Key takeaways
- Roof sarking is a secondary weatherproofing layer that manages water that gets past the roof covering and water vapour moving out of the building, so its lap direction and drainage path must be planned before the first sheet goes down.
- Laps must be arranged so each course overlaps the one below it in the direction water runs, which means working uphill in most roof geometries and never relying on a lap that sheds into the batten line.
- Drape must be consistent and slight, with the sarking laid over the battens rather than pulled tight, so water can run to the gutter or eaves without ponding or tearing at fixings.
- Penetrations — flues, vents, pipes, cables, solar mounts and exhausts — are the most common leak point and must be sealed with a compatible, durable method rather than relying on the roof covering alone.
- Sarking that is installed with the wrong lap direction or a reversed vapour control layer can trap moisture in the roof space, so the sequence of layers and the direction of the lap must be checked before the covering goes on.
Roof sarking is the layer that catches what the roof covering lets through. Tiles, metal sheet and every other covering will pass some water in a driven storm, and sarking is what stops that water reaching the ceiling lining, the insulation and the structure. It is also part of how a roof manages water vapour moving out of the building. Both jobs depend on the same thing: the sarking has to be installed so water always has a path downhill and out.
Most sarking failures on NSW sites are not product failures. The roll was fine when it was delivered. The problem is the lap direction, the drape, or a penetration that was sealed with whatever was in the ute. Those are detail decisions made in the first few hours of the roof, and they are very hard to correct once the covering is fixed.
This article deals with the weatherproofing layer itself — how it is lapped, how it is draped, and how penetrations and junctions are sealed. It does not cover roof covering selection, which is a separate decision made earlier in the job.
What sarking actually does, and why that sets the rules
Sarking has two jobs and they pull in slightly different directions. The first is as a secondary weatherproofing layer: it collects water that gets past the covering and delivers it to the gutter or eaves. The second is as part of the roof’s moisture management, controlling how water vapour moves between the roof space and the outside. WoodSolutions material on timber and moisture makes the general point that timber performs best when it can dry, and that trapping moisture against timber is what causes decay and movement. A roof space that cannot dry is a roof space that will eventually show problems in the battens, trusses and lining.
Those two jobs explain nearly every installation rule. Water has to be able to run downhill on the outer face of the sarking, which is why laps run a particular way. Vapour has to be able to move to the right side of the assembly, which is why the position of a vapour control layer matters. Wind has to be kept from getting under the laps, which is why laps are fixed over a batten. If you understand what the layer is doing, you can reason about a detail the article never mentions.
The related article on roof ventilation and condensation risk in NSW covers the ventilation side of this in more detail. The point here is that sarking is not a wrap; it is a drainage plane with a direction.
Installing sarking so water has a path to the gutter
- 1Set the starting courseStart at the eaves and work uphill so each new course overlaps the one below it in the direction water runs.
- 2Run the sarking over the battensLay the sarking over the top of the battens, not tucked behind them, so water running down the sheet stays on the outer face.
- 3Keep a slight, even drapeAllow a consistent sag between supports rather than pulling the sheet tight, so water tracks to the gutter instead of ponding on a flat span.
- 4Form the lap at the battenBring the lap over a batten and fix through both layers so wind cannot lift the lap apart.
- 5Seal penetrations as you goDetail flues, vents, pipes and mounts before the covering is fixed, not after, so access is still possible.
- 6Check the valley and ridge runsConfirm the lap reverses correctly at valleys and that the ridge closure does not block the drainage path.
Lap direction: the rule that decides whether water gets in
Laps must run so that each course overlaps the one below it in the direction water travels. On a simple gable, that means starting at the eaves and working up the roof, with each new course laid over the top of the previous one. The lap sheds water down the roof, onto the next lower course, and eventually into the gutter. Reverse it and every lap becomes a small dam holding water against the layer below.
The complication is that roofs are not all one plane. Valleys, hips, changes of pitch and junctions with lower roofs all change the direction water runs. At a valley, the drainage path turns, and the sarking has to turn with it. A lap that was correct on the main plane can be wrong once it crosses the valley line. This is where a lot of defects start, because the installer is working to the main roof geometry and the valley is treated as a detail to be sorted out later.
The practical rule is to mark the drainage direction on the roof before laying anything. On a complex roof, sketch the path water takes from ridge to gutter, including the valley runs, and lap accordingly. Where two planes meet, work out which sheet sits on top and make sure the under-sheet still has a path out. If it does not, the detail is wrong regardless of how neatly the lap is fixed.
Drape, tension and fixing: why a tight sheet fails
Sarking is not meant to be pulled tight like a drum. It is meant to sit over the battens with a slight, consistent drape so that water running down the outer face tracks down the roof rather than sitting on a flat span. A sheet pulled tight across a bay can pond water at the low point, and a sheet pulled tight over a fixing can tear at the fixing when it moves thermally or when someone walks the roof.
The drape also matters for the covering above it. Tiles and sheets are laid to a line, and a sarking layer that is bunched in one bay and tight in the next changes the surface the covering sits on. On metal roofs, a tight sheet can telegraph through the sheeting. On tile roofs, inconsistent drape can affect how the battens and tiles sit.
Fix through the lap, over a batten, so the fixing holds both layers and the lap cannot lift in wind. Fixing through the middle of a sheet instead of the lap is a common shortcut and it leaves the lap free to flutter, which is exactly how laps open up in a storm. The related article on roof truss bracing and tie-down checks covers the structural side of the same roof; the sarking has to survive the same wind loads without coming apart.
Penetrations are where sarking jobs fail
Every hole in the sarking is a potential leak, and roofs have more of them than most builders count: flues, exhaust vents, soil and vent pipes, cable entries, solar mounts, aerial and anchor points, and sometimes access hatches. Each one interrupts the drainage plane and each one has to be sealed so water that reaches it is directed back onto the sarking and down to the gutter.
The most common mistake is to treat the penetration as the roof covering’s problem. The covering has its own flashing, and that flashing is designed on the assumption that the sarking below is doing its job. If the sarking is simply cut and left, water that gets past the covering flashing runs straight into the roof space. The sarking detail has to be formed first, then the covering flashing installed over it.
Seal with a method compatible with the sarking material. Some sarking products are not compatible with certain sealants or tapes, and a seal that looks neat on the day can fail within a season. Check the manufacturer’s instructions for the product actually on site, not the one you used on the last job. Where a penetration is large or repeated, a purpose-made collar or boot is more reliable than a hand-formed patch.
Lead in. The mistake that costs the most is sealing penetrations after the roof covering is fixed. Access is gone, the detail is done blind, and the first sign of a problem is a stain on a ceiling lining months later when the source is hard to find and the repair means removing covering.
Penetration and lap checks before the roof covering goes on
- Lap direction runs downhill at every course, including at valleys and changes of pitch.
- Sarking sits over the battens and is fixed through the lap, not through the middle of the sheet.
- Drape is consistent across the roof, with no tight spans that will tear at fixings.
- Flues, vents, pipes and cable entries are sealed with a method compatible with the sarking material.
- Solar mounts, aerials and anchors are sealed and flashed, not just screwed through the sarking.
- Ridge and hip closures do not block the drainage path or trap water against the sarking.
- The vapour control layer, where specified, is on the correct side of the insulation.
Junctions, valleys and changes of pitch
Junctions between roof planes are where the lap direction has to be thought through rather than repeated. At a valley, water from two planes converges and runs down the valley line. The sarking on both planes has to deliver water into the valley without either plane damming the other. At a change of pitch, the drainage path changes angle, and the lap has to follow it without creating a low point where water can sit.
At a junction with a lower roof or a wall, the sarking has to be continuous or correctly lapped into the adjacent detail. A wall junction is a common leak point because water running down the wall can reach the roof and the sarking detail has to hand it off to the roof drainage path. If the sarking stops short of the junction and the covering flashing is expected to do all the work, the assembly is relying on one line of defence instead of two.
Ridge and hip closures also need checking. A closure that blocks the drainage path can trap water against the sarking, and a closure that is too open can let wind-driven rain past the covering and onto a lap that was not designed for that volume. Follow the closure detail specified for the covering and check it does not interfere with the sarking path.
The vapour control side: getting the layer order right
Where a vapour control layer is specified, its position in the assembly matters as much as the lap direction. A vapour control layer on the wrong side of the insulation traps moisture in the roof space rather than letting it move. On a NSW roof, the position depends on the climate zone, the roof type and whether the roof space is ventilated or sealed, and it is set out in the design documentation for the job.
The mistake to avoid is treating the vapour control layer and the sarking as the same thing. They can be combined in one product, but the installation requirements are not identical, and the lap direction rules for a drainage plane do not automatically apply to a vapour control layer. Read the product documentation and the design documentation together, and if they conflict, resolve it before the covering goes on.
Your Home’s guidance on sealing your home is a useful reminder that sealing and vapour management are linked problems. A tightly sealed roof space with the wrong layer order is a moisture problem waiting to show up.
Correct and incorrect lap and drape detail
| Correct detail | Incorrect detail |
|---|---|
| Each course overlaps the one below in the direction water runs | Courses lapped uphill, so water sits on the lap and finds a path through |
| Sarking laid over the battens with a slight, even drape | Sarking pulled tight or tucked behind battens, so water ponds or tears |
| Laps fixed over a batten through both layers | Laps fixed mid-span, letting wind lift the lap apart |
| Penetrations sealed before the covering is fixed | Penetrations sealed after the covering is on, with poor access and no inspection |
| Vapour control layer on the correct side | Vapour control layer reversed, trapping moisture in the roof space |
Worked example: a western Sydney re-roof
A builder in western Sydney is re-roofing a 1990s tile roof on a hip-and-valley plan. The covering is replaced, the battens are checked and replaced where needed, and new sarking is specified. The roof has two valleys, a flue, two exhaust vents and a solar array being reinstalled on new mounts.
The sarking is laid from the eaves up on the main planes, which is correct. At the first valley, the installer laps the sheet from the shorter plane over the sheet from the longer plane because it is easier to fix. That lap runs against the direction water travels in the valley, so in a driven storm water sits on the lap and finds a path under it. The flue and vents are cut and left for the plumber and electrician to seal later, and the solar mounts are bolted through the sarking with a bead of sealant around each bolt.
Nothing shows up at practical completion. The first winter, a stain appears on a ceiling lining under the valley. The builder opens the roof, finds the reversed lap and the unsealed penetrations, and has to remove covering over the valley to redo the lap and reflash the flue. The solar mounts are checked and several are found to have sealant that has already failed. The cost is not just the repair; it is the covering removal, the reinstallation, the scaffold if it has to come back, and the client relationship.
The alternative was cheap. Mark the drainage direction including the valleys before laying. Lap accordingly. Detail every penetration before the covering goes on, with the right sealant or a purpose-made boot. That is a few hours of care in the first day of the roof, against a repair that surfaces after handover.
Inspection points before the covering goes on
The window to check sarking is before the covering is fixed. Once tiles or sheets are down, every lap and every penetration is hidden, and the only evidence of a problem is water inside the building. Build the inspection into the programme as a hold point, and do not let the covering crew start until it is signed off.
Walk the roof and check the lap direction on each plane, including valleys and changes of pitch. Check the drape is consistent and the laps are fixed over battens. Check every penetration is sealed with a method compatible with the product, and that the seal is formed before the covering flashing, not after. Check the vapour control layer, where specified, is on the correct side. Record what you checked, with photographs, so that if a problem appears later you can show what was installed.
For builders working through their CPD, the detail work on the weatherproofing layer is the kind of thing that is easy to assume and hard to prove later. The guide to how NSW builder CPD works sets out the framework, and the PointCPD course catalogue covers the technical areas in more depth. The related article on slab edge thickness and set-down tolerances makes the same point about tolerances on the other side of the building: the detail you cannot see later is the one worth checking now.
What to check when the product on site is not the one specified
Substitutions happen. A roll runs short, a merchant sends a different product, and the crew installs what is on the truck. The problem is that lap direction rules, compatibility with sealants and the position of a vapour control layer all depend on the product. A substitution that looks equivalent on the roll may not be equivalent in how it is lapped or sealed.
If the product changes, check the manufacturer’s installation instructions for the product actually on site before it is installed, not after. If the substitution changes the layer order or the sealing method, get it resolved with the designer before the covering goes on. If you are unsure whether the substitution is acceptable, say so plainly and check the current requirement in the design documentation and the manufacturer’s instructions.
The commercial reality is that a substitution made to save a day can cost a strip and re-roof of a section if it is wrong. The sarking is one of the cheapest components on the roof and one of the most expensive to get wrong, because correcting it means removing what is above it.
Questions builders ask
Which way should roof sarking laps run?
Laps must run so each course overlaps the one below it in the direction water travels, which usually means starting at the eaves and working uphill. Reverse the lap and every lap becomes a small dam that holds water against the layer below. Work out the drainage direction first, including at valleys and changes of pitch, and lap to suit.
Should sarking be laid over or behind the battens?
It should be laid over the battens with a slight, consistent drape, and fixed through the lap over a batten. Laying it behind the battens breaks the drainage path and leaves water against the timber. Fixing mid-span instead of at the lap lets the lap lift in wind.
How do I seal roof penetrations in sarking?
Seal each penetration with a method compatible with the sarking product, using the manufacturer’s instructions for the product actually on site. Form the sarking detail before the covering flashing is installed, because the flashing is designed on the assumption that the sarking below is doing its job. For larger or repeated penetrations, a purpose-made collar or boot is more reliable than a hand-formed patch.
Does sarking need to be tight or loose?
It needs a slight, even drape, not a tight pull. A sheet pulled tight can pond water at a low point and tear at fixings when it moves. A sheet bunched in one bay and tight in the next changes the surface the covering sits on and can telegraph through metal sheeting.
What is the difference between sarking and a vapour control layer?
Sarking is primarily a drainage plane that collects water getting past the covering; a vapour control layer manages how water vapour moves through the assembly. They can be combined in one product, but the installation requirements are not identical, and putting a vapour control layer on the wrong side of the insulation traps moisture in the roof space.
When should sarking be inspected?
Before the roof covering is fixed. Once tiles or sheets are down, every lap and penetration is hidden and the only evidence of a problem is water inside the building. Build the inspection into the programme as a hold point, check lap direction, drape, penetrations and the vapour control layer, and record what you checked with photographs.
Sources
- Sealing your home · Your Home (Australian Government)
- WoodSolutions: timber and moisture · Forest and Wood Products Australia
Published 18 July 2026. General information for builders, not legal, financial or engineering advice for a specific job. Check current requirements with the relevant regulator or standard before relying on them.
