
Why Sealant Is Not Flashing
Sealant cannot fix a reverse lap. It fills a gap with a flexible material that shrinks, cracks and lets go of metal over a few seasons of sun, cold and movement. A properly lapped joint does not depend on any adhesion: gravity does the work. When sealant is the only thing keeping water out, the joint is one failed bead away from leaking. See roofing sealants for where sealant belongs as a backup.
Sizing the Lap
The right lap depends on roof slope and exposure. Steeper roofs shed water fast and need less lap. Lower slopes shed slowly and need more, and standing water or wind-driven rain calls for more again. Common minimums are listed below, but the product instructions always govern.
- Step flashing: at least 3 inches (75 mm) of head lap between pieces, the BC Building Code minimum.
- Underlayment: commonly 2 to 6 inches on side laps and 6 inches on end laps, depending on the product.
- Ice and water shield: commonly 3 to 4 inches on side laps and 6 inches on end laps, with sealing of the lap by pressing it down firmly.
- Headwall and apron flashing: at least 4 inches of roof leg over the shingles, and at least 4 inches of wall leg behind the cladding.
- Valley and metal panel laps: set by the valley type or panel profile. See roof valley types.
Fraser Valley Notes
Low-slope roofs on additions and porches are common here, and they shed water slowly under long wet spells. Give them a larger lap, stay within the shingle manufacturer's minimum slope, and use sealed membranes at every wall. Wind off the Fraser River and outflow winds through the canyon push rain under shallow laps, so laps that would pass on a sheltered lot are marginal on an exposed one.
Common Mistakes
- Installing flashing before the underlayment so the membrane ends up on top of it and water runs behind the metal.
- Lapping the wall building paper behind the flashing, which turns the wall itself into a path for water behind the roof.
- Short laps on low-slope roofs, where slow water and wind can drive past a lap sized for a steep roof.
- Sealing a reverse lap instead of redoing it. It looks fine for a season or two, then opens.
- Nailing through the lap line. Fasteners must sit in the covered part of a lap or at the top of a piece, not in the exposed part where water flows.
- Skipping the dry fit. Discovering a backwards lap after the next course is nailed means tearing the work back out.
When to Call a Professional
If a leak shows up at the same spot after a repair, or you find sealant smeared over a joint, suspect a lapping error. A roofer can pull back the layers and trace the actual water path. It is usually a single reversed piece.
Roofs with several wall joints, valleys and penetrations stack many laps in one place. If you are not comfortable reading the order in which they go together, have a roofer check the details before you add another layer or a sealant.
Fraser Roofing does free roof inspections across the Fraser Valley, from Hope to Abbotsford. See our Roof Repair service or the towns we cover.
The Lapping Rule: How Every Roof Layer Sheds Water: Questions
What is the lapping rule in roofing?
The lapping rule says every upper layer of roofing must lie over the layer below it, so water flowing downhill always passes over the top of a joint and never gets underneath it. It applies to underlayment, flashing, shingles, panels and wall coverings alike. Roofs are built from the bottom up so each new piece naturally goes over the last.
What is a reverse lap on a roof?
A reverse lap is a joint where the lower piece lies on top of the upper piece, so water running down the roof is directed into the gap instead of over it. Examples are flashing tucked over the top of shingles at the wrong end, or wall paper lapped behind flashing. It leaks whenever it rains and cannot be fixed with sealant.
How much should roofing underlayment overlap?
Most underlayments call for horizontal side laps of about 2 to 6 inches and end laps of about 6 inches, but the exact figure depends on the product. Synthetic underlayments often print overlap lines on the sheet. Ice and water shield typically needs 3 to 4 inches on side laps. Always check the manufacturer's installation instructions.
Does drip edge go over or under underlayment?
Both, depending on the edge. At the eaves, drip edge goes under the underlayment so water running down the roof drops onto the metal and into the gutter. On the rakes, drip edge goes over the underlayment so wind-driven rain cannot get under the membrane. Both follow the lapping rule for the direction the water comes from.
Can caulking fix a bad roof overlap?
Only for a short time. Sealant fills a gap but does not change the direction the water flows, and it cracks and pulls away after a few seasons of sun and cold. A reverse lap has to be corrected by reworking the pieces so the upper one laps over the lower. Sealant is a backup at properly lapped joints, not a substitute.
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All Flashing Details guides|The full How-To Library
This guide is general education. Always follow the product manufacturer's installation instructions, the BC Building Code and WorkSafeBC requirements, which take priority over anything written here.
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