Trapezoidal sheet substructure: structure, costs & simpler alternatives

The substructure for trapezoidal sheet metal is the supporting framework to which the sheet metal cladding is attached. A carefully planned and executed substructure is crucial for stability, tightness and durability. However, the structure is complex - especially with insulated roofs and walls, the work steps add up.

This article explains the structure of trapezoidal sheet substructures for roofs and walls, highlights cost factors and presents sandwich panels as an alternative that significantly reduces the installation effort.

What is a trapezoidal sheet substructure?

The substructure is the load-bearing system that lies between the main structure (roof truss, steel frame, wall studs) and the trapezoidal sheet metal. It fulfils several functions:

Load-bearing function: The substructure absorbs the weight of the trapezoidal sheet and additional loads (snow, wind, maintenance personnel) and transfers them to the main structure.

Fixing level: The trapezoidal sheet is screwed directly to the substructure. The purlins (horizontal or vertical profiles) form the support surface.

Rear ventilation level: In the case of insulated constructions, the substructure creates the necessary distance between the insulation and the trapezoidal sheet for rear ventilation.

Materials: Substructures are typically made of wood (squared timber, square timber), steel (C or Z profiles) or, more rarely, concrete (for solid constructions). The choice depends on the span, loads and type of building.

Substructure for trapezoidal sheet roof

Purlins and purlin spacing

Purlins are the horizontal profiles on which the trapezoidal sheeting rests. They run at right angles to the roof pitch and parallel to the eaves.

Purlin spacing: The spacing between the purlins depends on several factors:

• Profile height of the trapezoidal sheet (the higher, the greater the spacing possible)
• Snow load of the region
• Wind load
• Material thickness of the sheet metal

Spacing that is too large leads to deflection and leaks. Spacings that are too small increase material and labour costs unnecessarily.

Statics and load-bearing capacity

The static calculation of the substructure is complex and should be carried out by specialists:

Load assumptions:

• Dead weight of the trapezoidal sheet (low, approx. 4-10 kg/m²)
• Snow load (varies from region to region, 75-400 kg/m²)
• Wind load (depending on height and exposure)
• Traffic load (maintenance personnel on the roof)

The purlins must absorb these loads and must not deflect too much. You can calculate the required snow load for your project in order to select the correct dimensioning.

Dimensioning: Different cross-sections are used depending on the span width and loads:

• Timber purlins: 60x80 mm to 100x120 mm
• Steel purlins: C or Z profiles 100-200 mm high

Work steps for roof construction

The construction of an uninsulated trapezoidal sheet metal roof is simple:

1. Create the main structure (roof truss, steel trusses)
2. Fasten purlins to the main structure
3. Lay trapezoidal sheet metal on purlins and screw together
4. Install end sheets (eaves, verge, ridge)

With an insulated trapezoidal sheet metal roof, it becomes much more complex:

1. Create the main structure
2. Insertinsulation between or on the beams
3. Glue thevapour barrier airtight
4. Installcounter battens for rear ventilation (40-60 mm)
5. Installsupport battens (purlins) for trapezoidal sheet metal
6. Lay and screw down trapezoidal sheet metal
7. Install end sheets

The insulated version requires four additional work steps and correspondingly more time, material and skilled labour. Each layer is a potential source of error.

Substructure for trapezoidal sheet walls

The wall substructure follows similar principles to the roof substructure, with a few differences:

Vertical or horizontal installation: wall purlins can be installed vertically (for horizontal sheet installation) or horizontally (for vertical sheet installation). Vertical installation is more common.

Lower loads: Walls do not have to bear snow loads, only wind loads and dead weight. The purlins can therefore be smaller.

Insulated wall construction: Here too, the layers add up:

1. Main construction (columns, beams)
2. Insulation between the supports
3. Vapour barrier
4. Counter battens for rear ventilation
5. Wall purlins
6. Trapezoidal sheet metal

The effort involved is comparable to insulated roofs - many work steps, many sources of error.

Costs of the trapezoidal sheet substructure

The costs for the substructure are made up of material and labour. Specific prices vary greatly from region to region, so only qualitative factors are listed here:

Material costs:

• Purlins (wood or steel)
• Fastening material (screws, brackets, connectors)
• For insulated constructions: Insulation, vapour barrier, counter battens, support battens

Labour costs:

• Planning and static calculation
• Installation of purlins (relatively simple for non-insulated constructions)
• For insulated constructions: Installing the insulation, bonding the vapour barrier, installing several batten levels

Time required: An uninsulated substructure can be installed in just a few days. An insulated construction with a vapour barrier and rear ventilation takes considerably longer - weeks, depending on the size of the building.

Cost factor complexity: The more layers, the higher the costs. The insulated trapezoidal sheet metal construction is more expensive than a simple sheet metal shell - but still cheaper than a solid construction.

The catch: the sum of material, labour time and risk of errors makes the multi-layer construction expensive. Simpler systems can save time and money here.

Complexity and expense of multi-layer construction

Many work steps for insulated roofs

The biggest challenge with trapezoidal sheet substructures: The work steps add up for insulated buildings:

7 layers instead of 2: While an uninsulated roof only requires purlins and trapezoidal sheet metal, an insulated roof requires insulation, vapour barrier, counter battens, support battens and trapezoidal sheet metal. Five additional layers mean five times more work.

Coordination of several trades: carpenters for the timber construction, roofers for the vapour barrier and trapezoidal sheeting, possibly steelworkers for metal substructures. Coordination is time-consuming.

Weather dependency: Every open construction phase (e.g. between the vapour barrier and trapezoidal sheet metal) is dependent on the weather. Rain stops the work, moisture in the insulation leads to structural damage.

Time required: What can be completed in days with sandwich panels takes weeks with multi-layer trapezoidal sheet metal constructions.

Sources of error and challenges

Every layer is a potential source of error:

Vapour barrier: airtight bonding at all connections, penetrations and joints is critical. Every leak leads to moisture ingress and structural damage.

Rear ventilation: Insufficient clearances, blocked ventilation cross-sections or a lack of air circulation lead to condensation and mould.

Thermal bridges: Fixing points of the counter battens penetrate the insulation. Thermal bridges are unavoidable and reduce the insulating effect.

Installation sequence: The correct sequence of the layers is essential. Errors are difficult to correct retrospectively.

The complexity of the trapezoidal sheet construction makes it susceptible to execution errors - especially for inexperienced tradesmen.

Simpler alternative: sandwich panel installation

Sandwich panels for roofs and sandwich panels for walls simplify installation considerably, as the insulation, vapour barrier and cladding are already integrated.

Simplified substructure

The substructure for sandwich panels is much simpler:

Only purlins required: sandwich panels are mounted directly on purlins - no counter battens, no supporting battens, no separate insulation layer.

Work steps:

1. Create the main structure
2. Install purlins
3. Lay sandwich panels directly on purlins and screw them in place (for timber, sealing tapes must be laid for steel)
4. Finished

Three work steps instead of seven. No vapour barrier to be glued, no rear ventilation to be planned, no multiple batten levels.

Purlin spacing: The spacing of the purlins must be determined on site by a structural engineer depending on the type of goods and conditions.

Weather resistance: Sandwich panels can also be installed in light rain. The closed construction is immediately watertight.

Comparison: Installation effort for trapezoidal sheet metal vs. sandwich panels

Criterion	Trapezoidal sheet metal (insulated)	Sandwich panels
Work steps	7 layers	3 steps
Trades	2-3 (carpenter, roofer, possibly steelworker)	1 (fitter)
Vapour barrier	Separate installation and bonding	Integrated
Rear ventilation	Planning and execution required	Not applicable
Thermal bridges	Unavoidable due to battens	Minimised
Installation time	Several weeks	Few days
Sources of error	Many interfaces	Complete system
Weather dependency	High	Low
Coordination effort	High	Low

Simplified assembly not only saves time, but also costs. Fewer work steps mean fewer labour hours, lower material costs for battens and less coordination effort. You can find more details on installation in our detailed guide to sandwich panels.

Conclusion: Plan efficient installation right from the start

The substructure for trapezoidal sheet metal is simple for uninsulated structures: fit the purlins, screw the sheet metal together and you're done. This remains an economical solution for simple warehouses, open halls and unheated buildings.

However, as soon as insulation comes into play, things get complex. Seven layers instead of two, several trades, complex vapour barrier bonding and error-prone rear ventilation make installation time-consuming and expensive. Each layer is a potential source of error.

Sandwich panels radically simplify installation. Three work steps instead of seven, no separate insulation, no vapour barrier, no rear ventilation. The integrated construction method significantly reduces installation time, coordination effort and the risk of errors. The higher material costs are amortised by drastically reduced labour costs.

Our recommendation: If you want to reduce time, costs and complexity, you should opt for sandwich panels right from the start. Let us advise you individually on which substructure is best for your project. Successful reference projects show efficient installations in practice.

Last updated Nov 2025