Building envelope · Roof Assemblies
Built-Up Twin-Skin Metal Roof Assembly
A reference for how a site-assembled metal roof divides weathering, thermal and air control duties between separately delivered layers, and why a break in the liner shows up as condensation rather than as a leak.
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 twin-skin metal roof is
The roof is built up in the order the layers are needed. A profiled liner sheet is fixed to the purlins and closes the building from below; a spacer or bracket system is set out on top of it; insulation is laid over and between the spacers; and a profiled outer sheet is fixed through the arrangement into the spacer beneath. Each layer arrives separately and is a separate decision.
That division is the point of the type. The outer sheet sheds water and takes the wind. The liner, with its laps sealed, is the air and vapour control element and simultaneously the visible ceiling of the space below. The spacer carries the outer sheet down to the purlins and at the same time governs how much room the insulation has. No single layer is the roof.
The nearest relative is the insulated composite panel roof assembly, and a question about sequence separates them on site: could the liner have been installed on its own, with the outer sheet following later? In a twin-skin roof it could, because the layers are independent components fixed at different times. A composite panel arrives with the same duties bonded into a single piece and cannot be sequenced at all, which is why its design attention moves to the joints between panels.
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.
- site-assembled metal roof
- built-up metal cladding roof
- liner and outer sheet roof
- twin skin roof system
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.
- Enclose a large single-storey volume quickly using light components carried on a widely spaced structural frame.
- Separate weathering, thermal and air control duties into layers that can each be selected and adjusted independently.
- Allow the building to be closed from below before the weathering skin is complete, so following work can begin.
- Provide a lined soffit that serves as the finished internal surface of the space beneath.
- Accommodate rooflights, ducts and plant within a repeating profiled module across a large roof area.
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 supportPurlins and structural frame
The steelwork the whole roof hangs from. Purlin position sets where the liner can be fixed, where spacers land, and where any opening has to be trimmed, so the frame effectively fixes the module the rest of the roof works to.
- Layer 2 of 7. Order is meaningful.Control layerProfiled liner sheet with sealed laps
The lower sheet, fixed directly to the purlins. Its sealed side and end laps and its closed perimeter make it the air and vapour control element for the assembly, and its underside is the ceiling that the occupied space actually sees.
- Layer 3 of 7. Order is meaningful.AttachmentSpacer, bracket and rail system
The components standing between liner and outer sheet. They transfer wind and imposed load from the outer sheet to the purlins, set how deep the insulation zone is, and form a repeated path across the build-up along every support line.
- Layer 4 of 7. Order is meaningful.Thermal layerInsulation laid over the liner
Quilt or board laid across the liner and dressed around the spacers. Its behaviour depends on being held at its intended loft rather than pressed down, particularly where the outer sheet is drawn tight along the spacer line.
- Layer 5 of 7. Order is meaningful.Finish surfaceProfiled outer weathering sheet
The visible upper sheet that sheds water, takes wind uplift and carries any traffic during work on the roof. Its profile depth and its lap arrangement determine how it behaves on shallow falls and in driving rain.
- Layer 6 of 7. Order is meaningful.Jointing and sealingSide lap, end lap and perimeter sealing
Sealing appears at both sheets and does different work at each. At the outer sheet it keeps driving rain out of the laps; at the liner it closes the air path. Confusing those duties is a recurring problem with this assembly.
- Layer 7 of 7. Order is meaningful.Edge and terminationEaves, ridge, verge and penetration closures
Filler pieces, flashings and formed closures that shut the profile where it stops or is interrupted. Because the sheets are corrugated, every termination has to close a shaped opening rather than a straight line.
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.
The liner is asked to do unrelated jobs, and only the visible one gets judged on site. As a ceiling it is assessed on line and finish; as the air and vapour control element it is assessed on whether its laps are sealed and its perimeter closed. A liner that looks faultless from below and is open at its side laps has failed at the duty that matters, and the symptom appears later, inside the build-up.
The spacer sets the insulation zone and crosses it at the same time. Every bracket forms a path from the outer sheet down to the purlin, so how the spacer is shaped and how the insulation is dressed around it decides whether the thermal layer stays continuous along each support line. Pressing the quilt down so the outer sheet can sit tight defeats the layer at exactly those positions.
Because the layers are fixed in separate operations, each later operation can undo an earlier one. Fasteners for the outer sheet pass through the insulation and land in the spacer; foot traffic before the outer sheet is on can flatten the quilt; and any tape applied to the liner has to survive everything the following trades do above it.
A break in the liner does not produce a drip beneath the break. It lets warm moist air from the space below into the cold upper part of the build-up, where it condenses on the underside of the outer sheet and runs to somewhere unrelated before it appears. Diagnosing that as a leak in the outer sheet is the mistake this assembly invites most often.
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.
Control layer
Liner sheets and the products that close their laps are commonly drawn from these families. Whether the liner alone is intended to serve as the vapour and air control element, or whether a separate layer is assumed, is a question for the designer and the system documentation.
Thermal layer
Insulation in these families is commonly encountered in the zone between liner and outer sheet. How it is retained, and what happens to it where a spacer crosses, is part of the system design rather than a property of the insulation.
Finish surface
Outer sheets are commonly formed from these materials and their coatings. Profile, coating and fixing arrangement are selected together for the exposure and the fall, and remain matters for the designer and the manufacturer's documentation.
Primary support
Purlins and their supporting frame are commonly made from these. What the frame can carry, and how the sheets are restrained against uplift on it, are structural engineering questions and are not addressed by this entry.
Attachment
Spacer components and their fasteners are commonly made from these metals. Fastener behaviour where it crosses the insulation zone, and compatibility with the sheets it passes through, are matters for the system documentation.
Jointing and sealing
These families are commonly encountered at the laps and closures of both sheets. Which product belongs at a given lap, and what it is being asked to do there, is defined by the system documentation and 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.
Purlins and the frame beneath
The roof is a cladding system hung on a structural frame, so purlin position, frame deflection and any sway of the structure all reach the sheets. Openings in the roof have to be co-ordinated with the frame rather than cut where they are convenient.
Read about Portal Frame →Rooflights in the sheeting module
Translucent panels and kerbed rooflights sit within the same profiled module, so the liner, the insulation and the outer sheet all stop at each opening. Each of those layers needs its own closure, and the liner's air control duty is the one most often forgotten.
Read about Rooflight Kerb →Eaves and gutter line
At the eaves both sheets terminate at different levels, the insulation zone has to be closed, and the gutter is often carried by the frame rather than by the roof. Getting the liner closed at this line is what stops warm air being drawn into the build-up here.
Read about Eaves System →Ducts, pipes and roof-mounted plant
Large single-storey roofs carry a great deal of plant, and every support and duct passes through both sheets and the insulation. The liner has to be resealed around each one, not simply cut, or the assembly's air control is undone item by item.
Read about Penetration Sealing →Support lines crossing the insulation zone
Every spacer and bracket forms a repeated crossing of the thermal layer, and the wall build-up meets this roof along the same lines. What that means for the assembly is analysed by a qualified professional and is not a property of any single component.
Read about Thermal Bridging Control →
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
- The dominant risk in this assembly is moist internal air reaching the cold upper sheet rather than rain getting past it. That depends on the continuity of the liner and its closures, on the internal conditions, and on assessment by a qualified professional.
- Thermal
- The thermal layer is crossed at every support line and compressed wherever the outer sheet is drawn down. The behaviour of the completed assembly is a whole-system question, and this entry states no value for it.
- Buildability
- Each layer is exposed to the trades that follow it. Sealing done early can be damaged before it is covered, insulation can be walked on, and the liner's finish is on show, so protection between operations is a genuine design consideration.
- Interfaces
- The wall cladding, the gutter, the plant supports and the rooflights all cross the same layers in different ways. Whether each of them closes the liner, the insulation and the outer sheet is where most of the assembly's real detail effort goes.
- Maintenance and access
- The roof will be walked for plant maintenance, and the outer sheet is not a floor. Where traffic is expected, how it is routed and what protects the sheet and the insulation beneath are questions to settle before the building is occupied.
- Durability
- Fasteners, laps and cut edges deteriorate on a different timetable from the sheets themselves. Exposure, run-off from adjacent materials and any accumulation of debris in the profile all bear on how the roof ages.
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.
- Which element is intended to be the air and vapour control layer, and how is it closed at every edge?
- How is the insulation retained at its intended loft where the spacers cross it?
- What sequence of trades is assumed, and what protects each completed layer until it is covered?
- How do the roof layers continue into the wall cladding at the eaves and the verge?
- Where will roof-mounted plant sit, and how is each support taken through both sheets?
- What foot traffic is expected on this roof after handover, and how is it accommodated?
- Are rooflight positions co-ordinated with the purlin layout and with the sheeting module?
Boundaries
Commonly misunderstood points
Distinctions that are easy to blur, and the places where a familiar term means something narrower than it sounds.
- Condensation inside the build-up is regularly reported as a roof leak. They arrive by different routes and are corrected in different layers.
- The liner is treated as a ceiling with no other duty. It is usually the air and vapour control element as well, and its laps are part of that.
- Insulation is assumed to perform simply because it is present. What matters here is whether it is continuous and uncompressed across the support lines.
- The outer sheet is assumed to be a working surface. It carries weather and wind, and its ability to carry people is a separate question for the designer.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- Which layer in this build-up is doing the air control work, and how has its continuity been drawn?
- How has the risk of condensation within the build-up been assessed for the internal conditions expected here?
- How is the insulation held clear of compression along the spacer lines?
- What is the fixing arrangement at the perimeter zones where uplift is highest?
- How will roof-mounted plant supports be taken through the assembly and resealed?
- What safe access is planned for cleaning gutters and maintaining plant on this roof?
What this page does not do
- Air tightness and watertightness belong to the completed assembly and its closures, not to the liner or the outer sheet considered separately.
- This entry states no thermal or fire performance. Both depend on the tested arrangement, the insulation and the whole build-up, and rest with the relevant authority.
- Whether this roof can be walked on, and where, is a determination for the designer and the manufacturer's documentation rather than a general assumption.
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 →