Building envelope · Roof Assemblies
Tiled and Slated Roof Covering Assembly
Explains how a discontinuous roof covering is organised as a set of interacting layers, why the underlay is a secondary drainage plane rather than a waterproofing sheet, and what an owner should raise with a roofing designer.
Educational reference entry. How this system behaves depends on climate, jurisdiction, loading, substrate, the adjacent systems it meets, how the building is used, the manufacturer's instructions and qualified professional design.
Overview
What tile and slate covering is
This assembly sheds water by overlap. Each unit covers the head of the unit below it and part of the units either side, so the covering is deliberately discontinuous and water travels down the slope in a series of short journeys. Pitch, the amount left exposed on each course and the shelter the site offers are bound together rather than chosen separately.
Beneath the units sits an underlay carried across the structure. Its purpose is to catch what the covering lets through - wind-driven rain, snow entry, condensate, debris - and carry it out at the eaves. It is a drainage plane, not a waterproofing layer, and the difference shows up wherever the underlay is trimmed short, punctured, or lapped against the direction of flow.
The nearest relative in this library is the standing seam metal roof assembly, and the pair can be told apart from the ground. A lapped covering shows repeating open joints that rely on overlap and gravity; a seamed metal roof shows continuous trays whose joints are folded shut. That single observation explains why one type carries a genuine secondary drainage plane beneath it and the other does not need one in the same sense.
Where the insulation sits, and whether the space beneath is ventilated, belong to the roof build-up entries rather than to this one. The same covering can appear over a ventilated cold roof, over insulation held between and under rafters, or over a warm deck, and nothing about the covering itself settles that question. It is not the materials-library entries clay-roof-tiles, natural-slate-roofing or fibre-cement-slate, which are the covering units themselves rather than the drainage arrangement they are laid into.
Terminology
Common names and aliases
These names describe the same assembly. The encyclopedia keeps one entry per system so that a trade term or a regional name never becomes a second, thinner page.
- discontinuous roof covering
- battened roof covering
- lapped-unit roof covering
- slated and tiled roof build-up
Purpose
What this system is intended to do
The job the assembly exists to perform, conceptually. An intention is not a guarantee that any particular build achieves it.
- Shed the great majority of rainfall down the slope through overlapping units, without relying on any joint being sealed.
- Provide a secondary path at underlay level for the water, snow and condensate that inevitably get past the units.
- Hold individual units against wind uplift and against sliding, using fixings and clips matched to the covering and the exposure.
- Give a durable outer surface in which damaged units can be changed without disturbing the covering as a whole.
Composition
The roles this system is made of
What each part does in the assembly and what it depends on — never a product, a thickness, a fixing or a determination that anything is adequate for the role.
Order is meaningful in this system. The roles below sit in sequence, and moving one relative to another changes what the assembly does. The sequence is conceptual only: it states no thickness, no dimension, no fixing and no order of work on site.
- Layer 1 of 7. Order is meaningful.Primary supportRafters, trusses or purlins
The structural carcass that carries the covering and everything above it, and that fixes the plane the covering will follow. Its regularity and alignment govern whether battens sit true and whether the finished surface reads flat or wavy from the ground.
- Layer 2 of 7. Order is meaningful.Drainage planeUnderlay as secondary drainage layer
Laid across the structure beneath the battens, it collects the water, snow and condensate that pass the covering and delivers them to the eaves. Whether it is intended to breathe, or to have air moving behind it, changes what the rest of the roof has to do.
- Layer 3 of 7. Order is meaningful.SubstrateTiling battens
Timber sections fixed over the underlay, giving the covering a line to hang or nail to and setting the exposure of each course. They also hold the underlay down and shape the drape between supports that lets water run instead of pond.
- Layer 4 of 7. Order is meaningful.Cavity or voidCounterbatten space
A batten run along the rafter line beneath the tiling battens, lifting them clear of the underlay so water can travel freely and air can move behind the covering. It matters most where the underlay is fully supported, since there it has no drape of its own to drain through.
- Layer 5 of 7. Order is meaningful.Finish surfaceCovering units
The tiles, slates or shingles that take the weather, the ultraviolet exposure and the mechanical wear. They also set the visual grain of the roof, since unit size drives the course spacing and the shadow line seen from below.
- Layer 6 of 7. Order is meaningful.AttachmentNails, hooks and wind clips
Fixings that hold each unit to its batten, and clips that restrain the free edges most exposed to uplift. Their metal has to be compatible with the covering and with any flashing metal it will sit against, or corrosion becomes the failure route.
- Layer 7 of 7. Order is meaningful.Edge and terminationRidge, hip, verge and valley weatherings
The formed pieces and flashings that close the covering where courses stop or change direction. These are the places where the overlap logic runs out and a deliberate detail has to take over from it.
Interaction
How the parts work together
The reason this is a system rather than a list of parts: what depends on what, and what stops working when one part is changed.
What the counterbatten does depends on how the underlay is carried. Where the underlay is draped between rafters, the sag below the batten line is the channel water runs in, and the tiling battens hold that drape in shape. Where it is laid over sarking or a continuous deck there is no drape: the batten bears on it along its whole length and each crossing becomes a small dam, so a counterbatten is what restores a drainage path and an air space above the sheet - the same space other parts of the build-up often rely on for ventilation.
Batten position and covering overlap are a single decision, not a pair of them. Moving the battens further apart increases what is left exposed on each course and shortens the head lap, so the covering becomes more open to wind-driven rain at exactly the moment the underlay is being asked to do more. Pitch pulls in the same direction, since a shallower slope keeps water on the covering longer and increases what the lap is being asked to do.
The underlay only works if it stays continuous to the outside. Where it is cut back behind a gutter, stopped short at a valley, or lapped against the flow, water that the covering handed over correctly is delivered into the structure instead of out of it. The symptom looks like a covering problem and is not one.
Fixings tie the covering to the wind rather than to the roof. Uplift concentrates at eaves, verges, ridges and around anything that interrupts the surface, so the restraint pattern is not uniform even though the covering looks uniform. Where clips are left out at those edges, units lift and the underlay is exposed to a duty it was never the primary defence against.
Materials
Material families commonly met in each role
Commonly encountered, not recommended. Whether a material suits a given project depends on the whole assembly, the exposure, the manufacturer's documentation and qualified professional review.
Finish surface
These covering families are all commonly encountered as the lapped outer units. Their weights, workable pitches and fixing habits differ sharply, and whether any of them belongs on a given roof is a matter for the manufacturer's documentation and the designer.
Drainage plane
Underlay products are commonly encountered beneath battened coverings, and they are not interchangeable in behaviour. Which type belongs over a given roof space, and whether ventilation is still assumed alongside it, is settled by the designer and the product documentation.
Primary support
Battens and carcass timbers are commonly drawn from these families. Grading, moisture condition and any preservative treatment are engineering and durability questions that sit with the structural designer rather than with the choice of covering.
Attachment
Fixings and clips are commonly made from these metals. Compatibility between the fixing metal, the covering and any adjacent flashing or rainwater goods is the governing issue, and the combination should be confirmed against the manufacturers' documentation.
Edge and termination
Weatherings at abutments, valleys and hips are commonly formed in these materials. Which is appropriate depends on the covering, the movement expected at the junction and the adjacent construction, and remains a matter for the designer.
Junctions
Where this system meets others
Interfaces are where most assemblies actually fail, so they are set out explicitly rather than left inside the prose. What resolves a junction is a detail designed for the specific building — not a rule of thumb.
Eaves, fascia and gutter line
The eaves is where the underlay has to discharge outside the wall and into the gutter, where the lowest course is tilted or doubled, and where uplift is highest. Getting the underlay to overshoot correctly here is what stops water tracking back onto the wall head.
Read about Eaves System →Abutment against a wall or chimney
Where the covering runs into vertical construction the overlap logic stops and a flashing takes over, working with a cover detail formed in the wall. The underlay must be turned up behind that flashing, or the secondary plane simply ends at the junction.
Read about Abutment Detail →Rooflight and penetration upstands
Every opening interrupts both the batten grid and the underlay. The covering has to be closed around a raised kerb and the underlay dressed onto it, so that water arriving from above is diverted around the opening rather than gathered into it.
Read about Rooflight Kerb →Air path behind and beneath the covering
Where the design relies on air moving behind the covering or through the roof space, the counterbatten void and any eaves and ridge openings form part of that path. Filling them with insulation, mortar or debris removes a function the build-up assumed was there.
Read about Roof Void Ventilation →The roof build-up beneath the covering
The same covering sits over quite different arrangements of insulation, vapour control and ventilation. What the covering is asked to tolerate, and how much drying the roof can do, is set by that build-up rather than by the units themselves.
Read about Cold Pitched Roof →
Considerations
Topics worth discussing
Which topics genuinely apply to this system and what to raise about them. Measured values, classes and ratings come from a qualified professional's design for the specific building, not from a reference page.
- Moisture
- Water passing the units is normal and expected. The question a designer answers is not whether it happens but where it goes, which is why the route from underlay to eaves matters more than the apparent tightness of the covering itself.
- Movement
- A covering of small units accommodates movement by shifting slightly against itself. Rigid mortar bedding at ridges, hips and verges removes that freedom and concentrates movement into a line that then cracks, which is why dry-fixed alternatives are so often discussed.
- Durability
- Covering, underlay and fixings do not weather at the same rate. An underlay degraded by ultraviolet exposure at the eaves, or fixings corroding against an incompatible metal, can end a roof whose visible units still look entirely sound.
- Maintenance and access
- Individual units can be replaced, which is one of the strengths of the type, but that depends on safe access and on the fixing method allowing a unit to be released. Roofs clipped throughout are less simple to open than they look.
- Interfaces
- The field of the roof is the straightforward part. Valleys, abutments, verges and penetrations are where the overlap principle stops working and a designed detail replaces it, and they account for a disproportionate share of the trouble reported on this kind of roof.
Design
Questions the design has to answer
The decisions this assembly turns on. They are questions rather than answers, because the answer depends on the building.
- What secondary drainage path is assumed at underlay level, and where exactly does it discharge?
- Is a counterbatten void part of this build-up, and if it is not, what takes over its function?
- How is the covering restrained at eaves, verge and ridge, where wind uplift is concentrated?
- Are ridge, hip and verge details dry-fixed or bedded, and what movement does each approach assume?
- Which metals will sit in contact at fixings, flashings and rainwater goods, and are they compatible?
- Does the build-up beneath rely on air paths that this covering arrangement has to keep open?
Boundaries
Commonly misunderstood points
Distinctions that are easy to blur, and the places where a familiar term means something narrower than it sounds.
- The underlay is often described as a waterproof layer. It is a drainage plane that carries water back out, and it is not asked to be the primary defence.
- A steeper roof is assumed to need less care at the laps. Exposure, prevailing wind and the covering itself all bear on the lap, and pitch alone does not settle it.
- Replacing a covering is assumed to be a like-for-like job. A change of unit weight or fixing method reaches back into the battens, the structure and the wind restraint.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- How will the underlay be terminated at the eaves so that water discharges into the gutter rather than behind it?
- What wind exposure has been assumed here, and which courses and edges are receiving clips as a result?
- Is the existing structure being asked to carry a covering of different weight from the one it was designed for?
- How are the valleys and abutments formed, and what happens to the underlay at each of them?
- Does the specified underlay assume ventilation elsewhere in the roof, and is that ventilation actually present?
- Who is confirming compatibility between the fixings, the flashings and the rainwater goods?
What this page does not do
- Watertightness is a property of the completed, correctly detailed and correctly installed assembly, not of the covering units or the underlay taken alone.
- Fire performance is a property of a tested assembly and of the roof as a whole. This entry states none, and the relevant authority and the tested system govern.
- Whether a covering can be used at a given pitch and exposure is a design determination made against the manufacturer's documentation, not a general rule.
Related systems
How this system relates to others
Every link states what the relationship actually is, rather than leaving a bare list of related pages to be read as a suggestion.
Alternatives
Different systems answering the same need. Listing them together is not a comparison and does not suggest one is better — which, if either, suits a project is a design decision.
Meets these systems
Systems this one physically meets. The junction is usually where the design problem lives, so these are worth reading together.
Commonly built alongside
Systems routinely present in the same building without necessarily touching this one.
Go deeper
Related Build Design Hub guides
Planning guidance behind the decisions this assembly involves.
Preparation
Related planning checklists
Owner-side preparation before the conversation where this system comes up.
Inspiration
Related Ideas Library pages
Design directions where this system commonly appears.
Roof Assembly Systems
Roofs answering two questions at once: where the insulation sits relative to the deck, and how the water-shedding layer achieves continuity across the whole surface.
Browse all roof assemblies entries →