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Seaweed Houses and Sea-salt Huts, Laesoe Island

Date de soumission : 06/09/2023
Critères: (ii)(v)
Catégorie : Culturel
Soumis par :
The Danish Agency for Culture and Palaces
État, province ou région :
Northern Jutland
Ref.: 6692

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Les noms des biens figurent dans la langue dans laquelle les États parties les ont soumis.


Seaweed houses:
1. Museumsgården, Museumsvej 3. N57 15 47 – E11 01 57
2. Hedvigs Hus, Linievejen 36. N57 18 27 – E11 07 32

Sea-salt production cultural landscape:
3. Rønnerne (nature- and sites and monuments-protected areas, approx. defined by the points:) N57 13 33 – E11 00 33, N57 14 16 - E11 03 27 and N57 11 48 E11 07 43 Additional component parts (Seaweed houses) to be confirmed

Laesoe Island (Danish: Læsø) is a small lowland archipelago situated in the northern part of Kattegat, the strait which separates the North Sea and the Baltic Sea between Denmark and Sweden. The property is a serial nomination which demonstrates exceptional ingenuity and creativity from the early Middle Ages to the mid-twentieth century in the exploitation of the marine resources of salt and seaweed (eelgrass, Zostera marina) by a small and relatively isolated island community.

The property broadly comprises a large coastal heath and wetland landscape of medieval sea-salt production in the south of the island, together with two clusters of ‘seaweed’ houses in the east and northeast of the island, and an area of sandflats and sea in the southeast associated with the source of eelgrass used in roofing.

Cultural landscape of large-scale medieval (12th-16th century) sea-salt production

Shallow and patchy concentrations of saline groundwater (2-17%) occurred in superficial postglacial marine sands and gravels resting on impermeable clay beneath coastal sandflats, salt marshes and meadows in the south of Laesoe. Archaeological sites of around 1,700 sea-salt production huts (salt cotes) are recorded in the component part. Such sites typically comprise stone furnaces, embankments and mounds distributed in distinctive landscape patterns that represents successive exploitation near a retreating coastline. This corresponds to successive raised and stranded palaeo-shorelines that indicate the extent and direction of coastal change caused by centuries of natural marine regression related to post-glacial rebound. So many salt cotes, and in unique arrangement, is a rare manifestation of open-pan salt production once common in medieval Europe where, and when, salt was essential for preserving food, notably fish (especially herring from the Baltic and North seas), and in other aspects of life and commerce.

‘Seaweed’ houses 

Two clusters of around twenty ‘seaweed’ houses located in the east and northeast of the island, together with their associated elements and essential setting, represent an outstanding vernacular building typology that is globally rare in terms of architecture and construction. Moreover, this sustainable building tradition is a fundamental expression of a population group that responded to decisive conditions created by salt production, its cessation, and an ensuing ecological and economic crisis that illustrates an important stage in the natural and cultural-historical development of Laesoe.

‘Seaweed’ (eelgrass) farmhouses, both singly or in groups in the same complex, are scattered in the landscape and represent the contemporary settlement tradition (urban character on the island dates from recent times). The houses are accompanied by shared ‘gardens’ (fenced areas), surrounding peat dikes, and immediate uncultivated natural setting (‘outfield’) adjoining historic tracks/roads. The layout of individual properties is typically in a courtyard enclosed on three sizes and located on the periphery of the large, shared garden.

Roof constructions with distinctive dominant gables are adorned with seaweed, up to two metres thick and weighing some 35-40 tonnes, that spills over walls commonly made from local rubble, clay, and shipwrecked timbers retrieved from the shores and shallow sand banks that surround Laesoe. Seaweed is a sustainable natural renewable building material that serves as a climatic screen that can withstand the heaviest rain and stormy winds while also providing natural insulation, which keeps the house warm in icy winters, and cool in summer. It is air-permeable, creating construction breathability and a higher internal air quality, and has a high salt content that is pest- and rot-resistant and provides natural fire proofing that is not present in straw thatch that was also too valuable as fodder. Interiors follow the North Jutland tradition. 

Eelgrass habitat of sandflats and sea

Adjoining the sea-salt production landscape is a tidal and shallow-water seagrass habitat in the southeast of the island. Women collected the ‘seaweed’ on the shores after winter storms. It was also women who developed the roofing and thatching technique and, in groups of forty to fifty, who constructed the roofs, which can last three hundred years or more.

Justification de la Valeur Universelle Exceptionnelle

Laesoe Island is situated in the northern part of Kattegat between Denmark and Sweden. It is a lowland archipelago that is continuing to rise by post-glacial rebound in the strait, which separates the North Sea and the Baltic Sea. From around 1150 CE to 1950, a particular consequential relationship between nature and culture produced an organically evolved cultural landscape centred on marine resources of salt and seaweed (» eelgrass, Zostera marina).

Seaweed Houses and Sea-salt huts, Laesoe Island comprises a series of distinctive landscapes and sites, including shallow sea where eelgrass continues to grow. Across swathes of heather moorland and saltmarsh, around 1,700 archaeological sites of medieval sea-salt production huts trace continuously formed hypersaline groundwaters and successive raised palaeo-shorelines in marine regression. This landscape provides testimony to the world’s northernmost industry of its kind, the fuelling of which with locally cut wood led to an extended phase of self-imposed catastrophic ecological and economic collapse.

One outcome of such unsustainable exploitation and consequent collapse is a unique architectural ensemble of half-timbered farmhouses with extraordinary eelgrass roofs. These, some dozen of which represent the best examples of 30 that survive out of a total of around 300 built between 1600 and 1950, demonstrate survival and resourceful adaption to harshening living conditions through the salvaging of shipwrecked timbers from the reefs and shallows surrounding Laesoe, the exploitation of local marine clays, and the collection of eelgrass from beaches to form seaweed roofs up to 2 metres thick and many tonnes in weight.

Laesoe reveals in its special landscapes and sites a causal chronology of geological, maritime, and climatic factors, combined with political, economic, and innovative events, both internal and external. This is a nature-culture story, with multiple messages of sustainability, which set the framework for the islanders' lives and conditions in this remote community. It continues to do so.

Criterion (ii): Seaweed Houses and Sea-salt huts, Laesoe Island exhibits an important interchange of human values between 1600 and 1950, within an isolated island community, on developments in architecture and construction technology.

The exceptional human-nature story of this isolated small island community is illustrated in an integrated series of component parts, which convey the hardship caused by the catastrophic ecological consequences of unsustainable practice in natural resource exploitation (sea-salt), and the ability for humans to adapt to changed living conditions through innovation in sustainable building.

Criterion (v) Seaweed Houses and Sea-salt huts, Laesoe Island is an outstanding example of traditional land-use and sea-use, which is representative of human interaction with the environment especially when it has become vulnerable under the impact of irreversible change.

Salt production from a large area of coastal saltmarsh sustained the small community of Laesoe for centuries, yet the consequent deforesting of the island for fuel triggered an ecological crisis that prompted creative human interaction with both land and sea that enabled inhabitants to endure the hardships of the economic crisis of living that ensued.

Déclarations d’authenticité et/ou d’intégrité


Overall, authenticity of the seaweed houses is high, and of the archaeology of the sea-salt production huts very high. While eelgrass roofs and their timber structures need maintenance and periodic renewal their authenticity is ensured through a proactive conservation programme that was initiated decades ago by the Danish Agency for Culture and Palaces. Greatest respect for authenticity is also followed while adapting the building interiors to modern standards of living.


Overall, the property is a substantial and well-preserved representation of the values and way of life that distinguishes this remote island and its remarkable natural and historical-cultural specificities.   

Most of the salt production landscape remains in a stranded, relict, form, while the extent of cadasters in the vicinity of the historic seaweed farmhouses have remained comparatively unchanged and are well defined in the landscape. Twelve houses, protected by Listing, are nominated out of around thirty that survive from a total of around three hundred that once existed. The area of seagrass habitat remains subject to shoreline regression but, overall, the property is not under threat, with industrialization and modern development comparatively light.

Comparaison avec d’autres biens similaires

The unique combination of attributes that reflect the small island community’s way of life and response to survival under closely linked natural and cultural conditions and consequences means that comparative analysis on a like-for-like property basis is not possible. However, taking key attributes manifest in the discrete component parts, individually, meaningful comparisons can be made.

There are significant salt-related properties on the World Heritage List, such as Wieliczka and Bochnia Royal Salt Mines (Poland), open-pan salt works such as From the Great Saltworks of Salins-les-Bains to the Royal Saltworks of Arc-et-Senans, the Production of Open-pan Salt (France), and Hallstatt-Dachstein/Salzkammergut Cultural Landscape (Austria). There are also some minor salt works included as elements within the serial The Great Spa Towns of Europe (e.g. Bad Kissingen, Germany). A salinas (solar evaporation or ‘sunworks’) is on the Tentative List – Salines de Pedra de Lume (Cape Verde). Collectively, these sites are testimony to the importance of salt production as part of our universal heritage. Comparing the typology of salt exploitation on Laesoe island more specifically, while archaeological survivals of medieval sea-saltworks are not uncommon in several maritime countries, e.g. the UK, the sheer number and landscape pattern of vernacular salterns or groups of salt cotes responding to marine regression on Laesoe Island, is exceptional.

When comparing seaweed houses, and their context together with associated sources of construction material, then these are globally rare, although some no longer extant examples are known to have existed in Scandinavia and elsewhere. The closest comparable property, however, is a coastal area in the Shandong (Jiadong) Peninsula in eastern China, although materials and construction techniques are very different compared to Laesoe. In China, dense rows of houses are roofed with local seaweed (kelp), not least through regular stone wall construction (as opposed to local shipwrecked timber, driftwood, and stone rubble and clay), consistent fifty degree roof pitches with high ridgelines, and – laid on a tile roof in clay – a comparatively shallow and light seaweed layer which is actually an alternating combination of kelp, grass and wheat straw, neither sewn, tied, nor twisted. In the Chinese houses, this layer weighs up to five tonnes, as opposed to 35-40 tonnes, and lasts up to one hundred years as opposed to at least two hundred and fifty years in the case of Laesoe.