Who this guide is for
- Clients commissioning an industrial, agricultural or large single-storey building
- Owners investigating water appearing inside a metal-clad building
- Anyone planning rooflights, plant or ducts through a metal roof
- Readers comparing a built-up and a composite panel proposal
- Anyone responsible for maintaining a metal envelope
Where the duties sit in each arrangement
In a site-assembled build-up the duties are divided between separately fixed layers. 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 structure and at the same time governs how much room the insulation has. The record states it plainly: no single layer is the roof.
In a composite panel the same duties are bonded into one component, and the core also transfers shear between the faces so the panel spans between supports as one member. That is why damage to the bond is a structural matter as well as a thermal one, and why cutting a panel interrupts a spanning member rather than only a weathering layer.
The liner does the job nobody inspects
On a site-assembled envelope the liner is asked to do unrelated jobs, and only the visible one gets judged. 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.
The consequence is the defect this family is most known for. 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 somewhere unrelated before it appears. The wall record repeats the same warning — water appearing at an outer lap is commonly internal air that passed an unsealed liner, which is a defect in a layer nobody can see.
The spacer sets the insulation and crosses it
The insulation is only as thick as the spacer system lets it be. Every bracket forms a path from the outer sheet down to the support, so the thermal layer's continuity is a function of the spacers rather than of the boards or rolls, and pressing the quilt down so the outer sheet can sit tight defeats the layer at exactly those positions.
The records note that the two effects coincide rather than average out: the places where the insulation is thinnest are also the places where a conductive component crosses it. That is the built-up envelope's version of a question that every insulated assembly has.
On a panel envelope, the joint does everything at once
A composite panel's joint has to do in one geometry what a layered build-up does in separate places: shed water at the outer profile, close the core against a gap, and stop the air path at the liner face. A joint drawn open compromises all of those together rather than one at a time.
Because the panel is rigid and its joints have little tolerance, frame and support setting-out becomes a panel issue as much as a structural one. A panel forced onto an out-of-line support cannot be adjusted layer by layer and instead opens its joint with the panel beside it, and deflection under load reaches the joints because a panel cannot take up movement within its own thickness.
What can be sequenced, and what that is worth
A site-assembled envelope allows the building to be closed from below before the weathering skin is complete, so following work can begin. It also lets the insulation be changed, the spacer altered or a liner lap resealed as independent decisions.
The cost is that 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 composite panel closes the roof in a single operation and moves that variability into a factory, which is much of the reason the type exists.
- Built-up: layers adjustable individually, and each exposed to the trade that follows
- Panel: the field is repeatable because it was made in a factory
- Built-up: sealing of the liner decides airtightness, and it is covered immediately
- Panel: air control across the roof depends on the joints and closures, not the panel
Every interruption is where the difficulty went
On a panel envelope the record is direct: the field is simple precisely because all of the difficulty has been pushed into the joints and interfaces. Rooflights, ducts, ridge closures and the junction with the wall all substitute flashings, fillers and sealants for a bonded continuity, and each has to reproduce weathering, thermal continuity and air control simultaneously.
On a site-assembled envelope the equivalent is the number of closures a profiled sheet requires. A profiled sheet leaves a shaped opening wherever it stops, so every termination needs a closure made for that profile, and the record says it is the number of such closures, rather than the area of sheeting, that makes one of these walls complicated.
What a later alteration costs
A damaged composite panel cannot be repaired layer by layer, because the layers are bonded. Replacement usually means disturbing neighbouring panels, so access routes and the availability of matching panels are worth establishing early. Cutting a panel for new plant is also a structural question, because it interrupts a member that was spanning between supports.
A site-assembled envelope can be opened layer by layer, but the liner has to be resealed around every new penetration rather than simply cut, or the assembly's air control is undone item by item. Large single-storey roofs carry a great deal of plant, and every support and duct passes through both sheets and the insulation.
What neither envelope is
The sheeted wall record is careful to distinguish itself from a rainscreen. A rainscreen's premise is that the outer screen is not relied on to exclude water, with a drained ventilated cavity and a separate control layer behind it. Here the outer sheet is the weather line and the liner is the control layer, which reverses where the duties sit.
It is also not a precast panel facade, where the joints between large units do the weathering and the units are hung or restrained back to the structure. These sheets lap over one another, are fixed through to rails, and are continuous rather than jointed at intervals.
Questions for a metal envelope
- 1Establish whether the layers arrive separately or bonded into one component
- 2Ask which element is intended to be the air and vapour control layer
- 3Ask how that element is closed at side laps, end laps and every perimeter
- 4Ask who does that sealing and what inspection happens before it is covered
- 5Ask what depth is available for insulation and what holds it at that depth
- 6Ask what happens to the insulation where a spacer crosses it
- 7On a panel envelope, ask how each joint closes the outer face, the core and the liner face
- 8On a panel envelope, ask what setting-out tolerance the system requires from the frame
- 9List the closures and fillers the chosen profile needs at every termination
- 10Ask how roof-mounted plant supports pass through the assembly and are resealed
- 11Ask how the roof layers continue into the wall at the eaves and the verge
- 12Ask what foot traffic is expected after handover and how it is accommodated
- 13Ask what the plan is for replacing a damaged panel or sheet later
Common mistakes to avoid
- Reading condensation inside the build-up as a roof leak
- Treating the liner as a ceiling with no other duty
- Assuming insulation performs simply because it is present
- Pressing insulation down at the spacers so the outer sheet sits tight
- Assuming a panel roof has no detailing to worry about
- Cutting a panel and treating it as a weathering problem alone
- Assuming the bonded soffit makes the roof airtight by itself
- Treating the outer sheet as a working surface
When to involve a professional
- Ask which layer carries the air control duty and how its continuity has been drawn
- Ask how the risk of condensation within the build-up was assessed for the expected internal conditions
- Ask how the insulation is held clear of compression along the support lines
- Ask what fire requirements apply to this building and how the proposed arrangement addresses them
- Ask how the junction with the wall has been drawn for thermal continuity and air control
- Ask what safe access is planned for gutters, plant and later inspection
Frequently asked questions
Questions readers ask about this topic
How can I tell which kind of metal envelope a building has?
Looking up settles it in most cases: a site-assembled roof shows a liner with visible sealed laps and a spacer zone above it, while a panel roof shows a continuous bonded soffit. The underlying test is whether the liner could have been installed on its own with the outer sheet following later.
Why does water appear at a lap that is not leaking?
Because on these build-ups warm moist internal air that passes an unsealed liner reaches the cold outer sheet and condenses on it. What runs down the inside of that sheet and appears at a lap is commonly read as rain that has come in, when the defect is in a layer nobody can see.
Is a composite panel roof simpler to design?
The field of it is, because the variability has been moved into a factory. The record notes that the difficulty has been pushed into the joints and interfaces instead, and that every rooflight, duct, ridge closure and wall junction replaces a factory-made relationship with site-made ones.
Can plant be added to a metal roof later?
It can, but not casually. On a site-assembled roof the liner has to be resealed around each new penetration rather than simply cut. On a panel roof, cutting also interrupts a member that was spanning between supports, so the structural consequence is as real as the weathering one.
Is a sheeted metal wall a rainscreen?
No, and the record is explicit about the difference. A rainscreen's outer screen is not relied on to exclude water, with a drained ventilated cavity and a separate control layer behind it. On a sheeted wall the outer sheet is the weather line and the liner is the control layer, which reverses where the duties sit.
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