A sod roof or turf roof is a traditional Scandinavia

Scandinavia

Scandinavia is a cultural, historical and ethno-linguistic region in northern Europe that includes the three kingdoms of Denmark, Norway and Sweden, characterized by their common ethno-cultural heritage and language. Modern Norway and Sweden proper are situated on the Scandinavian Peninsula,...

n type of green roof

Green roof

A green roof is a roof of a building that is partially or completely covered with vegetation and a growing medium, planted over a waterproofing membrane. It may also include additional layers such as a root barrier and drainage and irrigation systems...

 covered with sod

Sod

Sod or turf is grass and the part of the soil beneath it held together by the roots, or a piece of thin material.The term sod may be used to mean turf grown and cut specifically for the establishment of lawns...

 on top of several layers of birch bark

Birch bark

Birch bark or birchbark is the bark of several Eurasian and North American birch trees of the genus Betula.The strong and water-resistant cardboard-like bark can be easily cut, bent, and sewn, which made it a valuable building, crafting, and writing material, since pre-historic times...

 on gently sloping wooden roof boards. Until the late 19th century it was the most common roof on rural log houses in large parts of Scandinavia. Its distribution roughly corresponds to the distribution of the log building technique in the vernacular architecture

Vernacular architecture

Vernacular architecture is a term used to categorize methods of construction which use locally available resources and traditions to address local needs and circumstances. Vernacular architecture tends to evolve over time to reflect the environmental, cultural and historical context in which it...

 of Finland

Finland

Finland , officially the Republic of Finland, is a Nordic country situated in the Fennoscandian region of Northern Europe. It is bordered by Sweden in the west, Norway in the north and Russia in the east, while Estonia lies to its south across the Gulf of Finland.Around 5.4 million people reside...

 and the Scandinavian peninsula. The load of approximately 250 kg per m² of a sod roof is an advantage because it helps to compress the logs and make the walls more draught-proof. In winter the total load may well increase to 400 or 500 kg per m² because of snow. Sod is also a reasonably efficient insulator in a cold climate. The birch bark underneath ensures that the roof will be waterproof.

The term ‘sod roof’ is somewhat misleading, as the active, water-tight element of the roof is birch bark. The main purpose of the sod is to hold the birch bark in place. The roof might just as well have been called a "birch bark roof", but its grassy outward appearance is the reason for its name in Scandinavian languages: Norwegian

Norwegian language

Norwegian is a North Germanic language spoken primarily in Norway, where it is the official language. Together with Swedish and Danish, Norwegian forms a continuum of more or less mutually intelligible local and regional variants .These Scandinavian languages together with the Faroese language...

 and Swedish

Swedish language

Swedish is a North Germanic language, spoken by approximately 10 million people, predominantly in Sweden and parts of Finland, especially along its coast and on the Åland islands. It is largely mutually intelligible with Norwegian and Danish...

 'torvtak', Icelandic

Icelandic language

Icelandic is a North Germanic language, the main language of Iceland. Its closest relative is Faroese.Icelandic is an Indo-European language belonging to the North Germanic or Nordic branch of the Germanic languages. Historically, it was the westernmost of the Indo-European languages prior to the...

 'torfþak'.

A sod roof is well suited to a barter

Barter

Barter is a method of exchange by which goods or services are directly exchanged for other goods or services without using a medium of exchange, such as money. It is usually bilateral, but may be multilateral, and usually exists parallel to monetary systems in most developed countries, though to a...

 economy because the materials are ubiquitous and cost nothing, although the work is labour intensive. But a household would usually have a lot of manpower, and neighbours would usually be invited to take part in the roofing party, similar to a barn raising

Barn raising

A barn raising is an event during which community men come together to assemble a barn for one or more of its households, with the support of women. The event was particularly common in 18th- and 19th-century rural North America. In the past, a barn was often the first, largest, and most costly...

 in the United States

United States

The United States of America is a federal constitutional republic comprising fifty states and a federal district...

. The Norwegian term dugnad denotes an established custom in rural communities, where large undertakings were accomplished with help from neighbours.

History

Roofs in Scandinavia have probably been covered with birch bark and sod since prehistory

Prehistory

Prehistory is the span of time before recorded history. Prehistory can refer to the period of human existence before the availability of those written records with which recorded history begins. More broadly, it refers to all the time preceding human existence and the invention of writing...

. During the Viking

Viking

The term Viking is customarily used to refer to the Norse explorers, warriors, merchants, and pirates who raided, traded, explored and settled in wide areas of Europe, Asia and the North Atlantic islands from the late 8th to the mid-11th century.These Norsemen used their famed longships to...

 and Middle Ages

Middle Ages

The Middle Ages is a periodization of European history from the 5th century to the 15th century. The Middle Ages follows the fall of the Western Roman Empire in 476 and precedes the Early Modern Era. It is the middle period of a three-period division of Western history: Classic, Medieval and Modern...

 most houses had sod roofs. Churches and other buildings with steeper roofs were covered with board

Lumber

Lumber or timber is wood in any of its stages from felling through readiness for use as structural material for construction, or wood pulp for paper production....

s, wood shingle

Wood shingle

Wood shingles are roof shingles made of cut wood, used for roofing material. Such roofing material made from split wood is referred to as "shakes"....

s, or lead

Lead

Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...

.

In rural areas sod roofs were almost universal until the beginning of the 18th century. Tile

Tile

A tile is a manufactured piece of hard-wearing material such as ceramic, stone, metal, or even glass. Tiles are generally used for covering roofs, floors, walls, showers, or other objects such as tabletops...

 roofs, which appeared much earlier in towns and on rural manors, gradually superseded sod roofs except in remote inland areas during the 19th century. Corrugated iron and other industrial materials also became a threat to ancient traditions. But just before extinction, the national romantics proclaimed a revival of vernacular traditions, including sod roofs. A new market was opened by the demand for mountain lodges and holiday homes. At the same time, open air museum

Open air museum

An open-air museum is a distinct type of museum exhibiting its collections out-of-doors. The first open-air museums were established in Scandinavia towards the end of the nineteenth century, and the concept soon spread throughout Europe and North America. Open-air museums are variously known as...

s and the preservation

Historic preservation

Historic preservation is an endeavor that seeks to preserve, conserve and protect buildings, objects, landscapes or other artifacts of historical significance...

 movement created a reservation for ancient building traditions.

From these reservations, sod roofs have begun to reappear as an alternative to modern materials. The more recent idea of the green roof

Green roof

A green roof is a roof of a building that is partially or completely covered with vegetation and a growing medium, planted over a waterproofing membrane. It may also include additional layers such as a root barrier and drainage and irrigation systems...

 is developed independently from the traditional sod roof, but could benefit from the experience gathered during hundreds of years in Scandinavia.

Birch bark

Birch bark is eminently suitable as a roof covering because it is strong, water-resistant and soil-resistant enough to last for generations, although 30 years was considered the normal lifespan of a sod roof in most places. Birch is common everywhere in Northern Europe

Northern Europe

Northern Europe is the northern part or region of Europe. Northern Europe typically refers to the seven countries in the northern part of the European subcontinent which includes Denmark, Estonia, Latvia, Lithuania, Norway, Finland and Sweden...

, and its bark is easily stripped from the trunk in spring or early summer, while the sap is running. One incision with a knife lengthwise down the trunk between two branches will produce a correspondingly wide sheet, its length equal to the circumference

Circumference

The circumference is the distance around a closed curve. Circumference is a special perimeter.-Circumference of a circle:The circumference of a circle is the length around it....

. The outer, flaky (white) bark is pried loose from the inner (green or brown) layer, the phloem

Phloem

In vascular plants, phloem is the living tissue that carries organic nutrients , in particular, glucose, a sugar, to all parts of the plant where needed. In trees, the phloem is the innermost layer of the bark, hence the name, derived from the Greek word "bark"...

. Removing the phloem will cause the tree to die, but removing the outer bark will not harm the tree, although the scar will remain for many years. A new, coarser bark will replace the stripped bark.

The sheets of bark must be stored flat under pressure to prevent curling. Left alone, a sheet will curl up into a tight roll, in the opposite direction of the natural curve of the trunk.

The bark is laid inside up directly onto the roof boards without any nails or other means of fastening. On roughly hewn or sawn roof boards, the friction alone will hold the layers of birch bark in place. But they have to be weighted down with a heavier material to prevent them from curling or blowing away. Planks of split logs have been used, but sod has an additional advantage because it is an insulator.

Bark sheets were laid from the eaves upward, overlapping like shingles, and straddling the ridge. 6 layers of birch bark were considered adequate, but up to 16 layers have been recorded in roofs of high quality. The first layers project about 8 cm along the eaves, where they will curve down around the edge of the outer board to form a throating. Extra long sheets will straddle the roof ridge.

Sod

Sod or turf for roofing was cut from good pasture land, preferably with sandy soil. A naturally grown grass with a deep root system was desirable. The turf was cut into portable pieces, each about one foot (30 cm) square and about 3 inches (7.5 cm) thick, half the thickness of the finished covering. It was hoisted or carried up on the roof, or on a scaffolding.

Pieces of sod must be laid out immediately after the birch bark, to keep it in place. For this reason, the process must be carefully coordinated. Two workers will usually cooperate on the same strip of the roof, wide enough to be comfortably within reach. One will lay out the bark, the other will put sod on top.

The first layer of sod was traditionally placed with the grass down, as the wilted grass would protect the bark from acid

Acid

An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...

 humus and act as a drain. The grass of the second layer faced upwards to establish a solid surface. Grass roots would eventually permeate the bottom layer to create one solid structure. The finished roof would in time look just like a flower-studded meadow.

A total thickness of about 15 cm is enough to make the sod survive a dry summer. More than 20 cm is superfluous, and the weight would be excessive. If the sod is too shallow, a severe drought will kill the vegetation, causing erosion

Erosion

Erosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...

 and soil creep. Most of Scandinavia has enough precipitation to let an established root system survive an occasional dry spell. In very dry areas, it was customary to cultivate certain drought resistant plants - notably Sedum rosaeum, Allium fistolosum and sempervivum tectorum

Sempervivum tectorum

Sempervivum tectorum is a succulent evergreen perennial whose many common names include Common houseleek,Jove's beard, Jupiter's eye, Thor's beard, St. George's Beard, and...

.

Sod support along the eaves and verges

To stay in place the sod must be supported by strong beams along the eaves. These logs or beams (Norwegian: torvvol) come in a variety of shapes and sizes, depending on the quality and status of the building, and on local tradition. An equivalent English term would be turf log.

The most common turf log is an undressed fir log 5 to 6 inches across. Also fairly common are squared timbers of similar size. In some districts planks raised on edge were used. Other traditions called for a more elaborate cross section with the appearance of a fascia

Fascia (architecture)

Fascia is a term used in architecture to refer to a frieze or band running horizontally and situated vertically under the roof edge or which forms the outer surface of a cornice and is visible to an outside observer...

, approaching "real" architecture

Architecture

Architecture is both the process and product of planning, designing and construction. Architectural works, in the material form of buildings, are often perceived as cultural and political symbols and as works of art...

.

The turf log was fastened to the roof boards and underlying rafter

Rafter

A rafter is one of a series of sloped structural members , that extend from the ridge or hip to the downslope perimeter or eave, designed to support the roof deck and its associated loads.-Design:...

s with naturally grown wooden hooks, preferably from durable juniper

Juniper

Junipers are coniferous plants in the genus Juniperus of the cypress family Cupressaceae. Depending on taxonomic viewpoint, there are between 50-67 species of juniper, widely distributed throughout the northern hemisphere, from the Arctic, south to tropical Africa in the Old World, and to the...

. The hooks were fastened with wooden pegs and covered by additional sheets of birch bark. Turf logs are also known to have been held in place by rafters hewn from logs with one root branch left to form a bracket. This method made for extremely cumbersome maintenance. When the brackets disintegrated, one would have to tear up the whole roof to renew all the rafters. A more refined fastening device was the kolv, an elaborately shaped stick of wood, around 60 cm long, pegged to the roof and extended through a hole in the turf log. Its wider protruding head would hold the log in place.

Erosion at the verges was in some districts prevented by means of similar logs, sometimes joined crosswise at the gable

Gable

A gable is the generally triangular portion of a wall between the edges of a sloping roof. The shape of the gable and how it is detailed depends on the structural system being used and aesthetic concerns. Thus the type of roof enclosing the volume dictates the shape of the gable...

 top. More common was a row of rocks gathered from the fields. Verge boards were not a feature of ancient log buildings with sod roofs, but were introduced during the 17th and 18th centuries through influence from urban architecture with tile roofs.

Modern turf roofs

In modern turf roof construction bitumen roofing felt combined with dimpled plastic drainage membranes are usually used in place of birch bark. Bitumen roofing felt is nailed to the sarking

Sarking

Sarking is the use of wood panels, or "sarking boards", under the shingles of a roof to provide support. It is a common term in Scotland, Australia, and New Zealand. The shingles or slates are nailed directly to the sarking boards without timber battens, providing a strong, wind-resistant roof...

and a dimpled cavity drainage membrane is laid over the top (dimples face down) to form a drainage layer. The turf is then laid over this drainage layer.