The exterior building envelope of a Bornholm half-timbered cottage balances thermal mass, vapor breathability, and weather shedding. Infill wall bays are constructed using flexible coppiced hazel withies woven around cleft oak studs, densely packed with an artisanal daub (klining) of glacial blue clay, sharp beach sand, cow dung, and chopped winter rye straw. Overhanging steep thatched roofs of hand-harvested winter rye straw (stråtækt tag) shed Baltic downpours and insulate interiors during freezing sub-zero winters.
At a Glance: Key Takeaways
- Daub Formula (Klining): 1 Part Glacial Blue Clay : 2 Parts Sharp Sand : 0.5 Part Dung + Rye Straw
- Roof Eave Projection: 400mm to 600mm deep overhang to shield clay bays
- Thatched Thatch Material: Long winter rye straw (Secale cereale), minimum 1.2m length
- Wattle Material: Coppiced Hazel (Corylus avellana) or Willow Withies
Hazel Wattle Hurdle Weaving (Pæle-Vidjer) in Timber Bays
In Bornholm half-timbering, infill bays (tavle) are not structural; they function as flexible thermal curtain screens. To anchor the infill securely without nails, carpenters chisel continuous triangular grooves along the inside faces of the vertical oak posts and horizontal sills.
Vertical split oak laths (stagepæle) are cut slightly oversized, sprung into the top and bottom grooves, and wedged tightly. Master weavers then interweave supple, green coppiced hazel or willow stems (vidjer) in an alternating basket-weave pattern. This creates a flexible, spring-like wooden mesh that mechanically interlocks with the clay daub while flexing with the timber frame during seasonal humidity changes.
The Klining Recipe: Glacial Blue Clay, Sand, Dung, and Rye Straw
Bornholm’s glacial moraine deposits yield dense, calcareous blue-gray clay rich in illite and smectite minerals. If used pure, this fat clay shrinks by over 15%, causing devastating drying cracks. The traditional formula (klining) controls shrinkage through careful aggregate balancing.
The clay is kneaded with two parts sharp quartz beach sand, clean fresh cow dung (which contributes natural enzymes, micro-fibers, and casein proteins that impart water resistance and flexibility), and chopped winter rye straw (30mm to 50mm lengths). The mixture is trodden by feet or horses until perfectly homogeneous, then cured in a damp pit for 48 hours to activate natural clay plasticity.
Two-Sided Packing Technique and Surface Keying
Applying daub is a coordinated two-person operation. One artisan stands inside the cottage while another stands outside. Working simultaneously, they press thick handfuls of damp daub into the hazel mesh from both sides at once, compressing the material firmly until the clay masses weld together around every hazel withy.
The daub is brought flush with the timber frame faces. Before the surface sets, the exterior face is lightly scored with diagonal fingernail or stick marks to provide mechanical grip for subsequent finish coats. The wall is allowed to dry slowly in shade for 3 to 4 weeks, with shrinkage fissures packed with fine clay-sand slurry as they appear.
Rye Straw Thatching (Stråtækt Tag) and Deep Overhanging Eaves
Clay daub walls can survive Baltic rain only if protected from direct water impact. This protection is provided by the thatched roof (stråtækt tag). Bornholm thatchers demand unbroken winter rye straw (rugstrå), harvested by hand scythe or vintage binder-reapers to keep hollow stems uncrushed.
Straw is bundled into tight sheaves (neg) and stitched to split fir roof laths using tarred hemp cords. The thatch is built to a dense 300mm thickness and dressed with a wooden thatching paddle (tækkefjæl). Extending 500mm beyond the wall line, the deep eaves cast driving rains clear of the clay panels, keeping the wall base completely dry.
Wattle, Daub, and Thatch Formulation & Dimension Matrix
Use this reference table to calculate exact physical dimensions, ratios, and specifications before purchasing:
| Building Element | Material Formulation | Nominal Thickness | Installation Method | Thermal / Breathability Metric |
|---|---|---|---|---|
| Wattle Hurdle | Coppiced hazel (15–20mm dia) | 40mm thick mesh | Woven into grooved oak posts | High structural flexibility |
| Base Daub Body (Klining) | 1 Clay : 2 Sand : 0.5 Dung + Straw | 120mm – 150mm | Simultaneous two-sided packing | Sd < 0.05m (ultra-breathable) |
| Finish Clay Wash | Fine blue clay + chalk + horsehair | 8mm – 10mm | Hand-troweled & timber-floated | Crack-free smooth substrate |
| Rye Straw Thatch | Long uncrushed winter rye straw | 280mm – 320mm | Tarred cord tie-down to battens | R-value ~4.2 m²K/W (Thermal roof) |
Practical Constraints
Every successful decoration project must account for physical and environmental constraints:
Fire Regulations for Historic Thatched Roofs
Requires modern fire-retardant barrier membranes installed beneath the thatch and strict clearance zones from smokehouse chimneys.
Drying Climates and Winter Frost Windows
Clay daub must be applied between May and August to ensure thorough drying before the arrival of autumn rains and sub-zero winter frosts.
Execution Blueprint & Craftsmanship Sequence
Execute this professional sequence to achieve flawless results without backtracking:
Grooving Posts and Lath Installation
Chisel 15mm triangular grooves into oak frame posts; cut split oak upright laths and spring into place at 150mm centers.
Hazel Hurdle Basket Weaving
Weave flexible green hazel withies horizontally in an alternating basket weave around the oak laths, packing tightly downwards with a wooden mallet.
Daub Compounding and Curing
Mix glacial blue clay, sharp pit sand, fresh cow dung, and chopped rye straw with water; tread thoroughly and let sweat for 48 hours.
Simultaneous Two-Sided Daub Packing
Pack daub into the hazel mesh from interior and exterior sides simultaneously, compressing firmly to ensure 100% core consolidation around twigs.
Surface Floating and Slow Curing
Level daub flush with timber faces; shield from direct sun with damp burlap, lightly misting daily for 21 days to prevent rapid shrinkage cracking.
Rye Straw Thatch Application
Lay winter rye sheaves over roof laths; tie down with tarred hemp twine, beating upward with a thatching paddle to create a 300mm weather-tight coat.
Common Mistakes & Countermeasures
Avoid these costly errors that compromise aesthetics, ergonomics, and budget:
Applying Daub from One Side Only
Using Combine-Harvester Straw for Thatching
Project Execution Checklist
Track your physical progress. Checkboxes persist automatically on this device:
Frequently Asked Questions
Does cow dung make the clay daub smell bad?
No. Once the clay daub dries and carbonation occurs, all organic odor vanishes completely within 48 hours, leaving an earthy, clean scent similar to sun-dried clay.
How often does a rye straw thatched roof need to be replaced on Bornholm?
A well-thatched rye straw roof pitched at 50 degrees lasts 40 to 50 years on the main roof slopes. The ridge (mønning), which takes the brunt of Baltic gales, is typically renewed every 6 to 8 years with fresh turf or rye straw.
