Blockbau corner detail showing the characteristic horizontal log stack with interlocking notches. Wikimedia Commons, CC licence.
Blockbau — literally "block construction" — is a method of horizontal log stacking in which structural walls are formed by laying square-hewn or round logs one above the other, each layer locked to the one below at the corners by a cut notch. The technique reached the Italian Alpine provinces through Germanic migration during the medieval period and remained the dominant structural system for rural buildings in South Tyrol and parts of the Trentino until the late nineteenth century.
Corner Notching: The Central Technical Problem
The corner joint carries the full lateral stability of a Blockbau structure. Unlike timber-frame buildings in which diagonal bracing resists racking forces, a Blockbau wall relies on the friction and interlock of the notch itself. Three principal notch families survived in the Italian Alpine inventory.
The Saddle Notch
The saddle notch is a concave cut made into the underside of each log, shaped to accept the curved top of the log below. In well-preserved structures around Sesto (Sexten) and Dobbiaco (Toblach), saddle notches extend 30–40 mm deep into logs of 20–28 cm diameter. Builders used a scribing tool — a simple compass-like gauge — to transfer the exact profile of the lower log onto the upper before cutting. This full-scribe method leaves a gap of less than 3 mm when the building is assembled dry, tightening further under compressive settlement over the first decade.
According to fieldwork records published by the Istituto Universitario di Architettura di Venezia, more than 60 percent of surveyed pre-1800 Blockbau structures in the Puster Valley used full-scribe saddle notching on primary load-bearing walls.
The V-Notch
The V-notch is a simpler triangular cut placed at both the upper and lower surfaces of each log at its end. Two opposing V-shapes interlock when the logs cross. This method requires less scribing skill and can be executed with a hatchet and chisel without a gauge. The trade-off is a reduced bearing surface and a tendency to collect water in the upward-facing notch, which accelerates fungal degradation unless the corner overhang is sufficient to shed rain.
V-notched buildings appear predominantly in secondary agricultural structures — granaries, hay barns, and tool stores — where dimensional precision was less critical and speed of construction was prioritised.
The Dovetail Notch
The dovetail notch adds a trapezoidal profile to the corner interlock, preventing vertical separation of the logs without relying solely on gravity and compression. It is the most labour-intensive of the three systems and appears in residential buildings of higher status — manor farms, rectories, and buildings associated with monastic land holdings. Several surviving examples near Brunico (Bruneck) date to the seventeenth century and retain their original dovetail corners without any subsequent iron reinforcement.
Blockbau farmhouse with characteristic shingled exterior cladding, Bezau, Vorarlberg — a closely related regional variant of the South Tyrolean tradition. Wikimedia Commons, CC licence.
Log Preparation and Seasoning
Raw logs were typically felled between November and February, when sap content is lowest and the risk of checking (longitudinal cracking during drying) is reduced. Builders preferred logs with a straight grain and minimal spiral, selecting Norway spruce (Picea abies) and European larch (Larix decidua) for their workability and resistance to surface mould.
Bark removal was performed immediately after felling, followed by a seasoning period of one to three years in open-sided log stacks. Uneven seasoning — common when logs were rushed into construction — produced longitudinal checks that were traditionally packed with moss or hemp fibre, a technique documented in the technical literature of the Consiglio Nazionale delle Ricerche as providing measurable draught reduction in field tests.
Wall Sealing and Chinking
Even with precise saddle notching, the flat mid-section of each log does not bear continuously against the one below. The gap — typically 5–20 mm for well-fitted walls — was chinked with materials available locally. Across South Tyrol, the most common historic chinking materials were:
- Dried sphagnum moss, compressed into a gasket-like bead along the log bearing line
- Hemp fibre twisted into rope and driven into wider gaps with a blunt chisel
- Clay-lime mixtures applied as a wet paste and allowed to cure in place, visible in section in many restoration projects
- Riven wood strips — thin larch shims wedged into the gap to maintain a consistent bearing line
Modern conservation practice, outlined in guidance from the Autonomous Province of Bolzano, now requires that chinking replacements in listed structures match the original material type where analysis can determine it, typically by sampling the remaining residue under magnification before cleaning.
Settlement and Long-Term Behaviour
A freshly constructed Blockbau wall settles vertically as the logs compress, moisture equalises, and the notch surfaces bed in. Settlement rates of 15–25 mm per metre of wall height were recorded in monitored test structures at the University of Trento's timber research programme during the first twelve months after construction. This has implications for door and window frames: traditional builders fitted vertical "compression posts" — slotted upright timbers that allow the log wall to slide past the frame without transmitting load onto the lintel. The slot dimension was calculated at approximately 1/100 of the wall height above the opening.
Buildings that omit this allowance develop cracked lintels within five to ten years, and this failure pattern is visible in hastily built mid-twentieth-century reconstructions of traditional forms in several Dolomite valley municipalities.