Screwing trapezoidal sheet metal: instructions, tips & easier installation

Trapezoidal sheet metal is an economical solution for roofs and walls - but the correct screw connection is decisive for tightness, stability and durability. Too few screws lead to leaks, the wrong types of screws cause corrosion and over-tightening damages the seals.

Fastening trapezoidal sheet metal is technically demanding - the work steps add up, especially with insulated constructions. This article shows you how to screw trapezoidal sheeting together properly and presents an alternative that significantly reduces the installation effort.

Why correct screw fastening is crucial for trapezoidal sheet metal

The screw connection fulfils three critical functions:

Load-bearing capacity: Screws secure the trapezoidal sheet against wind and snow loads. Too few fixing points lead to deflection and, in extreme cases, to the detachment of entire sheets.

Tightness: Every screw penetrates the sheet metal. Without the correct sealing discs, water will penetrate. The gaskets must be compressed precisely - too loose will result in leaks, too tight will damage the rubber coating.

Durability: Incorrect screws or improper installation cause corrosion. The problem: trapezoidal sheet metal is only 0.5-0.75 mm thin. Every installation error is visible later or leads to consequential damage.

Which screws for trapezoidal sheeting?

Only special trapezoidal sheet metal screws with integrated EPDM sealing discs are used for fastening. These are made of hardened, galvanised steel with a point or drill point. The sealing disc seals the drill hole watertight. Typical lengths: 4.8 x 35 mm (wood) to 6.3 x 120 mm (steel with insulation).

The substrate decides:

• Timber substructure: screws with point, at least 25 mm penetration depth
• Steel substructure: screws with drill point for profiles up to 3 mm
• For thicker steel profiles: Pre-drilling required

The choice of screw depends on the substructure. Never use standard wood screws without a sealing washer - every leaking screw is a leak.

Screw spacing for trapezoidal sheet metal

The number and spacing of the screws depends on the profile height, snow load, wind zone and building height. You can determine the required snow load for your project.

Typical screw spacing (examples only, a static screw plan is mandatory)

Profile height	Purlin spacing	Screws per sheet (6 m)	Screw spacing
20-35 mm	1,0-1,5 m	8-12	50-75 cm
35-60 mm	1,5-2,5 m	6-10	60-100 cm
60-100 mm	2,0-3,0 m	5-8	75-120 cm

Rule of thumb: roof 4-7 screws per m², wall 3-5 screws per m². You will also need screws for longitudinal joints (every 30-50 cm), overlaps (every wave) and edge areas (closer spacing). Order 10-15 % in reserve.

Screwing trapezoidal sheet metal: step-by-step

Preparation

Tools: Cordless screwdriver with torque setting, suitable bit, spirit level, straightedge, jigsaw for cutting to size.

Check the substructure: Purlins aligned, spacing correct, dry for timber (< 20 % moisture), rust protection present for steel.

Prepare material: trapezoidal sheets, screws with reserve, sealing tapes, fall protection.

Roof screw connection

Step 1 - Align the first sheet: Start against the main wind direction, align parallel to the eaves with a guide line, overhang 30-50 mm.

Step 2 - First screws: First screw in the centre raised bead near the eaves. Screw perpendicular to the purlin (not to the sheet metal). Sealing disc visibly compressed, but sheet metal not deformed.

Step 3 - Further screws: Work from the centre outwards. Always screw into the deep bead (bearing surface), never into the high bead. Observe distances according to structural analysis. Screw each purlin.

Step 4 - Overlap: Second sheet over the first (against the weather side), at least one wave overlap. Sealing tape between sheets. Screw the overlap to each corrugation (30-50 cm apart).

Step 5 - Longitudinal joints: Overlap 20-30 cm, screw sealing tape over the entire surface, transverse to the corrugation. Stagger the joints (chequerboard pattern).

Wall screwing

Similar principles to roof, but more generous spacing (lower loads). Vertical or horizontal installation possible. Plinth area and corner areas: narrower spacing due to higher wind load. The requirements for sandwich panels for walls are identical - correct screwing is crucial.

Common mistakes when screwing trapezoidal sheet metal

Too few fixing points

The most common mistake: "The sheet is on top, it will hold." The consequences: Sheet metal lifts in the wind, deflection with water pockets, in extreme cases complete detachment. This is usually caused by cost savings on screws - completely out of place. Screws cost cent, storm damage thousands.

Screws too tight

"Tight is good, tighter is better" - not with trapezoidal sheet metal. The consequences: Sealing disc loses elasticity, rubber swells out at the sides (UV damage), sheet metal deforms locally. Solution: Cordless screwdriver with torque limiter (3-5 Nm). Check: Sealing disc visibly compressed, but sheet metal not deformed.

Incorrect screw types

Standard screws without seals lead to leakage at each screw and corrosion. Solution: Only use trapezoidal sheet metal screws with EPDM sealing washer (15-25 cent per piece).

Missing pilot holes in steel

Self-drilling screws fail with steel profiles > 2.5 mm. Consequences: Screw does not drill all the way through, unclean hole, no secure fastening. Solution: Pre-drill (3-4 mm), deburr, then insert the screw.

Complexity with insulated trapezoidal sheet metal constructions

Multi-layered installation increases the risk of errors

For uninsulated halls, trapezoidal sheet metal is simple: fit the purlins, screw the sheet metal together and you're done. With insulated buildings, it becomes more complex:

Layers in an insulated roof construction:

1. Main construction
2. Insulation
3. Glue vapour barrier airtight
4. Counter battens (40-60 mm rear ventilation)
5. Support battens (purlins)
6. Screw trapezoidal sheet metal
7. End sheets

Seven layers instead of two. Each layer is a work step and a source of error. Typical problems: Vapour barrier leaking → Moisture in insulation → Mould. Counter battens penetrate vapour barrier → Thermal bridges. Several trades required → Coordination effort. In addition to thermal insulation, sound insulation also plays a role with trapezoidal sheet metal - another aspect where multi-layer systems reach their limits.

Vapour barrier and screw connection in conflict

The basic problem is that the screw connection penetrates all layers - including the vapour barrier. This must be airtight (DIN 4108-7), but every penetration is a potential leak. Attempted solutions (sealing sleeves, special bonding) are time-consuming and never perfect. Many trapezoidal sheet metal constructions fail blower door tests.

Simpler alternative: sandwich panel installation

Integrated insulation reduces the amount of screwing required

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

Structure of sandwich panels: Outer shell (galvanised sheet steel 0.4-0.6 mm), insulation core (PU or PIR, 40-200 mm), inner shell (most commonly sheet steel). The insulation core is glued at the factory. Separate vapour barrier is not required - inner shell is vapour-tight. Rear ventilation unnecessary - closed construction prevents condensation.

Assembly:

1. Create the main structure
2. Install purlins
3. Screw sandwich panels directly together
4. Finished

Three work steps instead of seven. No vapour barrier, no counter battens, no supporting battens. Fewer screws required (insulation core reinforced), no penetration of separate vapour barriers. Further details can be found in our comprehensive guide to sandwich panels.

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

Criterion	Trapezoidal sheet (insulated)	Sandwich panels
Work steps	7 layers	3 steps
Trades	2-3	1
Vapour barrier	Separate installation	Integrated
Rear ventilation	Required	Not applicable
Thermal bridges	Unavoidable	Minimised
Screws per m²	5-8	3-5
Installation time	Several weeks	A few days
Sources of error	Many interfaces	Complete system
Airtightness	Difficult	Tight at the factory

The table shows that sandwich panels are not only quicker to install, but also less prone to errors. The higher material costs (approx. 30-50 % surcharge) are amortised by drastically reduced labour costs.

For whom are sandwich panels worthwhile?

• Heated halls and workshops
• Projects with time pressure
• Builders without specialist knowledge
• Buildings with blower door requirements
• Projects in noisy areas

When does trapezoidal sheeting make sense?

• Unheated halls without insulation
• Very tight budgets
• Temporary buildings

The decision depends on the requirements. Sandwich panels are the more efficient solution for insulated buildings with quality requirements. Successful reference projects demonstrate this in practice.

Conclusion: Plan the screw connection correctly from the outset

Screwing trapezoidal sheet metal is technically demanding. Correct screw types, exact spacing, precise torque and clean execution are crucial. Frequent mistakes - too few or too many screws, incorrect torque, missing sealing tapes - lead to leaks and storm damage.

Trapezoidal sheeting remains economical for uninsulated constructions. However, as soon as insulation is required, things become complex. Seven work steps instead of two, several trades, complex vapour barrier bonding and error-prone rear ventilation make installation time-consuming. The screw connection penetrates all layers - every penetration is a weak point.

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. Screwing is simpler - fewer screws, no penetration of separate vapour barriers, factory quality.

Our recommendation: If you want to reduce time, costs and complexity, you should consider sandwich panels right from the start. The higher material costs are amortised by drastically reduced labour costs and better building quality. Let us advise you individually on the best solution for your project.

Last updated Nov 2025