Envelope and protection · Roofing
Roofing Underlay (Secondary Layer Beneath a Covering)
This entry explains what a roofing underlay does as a secondary layer rather than a primary barrier, and why the choice between vapour-permeable and impermeable types is a design decision bound up with ventilation.
Educational reference entry. Suitability depends on the complete assembly, substrate, exposure, installation method, manufacturer documentation, local requirements and qualified professional review.
Overview
What roofing underlay is
A roofing underlay is the sheet laid beneath a tiled, slated, shingled or sheeted covering, either draped over the rafters or laid over a continuous deck or sarking board. It is a secondary layer: its part is to deal with the water, wind-driven rain and dust that get past the covering above and to carry them out at the eaves, not to serve as the roof's primary barrier.
Two broad families exist and they are not interchangeable. Vapour-permeable underlays allow water vapour to pass outwards through them, while impermeable ones, traditionally bitumen-based and now often reinforced plastic sheets, do not. The choice interacts directly with how the roof space beneath is ventilated and with where the insulation and any vapour control layer sit, which is why it is a design decision rather than a supply decision.
Products differ in more than permeability. Some expect support at every rafter, others suit being laid over a deck; some have integral adhesive edges or taped laps; and most state a limited period during which they may be left exposed to sunlight before the covering goes on. An underlay is also not the same thing as a breather membrane or a vapour control layer, even where the sheets themselves look similar.
Terminology
Common names and aliases
These names describe the same thing. The library keeps one entry per material so that an acronym, a trade name or a regional spelling never becomes a second, thinner page.
- Roofing underlayment (US usage)
- Sarking membrane
- Roof underlay
- Breather underlay
Applications
Common applications
Where this material is typically encountered. A typical application is not a statement that it suits your project.
- Beneath tiled and slated pitched roofs, draped over the rafters and held by the battens above.
- Over a continuous deck beneath shingles or a sheeted covering as part of the same build-up.
- In roof build-ups where the insulation follows the rafter line rather than sitting at ceiling level.
- As a temporary weather cover during construction, within the exposure period the product allows.
- In re-roofing, where the covering is stripped and the underlay beneath is renewed at the same time.
- Over sarking boards in build-ups where a boarded layer sits between the rafters and the counterbattens.
Consideration
Where it is commonly considered
- Wherever a covering sheds water down a slope and a secondary layer beneath it forms part of the design.
- Where the ventilation strategy for the roof space has been settled and the underlay is chosen to suit it.
- Where the covering, the battens, the insulation and the underlay are being specified as one build-up.
- Where an existing roof is being stripped so that what lies beneath the covering can be seen and renewed.
Professional review
Where additional professional review is especially important
These are the situations where the answer depends on the specific building, and where a qualified professional should be involved before anything is decided.
- Where a vapour-permeable type is proposed without the ventilation and air-tightness arrangement being examined.
- Where an underlay is being treated as the barrier that keeps the building dry rather than as a secondary layer.
- Where the insulation is being moved to follow the rafters without the underlay and ventilation being reconsidered.
- Where a new covering is being laid over an old underlay whose type and condition are both unknown.
- Where the sheet would be left exposed for longer than the product documentation allows before covering.
Properties
Key properties to discuss
Qualitative topics worth raising. Measured values, classes and grades come from the manufacturer's technical documentation for the specific product, not from a reference page.
- Moisture behaviour
- The underlay deals with whatever gets past the covering and with vapour moving outwards from the building. Permeable and impermeable types handle the second of those quite differently, and the choice cannot be separated from how the roof space is ventilated.
- Installation dependency
- How the sheet is supported, lapped and terminated is what makes it work. The eaves detail in particular has to discharge water into the gutter rather than behind the fascia, and a support tray at the eaves is commonly part of that arrangement.
- Substrate dependency
- Whether the underlay is draped over rafters or laid on a boarded or panelled deck changes which products suit and how they are held. Counterbattens appear in some build-ups and alter the drainage path beneath the covering.
- Repairability
- The underlay is buried by the covering above it, so it is not repaired in isolation. Reaching it means lifting the covering, which is why it is normally renewed when a roof is stripped rather than patched later on.
- Maintenance
- Because it is hidden, an underlay only announces itself when something has gone wrong: damp patches, staining at the eaves or debris held above it. Any inspection means lifting part of the covering, which is work for a qualified professional.
- Documentation
- The product documentation states whether the underlay is vapour-permeable, what support it needs, what ventilation arrangement it assumes, how the laps are treated and how long it may stay exposed before the covering goes on.
Exposure
Exposure and environmental conditions
- Does driving rain reach this roof often enough to bear on which underlay is selected?
- How long is the roof likely to stand with the underlay exposed before the covering goes on?
- Is the roof space heated, unheated or partly converted, and how does moisture reach it from below?
- Does the building generate a lot of moisture internally, and where is that vapour expected to go?
- Is the climate here cold enough that condensation beneath the covering is a recurring concern?
Assembly
Substrate and system dependencies
What this material sits on, is fixed to or depends on. This is usually where performance is won or lost.
- The position of the insulation, at ceiling level or along the rafters, changes what the underlay has to do.
- The ventilation arrangement at the eaves and at high level is decided together with the type of underlay.
- An underlay is a secondary layer and does not take the place of a vapour control layer, which is a separate element with its own job in the build-up and its own continuity to maintain.
- Battens, and counterbattens where they are used, support the sheet and set the drainage path beneath the covering.
- The eaves detail, including any support tray, determines where water leaving the underlay actually goes.
Appearance
Appearance and finish considerations
- The only part normally seen is the edge at the eaves, which is visible from the ground on many roofs.
- A sheet left exposed during construction weathers and can look poor before the covering ever goes on.
- Underlay draped between rafters shows as a sag beneath the battens, seen only from within the roof space.
- Colour and printing on the sheet vary by product and show wherever the underlay is visible at verges and eaves.
Durability
Durability questions
Questions to raise rather than a service life to expect. No material has a universal lifespan — it depends on the assembly, the exposure and the upkeep.
- How long has this underlay been exposed, and what does the product documentation allow?
- Is the sheet supported in the way the product expects, given the arrangement of the battens above it?
- How are the laps treated, and does the product rely on tape or on integral adhesive at its edges?
- What happens at the eaves, and where does water leaving the underlay actually discharge?
- If the covering above is staying put, what is actually known about the sheet hidden beneath it?
Upkeep
Maintenance and repair considerations
- How would a problem with the underlay be identified from inside the roof space?
- If the covering is being repaired locally, what is being done about the sheet beneath it?
- Is the underlay being renewed as part of the re-roofing, or left in place, and on what basis?
- Who would assess the condition of a hidden layer like this, and what would that involve?
Installation
Installation dependencies
What the installation depends on — not how to do it. Method belongs to the manufacturer's instructions and the trades carrying out the work.
- The type of underlay and the ventilation arrangement are decided together, not one after the other.
- The eaves detail has to discharge into the gutter rather than behind the fascia board.
- Laps and terminations at ridge, verge, valley and abutment all follow the product documentation.
- Penetrations for flues, vent terminals and rooflights interrupt the sheet and need proper collars or dressing.
- The period for which the sheet may be left exposed before the covering goes on is limited by the product.
Documentation
Documentation to request
Ask for these in writing, for the specific product being proposed, and keep them with the project record.
- Product documentation stating whether the underlay is vapour-permeable and what support it requires.
- A description of the ventilation arrangement that the underlay is being used alongside.
- Details of how the sheet is treated at the eaves, ridge, valley and abutments.
- Confirmation of the exposure period allowed before the covering is laid over it.
- Details of the product noted alongside the covering references, so both are on record together.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- Is the underlay proposed vapour-permeable or impermeable, and what is the reasoning behind that?
- How is this roof space ventilated, and how does the underlay choice relate to that arrangement?
- Where is the insulation in this roof, and does the underlay suit where it has been placed?
- Does a vapour control layer form part of this build-up, and who is detailing its continuity?
- What happens at the eaves, and is a support tray part of the detail here?
- How are the laps formed, and does this product need them taped or bonded?
- How long will the underlay stand exposed before the covering goes over it?
- What condition is the existing underlay in if the covering is only being partly renewed?
Blind spots
Commonly overlooked points
- The underlay is a secondary layer, and treating it as the thing that keeps the building dry misreads the whole roof.
- Changing the underlay type without revisiting ventilation is among the commonest sources of trouble in a roof.
- The eaves detail decides where water from the underlay ends up, and it is often settled in minutes on site.
- Underlay, breather membrane and vapour control layer do different jobs, and the sheets can look much alike.
- An old underlay under a covering being retained is frequently in worse condition than anyone expects.
What this page does not do
- An underlay does not make a roof dry on its own; the covering, the junctions and the ventilation arrangement all bear on that.
- No lifespan is stated here, because it depends on the product, on how long it stood exposed and on the roof around it.
- The relationship between underlay, ventilation, insulation and vapour control is a design matter for a qualified professional.
- Requirements for roof build-ups vary by location and are confirmed locally rather than taken from a reference page.
Related entries
Related materials
Materials from the same family, an adjacent role in the assembly, or a shared application.
Inspiration
Related Ideas Library pages
Design directions where this material commonly appears.
Preparation
Related planning checklists
Owner-side preparation before the conversation where this material comes up.
Go deeper
Related Build Design Hub guides
Planning guidance and neutral comparisons behind the decisions on this page.
Roofing Materials & Systems
Reference entries for roof coverings and roof waterproofing — tiles and slates, shingles, sheet and standing-seam metal, single-ply and built-up membranes, liquid systems and green roofs.
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