Rolled titanium-zinc roofing relies on precise mechanical interlocking to achieve weather-tightness without rigid penetration. On pitches above 25°, standing seams (joint debout) are crimped in single or double folds over fixed and sliding clips. On pitches between 5° and 25°, the batten-cap system (système à tasseaux) uses tapered timber battens and interlocking zinc roll caps. Thermal movement of 2.2 mm/m/100°C is absorbed entirely through sliding clips and stepped transverse expansion laps.
At a Glance: Key Takeaways
- Standard Zinc Thickness: 0.80 mm (gauge 14) or 1.00 mm (gauge 16)
- Double Standing Seam Height: 25 mm finished height (upstand flanges at 35 mm & 45 mm)
- Batten (Tasseau) Dimensions: 40x50 mm trapezoidal spruce or Scots pine batten
- Clip Spacing & Typology: Fixed clips at high point; sliding clips every 330 mm
- Max Continuous Tray Length: 10 meters (longer runs require transverse step expansion laps)
- Minimum Roof Pitch: 5° for batten-cap systems; 15° for standing seam systems
Titanium-Zinc Metallurgy & Anisotropic Rolling Grain Direction
Architectural zinc is manufactured by continuous hot-rolling a micro-alloy of zinc with 0.08% titanium and 0.12% copper. The micro-alloying elements refine the crystalline grain structure: copper increases tensile strength (permitting tight 180-degree hem folds without cracking), while titanium raises the recrystallization temperature, conferring high resistance to permanent plastic deformation ('creep') under heavy winter snow accumulation.
However, rolled zinc possesses an anisotropic crystal grain structure running parallel to the direction of rolling. Master zinc smiths must account for rolling grain orientation during shop fabrication:
- Bending Parallel to Grain: Risk of micro-fracturing along grain boundaries; inside bend radius must be at least 1.5 times sheet thickness.
- Bending Perpendicular to Grain: Superior ductility; bends can achieve an inside radius equal to 1.0 times sheet thickness.
Furthermore, zinc must never be bent or profiled when the sheet temperature is below 10°C (50°F). At lower temperatures, zinc transitions from a ductile face-centered crystal structure to a brittle hexagonal lattice; attempting to fold cold zinc causes instantaneous edge snapping.
Standing Seam Mechanics: Double-Lock Profiles & Clip Geometry
The standing-seam system (joint debout) is the hallmark of fine French roofing. Adjacent zinc trays are fabricated with asymmetric upstand flanges: an under-flange measuring 35 mm high and an over-flange measuring 45 mm high. The trays are positioned side by side, secured with stainless-steel clips that fit over the 35 mm flange, and then folded together using seaming irons or specialized electric seaming rollers.
The joint is crimped in two sequential operations:
- Single Fold (Premier Pli): The 45 mm over-flange is bent 90 degrees down over the 35 mm under-flange and clip, creating an inverted L-profile seam. In vertical cladding applications, this single fold provides adequate water shedding.
- Double Lock (Double Pli): The inverted L is rotated and folded down a second time against the vertical tray body, creating a 25 mm high, water-tight double seam that completely encapsulates the fixing clip within the interlocked metal folds.
No solder, silicone, or chemical sealants are used; the capillary barrier formed by the 25 mm upstand height prevents wind-driven rain from breaching the roof deck.
The Batten-Cap (Système à Tasseaux) Method for Low Pitches
For shallow terrasson pitches (down to 5 degrees) or monumental château roofs where bold architectural shadow lines are desired, French zinc work specifies the système à tasseaux. The roof deck is fitted with trapezoidal wood battens measuring 40 mm wide at the base, 50 mm tall, and tapering inward to 35 mm at the top. The trapezoidal taper is critical: it prevents the zinc sheets from binding during thermal contraction.
Zinc trays are bent with 35 mm vertical upstands that rest snugly against the batten flanks. Zinc clips (pattes) nailed to the sides of the batten are folded over the tray flanges. Finally, a pre-profiled zinc roll cap (le couvre-joint) is slid or snapped over the batten. The lower edges of the cap are hemmed inward to interlock with the clips and tray flanges, creating an exceptionally robust joint that withstands heavy foot traffic, wind uplift, and ponding snowmelt.
Zinc Joinery System Selection Based on Roof Pitch and Exposure
Use this reference table to calculate exact physical dimensions, ratios, and specifications before purchasing:
| Roof Pitch Angle | Recommended Joinery | Tray Width | Clip Spacing | Transverse Expansion Detail |
|---|---|---|---|---|
| 5° to 15° (Shallow Terrasson) | Système à tasseaux (Batten cap) | 500 mm maximum | 330 mm along batten sides | Step expansion joint (ressaut) every 8 meters |
| 15° to 25° (Moderate Slope) | Double standing seam (Joint debout) | 500 mm or 600 mm | 400 mm along seam length | Double-lock welted seam with sliding clips |
| 25° to 60° (Steep Roof) | Double standing seam | 600 mm or 670 mm | 450 mm along seam length | Single-welt transverse lap joint |
| 60° to 90° (Vertical Brisis) | Single standing seam (Angled seam) | 500 mm | 500 mm along seam length | Simple 100 mm overlap with hemmed edge |
Practical Constraints
Every successful decoration project must account for physical and environmental constraints:
Acoustic Transmission from Rainfall on Metal Roofs
Heavy rain drumming on thin zinc can cause excessive ambient noise inside luxury mansard attic bedrooms. Solution: Install structured drainage underlayment membranes featuring acoustic dampening filament coils beneath zinc trays.
Condensation Attack on Underside of Low-Pitch Zinc
Warm interior moisture migrating into cold unventilated roof voids condenses on zinc backsides, causing white rust. Solution: Maintain a continuous, unimpeded 50 mm ventilated airflow cavity with continuous eaves intake and ridge exhaust vents.
Execution Blueprint & Craftsmanship Sequence
Execute this professional sequence to achieve flawless results without backtracking:
Sheet Cutting and Flange Profiling
Slit rolled titanium-zinc coils into specified tray widths. Run through a mechanical sheet brake or profiling roll former to create 35 mm under-flange and 45 mm over-flange.
Thermal Clip Calculation and Layout
Calculate tray expansion based on 2.2 mm/m/100°C. Install 3 to 5 fixed clips at the top third of the run; fasten sliding clips at 330 mm intervals along the remaining length.
Tray Alignment and Clip Securing
Lay the first zinc tray on the volige deck. Drive stainless-steel ring-shank nails through sliding clip holes. Hook clip tabs over the 35 mm under-flange.
Mating Adjacent Over-Flange
Position the adjacent tray so that its 45 mm over-flange covers the under-flange and clips of the first tray, leaving a 3 mm clearance gap at the base for cross-expansion.
First-Fold Seaming (Premier Pli)
Engage the upper seam with a handheld single-lock seaming iron or motorized runner, bending the 45 mm flange 90 degrees downward.
Double-Lock Final Crimping
Pass over the seam with double-action crimping pliers, compressing the joint into a 25 mm vertical double standing seam, checking for smooth, unwrinkled finish.
Common Mistakes & Countermeasures
Avoid these costly errors that compromise aesthetics, ergonomics, and budget:
Using Electro-Galvanized or Carbon Steel Nails for Clips
Installing Trays Longer than 10 Meters Without Step Expansion Joints
Crimping Seams Too Tightly with Mechanical Pliers
Project Execution Checklist
Track your physical progress. Checkboxes persist automatically on this device:
Frequently Asked Questions
What is 'oil canning' in a zinc roof and how is it prevented?
Oil canning is visible wavy distortion or buckling across the flat center of metal roof trays. It is caused by thermal expansion stress, uneven timber roof decking, or overtightened clips. To prevent oil canning, zinc smiths restrict tray widths to 500 mm, use thicker 0.80-1.00 mm gauge zinc, ensure the underlying timber volige deck is planed flat, and use sliding clips that move freely.
Why can't zinc sheets be soldered together to form a long roof run?
Zinc has a high thermal expansion rate. If long sheets are continuously soldered together, thermal expansion stress will concentrate at the soldered seam, tearing the softer zinc adjacent to the solder line. Roof trays must be allowed to slide independently within mechanically locked standing seams or batten caps.
What is 'white rust' on the underside of a zinc roof?
White rust (zinc hydroxide) occurs when stagnant condensation moisture contacts the underside of zinc in an unventilated roof space lacking carbon dioxide. Without CO2, the zinc cannot form its protective zinc carbonate patina; instead, it corrodes continuously into a powdery white residue that can perforate a metal roof within a few years. Proper 50 mm cross-ventilation prevents this completely.
