Crannog

A crannog is a partly or entirely artificial island built in a lake, loch, bog or estuary, found mainly in Scotland and Ireland. Unlike the pile dwellings of the Alps, which were built on lake shores and later flooded, crannogs were built out in the water. Most supported one or more roundhouses, so the typical crannog roof is the conical thatched roof of the British and Irish roundhouse.

Dates and numbers

Crannogs were used over more than five millennia, from the Neolithic to the 17th or early 18th century. Eilean Dòmhnuill on North Uist, with radiocarbon dates of 3650-2500 BCE, is the earliest known constructed crannog. In Ireland they appear in Middle Bronze Age layers at Ballinderry (1200-600 BCE) and were most common in the Early Historic period. In Scotland, construction peaked between about 800 BCE and 200 CE; Oakbank on Loch Tay belongs to the Late Bronze Age-Early Iron Age. Ireland has an estimated 1,200 crannogs; Scotland has 389 officially listed sites, though estimates range from about 350 to 500 depending on definition.

Construction

Methods depended on local materials. Where timber was plentiful, the island was often ringed by sharpened piles, sometimes joined by mortise and tenon, and built up with clay, peat, stone, timber and brush. In treeless areas such as the Outer Hebrides, builders used stone. Causeways or logboats gave access. Surviving crannogs are typically 8-25 m (26-82 ft) across.

Roofs

Waterlogging preserves timbers, piles and floor deposits far better than dry sites, but roof coverings rarely survive. At Oakbank, bracken, rushes or straw may have been used for thatch. The reconstruction built by the Scottish Crannog Centre near Kenmore on Loch Tay, following experimental work led by Nicholas Dixon and Barrie Andrian in 1994-1997, has a conical roof of water reed from the River Tay fixed with ties and hazel spars over rafters, ring beams and hazel purlins. A steep pitch was considered crucial so that snow and rain would not lie on the roof.

The Loch Tay reconstruction was seriously damaged by fire in June 2021. A replacement crannog, built with traditional skills including thatching and hazel coppicing, was unveiled on 22 May 2026.

Significance for roofing history

Crannogs show roundhouse roofs adapted to an exposed, wet setting, and their waterlogged deposits are among the best sources for the timber frames beneath prehistoric thatch. The 2021 fire also illustrated a long-standing hazard of thatched roofs on open hearths.