Inward-sloping lapping voussoir joints and the outward shedding slope (pente chassante) form the ingenious weatherproofing system of Périgord bories. Without mortar, tar, or membranes, bories shed heavy Atlantic rains because every limestone slab in the roof vault is tilted slightly downward toward the exterior at a 5° to 10° angle. Overlaid with thin, overlapping limestone shingles (lauzes) in a fish-scale pattern, rainwater cascades outward and downward off the eaves, keeping the interior bone-dry.
At a Glance: Key Takeaways
- Weatherproofing Method: Pente chassante (deliberate 5°–10° downward outward stone slope)
- Roof Cladding: Lauzes calcaires (thin, flat split limestone slabs, 15–30 mm thick)
- Lapping Geometry: Concentric fish-scale overlapping with 50% to 60% stone exposure overlap
- Mortar Presence: 100% mortar-free drystone jointing; completely gravity-drained
- Eaves Detail: Projecting drip overhang (larmier) discharging water clear of the wall base
The Physics of Gravity Weatherproofing: Zero Mortar, Zero Leaks
In modern construction, weatherproofing relies heavily on chemical barriers: waterproof membranes, silicone sealants, asphalt shingles, and polymer-modified mortars. To the modern eye, the idea of a stone building remaining completely dry inside through violent rainstorms without a single smear of mortar or waterproof barrier seems miraculous. Yet the drystone bories of the Périgord achieve exactly this through pure adherence to the laws of gravity and surface tension.
Drystone weatherproofing does not attempt to create an impenetrable, hermetic skin that traps water; rather, it creates a permeable labyrinth where water is continually guided downward and outward. In the marine-influenced climate of southwest France, where intense autumn downpours are common, this gravity-driven drainage system ensures that moisture never accumulates inside the structural shell, preventing damp rot and frost destruction.
The Pente Chassante: Engineering the Outward Angle
The foundational secret of borie weatherproofing is the pente chassante (the outward shedding slope). During construction of the corbelled roof vault, a novice mason might instinctively lay each stone dead-flat or level. Doing so is fatal: water seeping into the unmortared horizontal joints would pool on the flat stone beds and eventually track inward toward the interior living space.
The master drystone builder introduces a deliberate, calculated slope. Every single stone slab is bedded with an outward, downward tilt of 5 to 10 degrees toward the exterior. To achieve this angle uniformly across hundreds of stones, the mason inserts small, wedge-shaped limestone chips (cales) beneath the rear tail of each slab. As a result, the inner edge of each stone sits 15 to 25 millimeters higher than its outer nose. When rainwater penetrates the surface joints, gravity immediately pulls the water forward and downward along the slope, discharging it onto the face of the stone below.
Lauzes: The Fish-Scale Limestone Shingle Envelope
While the inner structural vault is formed by corbelled limestone rubble, the exterior envelope is clad in a specialized stone medium: lauzes calcaires. Lauzes are thin, naturally cleaved slabs of dense, fine-grained Jurassic limestone, typically measuring 15 to 30 millimeters in thickness. Unlike porous chalky stones, prime lauzes are hard, resonant when tapped, and virtually impervious to water absorption.
Masons lay these lauzes in overlapping horizontal courses reminiscent of the scales of a fish. Every course overlaps the course beneath it by 50% to 60% of its length (the double-lap rule). Furthermore, the vertical joints between adjacent stones are strictly staggered: the solid body of each upper lauze covers the seam between the two stones beneath it. This double-lap, broken-joint geometry ensures that even during driving gales, water cannot find a continuous path through the stone courses into the interior.
The Larmier and Capstone Detailing
The weatherproofing system culminates in two crucial architectural details: the eaves and the apex. At the base of the roof vault, where the corbelling springs from the vertical wall, the mason installs the larmier (the drip course). Composed of extra-large, thick lauzes, the larmier projects 100 to 150 millimeters outward beyond the wall face, forming a continuous stone cornice. Rainwater running down the roof gathers at the larmier nose and drops straight to the ground, preventing water from running down the exterior wall face and eroding the foundation.
At the very summit of the borie, where the converging lauzes meet, the mason places the clef de voûte—a massive, flat monolithic limestone capstone. Carefully centered over the apical eye, this capstone overhangs the top lauze ring on all sides. The junction beneath the capstone is often bedded in dense, local red clay (terre grasse), which acts as a flexible, waterproof gasket that blocks wind-driven rain while allowing the stone to breathe. Through this unified system of angles, laps, and overhangs, the borie remains an impenetrable lithic umbrella.
Drystone Roof Drainage Performance: Slope Angles & Lauze Overlaps
Use this reference table to calculate exact physical dimensions, ratios, and specifications before purchasing:
| Outward Slope Angle | Lauze Overlap Ratio | Rain Shedding Efficiency | Wind Resistance | Risk of Slab Slippage |
|---|---|---|---|---|
| 0° to 3° (Near Flat) | 30% (Single lap) | Very Poor; internal leakage guaranteed | Poor (wind drives rain into seams) | Zero slippage risk; unstable roof |
| 5° to 10° (Optimal Borie) | 50% to 60% (Double lap) | Exceptional; 100% shedding via gravity | High (dense shingle interlocking) | Zero slippage; locked by friction & weight |
| 15° to 20° (Steep Pitch) | 65% (Heavy overlap) | Maximum water velocity | Moderate (vulnerable to high-wind shear) | High risk of drystone slabs sliding down |
| Vertical (Wall Batter) | N/A (Battered wall) | Immediate gravity runoff | Maximum | Zero (vertical compressive stacking) |
Practical Constraints
Every successful decoration project must account for physical and environmental constraints:
Severe Freeze-Thaw Thermal Cycling in the Périgord
Water trapped in microscopic stone pores expands by 9% upon freezing, shattering fragile limestone. Mitigation: Maintain unmortared joints so all water drains immediately; avoid any pockets where standing water can pool.
Moss and Lichen Accumulation on Shaded Slopes
Vegetation traps moisture against stone surfaces and forces roots into drystone joints. Mitigation: Periodically scrape moss from the roof using wooden spatulas to ensure unobstructed water runoff.
Execution Blueprint & Craftsmanship Sequence
Execute this professional sequence to achieve flawless results without backtracking:
Lauze Sorting and Graduation
Sort split limestone lauzes by size and weight, organizing them into piles for bottom eaves (large 400x500 mm slabs), middle vault (300x300 mm), and upper apex (200x200 mm).
Drip Edge Eaves Course (Larmier)
Lay the foundational eaves course of heavy lauzes atop the vertical wall, projecting them 100 mm outward and tilting them downward at 8 degrees to throw water clear of the wall base.
Shim Bedding for Pente Chassante
Insert thin, angular limestone shims (cales) under the rear tail of each stone, elevating the back by 15 mm relative to the nose to establish the permanent outward slope.
Second Course Staggered Laying
Lay the second course of lauzes, ensuring each stone overlaps the lower course by at least 50% of its length and spans the vertical joint between the two lower stones.
Backfill Ballasting
Immediately pack the rear cavity behind the lauzes with dry limestone rubble and spalls, consolidating the backfill to prevent any backward movement.
Progressive Stepped Shingling
Ascend the exterior of the vault course by course, maintaining the fish-scale overlap and outward angle, matching the progress of the internal corbelled rings.
Apex Ring Sealing
As the lauzes converge at the peak, bed the final uppermost ring in a thin layer of impervious local red clay (terre grasse) to prevent wind-driven rain infiltration.
Monolithic Capstone Overhang
Lower the final monolithic capstone (clef de voûte) onto the top ring, ensuring its perimeter overhangs the upper lauze course by at least 50 mm on all sides.
Common Mistakes & Countermeasures
Avoid these costly errors that compromise aesthetics, ergonomics, and budget:
Laying roof stones dead-flat (horizontal) or sloping slightly inward
Aligning vertical joints of adjacent lauze courses directly over one another
Using soft, marly limestone lauzes that absorb water easily
Project Execution Checklist
Track your physical progress. Checkboxes persist automatically on this device:
Frequently Asked Questions
How does water behave on an unmortared lauze roof during a downpour?
When rain falls on a properly constructed borie roof, water drops hit the outer face of an inclined lauze and flow downward via gravity. Reaching the lower edge of the stone, surface tension and the outward slope cause the droplet to fall onto the face of the underlying overlapping lauze. This cascading process repeats down the entire exterior of the dome until the water discharges from the projecting eaves course (larmier) onto the ground, never penetrating the internal wall cavity.
What is a 'pente chassante' in French drystone terminology?
The term 'pente chassante' literally translates to 'driving slope' or 'shedding slope'. In drystone masonry, it refers to the deliberate technique of laying wall and roof stones with a slight downward tilt toward the outside of the building (typically 5° to 10°). This intentional slope ensures that any moisture that enters the joints between stones is immediately driven by gravity outward toward the exterior, preventing internal dampness.
Can moss or lichen damage a drystone lauze roof?
While a light patina of lichen is harmless and provides historic character, heavy moss buildup can be detrimental. Moss absorbs and retains moisture like a sponge, keeping the limestone damp for extended periods and increasing the risk of frost spalling during winter freezes. Traditional farmers cleared moss every few years with stiff broom sweeps on dry summer days.
Why are lauzes graded from largest at the bottom to smallest at the top?
Grading lauzes serves both structural and hydraulic purposes. At the bottom of the roof, large, heavy lauzes provide a stable, wind-resistant eaves projection and handle the cumulative volume of water draining from the entire dome. Near the apex, the radius of the dome is small; smaller lauzes allow the mason to fit the stones tightly into the sharp curve without leaving large triangular gaps.
