Multi-tiered canal tile corbelling is the core stereotomic masonry discipline required to build a stable Provençal génoise. By progressively cantilevering successive courses of curved terracotta canal tiles outward by 80 to 100 mm per course, masons construct cornices projecting up to 450 mm beyond the wall face. Stability relies on downward bedding angles (5 to 8 degrees), triangular cut terracotta inserts (claverettes), and flexible lime-sand mortars that absorb thermal expansion.
At a Glance: Key Takeaways
- Cantilever Progression: 80 to 100 mm outward projection per horizontal tier
- Bedding Angle: 5° to 8° downward pitch outward to ensure positive water drainage
- Infill Component: Claverettes (hand-cut triangular terracotta tile wedges filling void pockets)
- Mortar Specification: Natural Hydraulic Lime (NHL 2 / NHL 3.5) with coarse river quartz sand
The Statics of Tile Corbelling: Counterweight Equilibrium
Unlike a true stone vault that relies on mutual compression, a corbelled cornice functions on the law of levers. Every projecting canal tile acts as a cantilever beam, with the outer projecting horn seeking to pivot downward around the wall edge.
To maintain absolute equilibrium without mechanical ties, masons adhere strictly to the rule of thirds: at least two-thirds (66%) of the tile's total length (approx. 400 to 500 mm) must remain deeply embedded inside the solid masonry wall head, counterweighted by heavy limestone rubble. The outward cantilever is restricted to a maximum of one-third (33%), or roughly 80 to 100 mm per course.
Bedding Pitch: The 5-to-8 Degree Water-Shedding Slope
A fatal error in amateur génoise construction is laying the tile tiers dead level. If tiles are laid horizontally, wind-driven rain penetrates into the concave hollows, where standing water leaches lime mortar and freezes during winter cold snaps, popping tiles loose.
Master masons intentionally bed each tile with a downward forward slope of 5 to 8 degrees. This gentle outward tilt ensures that any rainwater striking the corbel is immediately flushed forward and shed over the lip of the next lower tier, keeping internal mortar beds permanently dry and sound.
The Claverette: Structural Wedge and Aesthetic Triangle
Because canal tiles are semi-cylindrical troughs, laying them side-by-side leaves large triangular gaps between their outer curves. Left unfilled, these voids would leave the subsequent tier unsupported, causing upper tiles to crack under dead weight.
Masons cut triangular wedges from scrap flat roof tiles or floor bricks, called claverettes. These triangles are bedded point-downward into the lime mortar between adjacent canal tiles. The claverettes act as rigid structural keystones that distribute the vertical weight of the next tier evenly while creating the iconic rhythmic sawtooth triangular relief that defines Provençal architecture.
Lime Mortar Plasticity: Absorbing Thermal Shock
In southern France, summer temperatures on terracotta roof surfaces regularly exceed 60°C under the blazing Mediterranean sun, dropping rapidly to 15°C at night. This extreme diurnal temperature swing causes substantial thermal expansion and contraction in terracotta tiles.
Using rigid Portland cement creates an unyielding bond that shears soft terracotta tile flanges. Instead, builders utilize natural hydraulic lime (chaux hydraulique naturelle NHL 2 or NHL 3.5) blended with slaked fat lime putty (CL-90) and washed quartz pit sand. This friable, micro-porous matrix possesses a low modulus of elasticity, absorbing thermal micro-movement without cracking.
Mortar Formulations for Multi-Tiered Canal Tile Corbelling
Use this reference table to calculate exact physical dimensions, ratios, and specifications before purchasing:
| Mortar Layer | Binder Formulation | Aggregate Grading | Mechanical Function |
|---|---|---|---|
| Base Wall Leveling Bed (Arase) | Pure NHL 3.5 hydraulic lime | Coarse river sand (0–4 mm) | High compressive load-bearing support on stone wall head |
| Tile Bedding Matrix (Corbel Core) | 50% NHL 2 + 50% CL-90 fat lime putty | Medium washed quartz sand (0–2 mm) | Plasticity, slow set, and high adhesion to wet terracotta |
| Claverette Pointing Grout | Pure CL-90 high-calcium lime putty | Fine marble dust + yellow ochre (0–1 mm) | Smooth, brilliant white-cream weather-shedding surface finish |
Practical Constraints
Every successful decoration project must account for physical and environmental constraints:
Dimensional irregularity in antique handmade salvaged canal tiles
Handmade 18th-century tiles vary in length by up to 30 mm and width by 20 mm, making modular stringline alignment challenging. Mitigation: Sort tiles on the ground into matched-length groups, reserving longer tiles for the lower tiers and slightly shorter tiles for upper courses.
Slow setting velocity of fat lime mortars in autumn dampness
In cold, damp weather, non-hydraulic lime mortar remains soft for days, limiting vertical erection speed to a single tier per 24 hours. Mitigation: Incorporate 10% to 15% pozzolanic calcined metakaolin or crushed brick dust into the lime putty, inducing hydraulic setting within 4 to 6 hours.
Execution Blueprint & Craftsmanship Sequence
Execute this professional sequence to achieve flawless results without backtracking:
Setting the Front Stringline Datum
Anchor brass guide pins into the corner quoin stones. Stretch a taut nylon stringline 100 mm proud of the wall face to establish the projection plane of the first tier.
Applying the Saturated Sottofondo Mortar Bed
Thoroughly wet the stone wall head. Trowel a stiff 30 mm bed of NHL 3.5 lime mortar sloped forward at 6 degrees toward the courtyard.
Bedding the First Tier of Canal Tiles
Press soaked canal tiles convex-side-down into the mortar bed at 250 mm centers. Adjust alignment until tile noses graze the stringline with an even 6-degree forward pitch.
Wedge Insertion with Terracotta Claverettes
Butter triangular cut claverette wedges with sticky lime-sand mortar. Press them point-downward between the curved tiles, packing mortar flush with the tile crowns.
Applying Backfill Masonry Counterweight
Pack limestone rubble and stiff mortar behind the embedded tail of the tiles, creating a solid 200 mm deep counterweight mass before attempting the second tier.
Erecting the Second and Third Stepped Tiers
Offset the stringline outward by an additional 90 mm. Lay the second tier of canal tiles centered over the claverette junctions below, repeating the process for subsequent tiers.
Tooling Joints with the Curved Pointing Iron
Tool all exposed lime mortar joints with a narrow pointing trowel, undercutting the bottom margins slightly to form crisp drip breaks.
Applying the Sacrificial Ochre Lime Wash
Allow mortar to firm for 48 hours. Brush a protective wash of slaked lime tinted with natural Roussillon yellow ochre over all mortar joints to unify color.
Common Mistakes & Countermeasures
Avoid these costly errors that compromise aesthetics, ergonomics, and budget:
Laying the next tile tier before backfilling the counterweight on the lower tier
Using dry, un-soaked terracotta tiles directly on lime mortar
Project Execution Checklist
Track your physical progress. Checkboxes persist automatically on this device:
Frequently Asked Questions
What is a 'claverette' and why is it essential?
A claverette is a flat, hand-cut triangular piece of terracotta tile (often cut from broken roof tiles or terracotta floor tiles). It is inserted point-downward into the triangular space between two adjacent curved canal tiles. It acts like a mini-keystone, distributing the weight of the next tier and creating a flat bearing surface for upper courses.
Can a génoise be built around curved or rounded wall corners?
Yes! Around rounded towers (tours d'angle) or bastide pigeonniers, skilled masons splay the canal tiles in a fan pattern (pose en éventail). The gaps between tiles at the outer rim widen, requiring custom-cut, wider claverettes to maintain smooth radial symmetry.
Why is natural hydraulic lime (NHL) preferred over quicklime for corbelling?
Pure slaked quicklime (CL-90) sets strictly by air carbonation, which takes weeks inside thick masonry joints. Natural hydraulic lime (such as NHL 2 or NHL 3.5) contains natural silicates that react with water, achieving firm initial compressive strength within 12 to 24 hours, allowing builders to erect multi-tiered cornices without structural sagging.
How do you calculate the total projection of a four-tiered génoise?
If each course steps outward by a standard 90 mm, a four-tiered génoise will project 360 mm (nearly 15 inches) beyond the wall face. Combined with the final roof starter tile overhang (approx. 80 mm), the total roof drip line projects 440 mm clear of the stone wall.
