Low-rise and multi-storey housing

For over a century, Sweden had a ban on building wooden homes above two storeys. This ban was lifted in 1994. Today, there are many examples of exciting new uses for wood in multi-storey blocks – both technical and architectural. Good solutions have been developed for sound insulation and fire safety, which were initially problem areas for high-rises.

Low-rise housing

Around 90 percent of all low-rise housing – small-scale homes with 1 or 2 storeys – is built around a wooden structural frame. Modern Swedish timber-frame houses can be seen as an evolution of the old systems using vertical posts or horizontal boards. In the early days, all wooden buildings were constructed on site, but now much is prefabricated off-site to create kits or box volumetric units. Since wood is low in weight in relation to its load-bearing capacity and strength, the material is ideally suited to industrial construction, making transport, erection and installation on site much easier. There is also growing interest in stick-built wooden houses, in traditional and more modern forms. Wooden homes are being redeveloped and extended both outwards and upwards.

The design of low-rise housing, which was formerly strongly tied to the material’s basic properties and the construction techniques of the time, is now more strongly defined by production techniques and more recent building practices. Many types of housing have sometimes also lacked cultural and historical links with the setting in which they are built. But today we are seeing a growing interest in low-rise housing that is more sympathetic to its surroundings, to the topography, nearby buildings, the local vernacular and so on. In addition, there is a growing demand for low-rise housing with a more “modern” look. Several developers have therefore adapted their production methods to allow for more individualised solutions. Interest in architect-designed wooden buildings is also on the rise.

Old low-rise housing

Wood construction techniques have a long and illustrious history in Scandinavia. Timber harvesting, sawing and construction methods have had a major impact on the technical and architectural solutions that have been chosen. Old wooden houses are invaluable features of the built environment and their preservation requires the retention of knowledge about historical wood construction. What is more, several of the wall structures that are common in old wooden houses still occur in newbuilds.

In older low-rise houses in rural Sweden, the outer walls usually terminate some way above the intermediate floor. In the upper level, there is often a furnished room in the centre of the building, with storage areas ranged along the outer walls. This type of house can still be seen as a functional home with many benefits. The buildings are relatively narrow, which provides good light indoors. There is also reasonable energy efficiency, in part because the rooms on the upper floor, usually bedrooms, only have a small section of external wall exposed to the elements.

These house types were built with timbers of a length that matched normal log lengths and ran the whole way from top to bottom.
This house type is no longer built very often, mainly due to changes in construction techniques.

Common format for old houses.

Dimensions and modules in older buildings
Wood construction has long been modularised in terms of dimensions.
In older buildings, the dimensions were in feet and inches. Feet were used for the frame measurements, for example the centre spacing between joists, and inches for the timber dimensions. These modules were used a long way into the 1970s, primarily because sheet material was measured in feet, for example a width of 4 feet or around 1220 mm. The older timber-framed houses are therefore built with a 2-foot centre spacing between the joists and between the beams.

The 1960s saw the introduction of 3M (300 mm) as a module in construction, and wood construction has been adapted accordingly. Lengths of timber, sheet formats, centre spacings between joists and beams and so on are thus almost always multiples of 3M.
 
Old outer walls
Some of the older wall structures are shown here.

The log wall is a solid wood structure, in which the horizontal logs are both load-bearing and protection against the elements. Log walls made from axe-cut logs (150–200 mm) were usually finished with external cladding and internal wallpaper on lining felt. The log cabins built today are generally holiday homes. Houses that are used all year round will have supplementary insulation either on the interior or the exterior. Many more recent “log cabins” are in actual fact timber-boarded houses made from horizontal tongue and groove boards, 50–75 mm thick.

The post and plank wall occurs in older buildings, particularly outbuildings. It comprises upright posts filled in with wooden boards. The boarding is set into grooves in the posts. In modern building, the post and plank wall is of most interest for its architectural look, since it lends an attractive form to the outside of a traditional stud wall.

Log wall.

 

The vertical log wall is an early form of modular building, with one-storey-high sections built on the ground and then lifted onto the sole plate. The wall comprises vertical axe-cut logs of 150 mm. Externally, the wall is finished with cladding or tiles and render. The internal wall is covered with lining felt and papered over. Vertical log walls can be found in houses built from 1880–1910.

 

Vertical log wall – no cladding.

 

The vertical boarded wall is a variant of the vertical log wall, but built using vertical boards instead.

 

The post wall has vertical wall posts measuring approx. 125x125 mm, and is a precursor to the stud wall. The distance between the posts is around 1200 mm. The inside of the wall is usually finished with horizontal cladding and then lined and papered, with horizontal cladding also fixed to the exterior, often with carved wooded features. The space between the posts and vertical studs is filled with wood shavings or sawdust to provide thermal insulation. The insulation is protected inside and out with felt.

 

The genuine post wall houses were built in the decades immediately before and after the year 1900. Designs using larger posts in solid wood or glulam are now found mainly in industrial buildings and in agricultural outbuildings.

Post wall – no cladding.

The boarded wall comprises a load-bearing and thermally insulating frame of 63–75 mm vertical tongue and groove boards. The exterior is finished just in cladding or in rendered cladding.

The inside is boarded and then clad in lining felt and wallpaper. The boarded wall appears in houses built between 1880 and 1950. Many of the older detached villas and two-storey apartment blocks are built using this technique.

Boarded wall, vertical boarded wall – no cladding.

 

Most Swedish low-rise housing is constructed using wood. It can take various forms, including detached houses, semi-detached and terraces. In terms of construction, low-rise housing differs from multi-storey buildings on a number of points. The main difference is that floor structures to separate apartments are not required. The fire safety regulations for low-rise housing are also different to those that apply to apartment blocks.

 

Multi-storey buildings

 

The Swedish tradition of building in wood is many centuries old, taking in everything from individual homes to larger buildings, churches and agricultural outbuildings. These provide architectural precedents that can be developed and interpreted using a modern design language.

 

When it comes to multi-storey buildings for everyday use such as housing and offices, there are, however, few sources of inspiration in that tradition. The sources that do exist are the homes of the wealthy, the manor houses and estates of the landed gentry and so on.

 

Sweden built practically no high-rise housing with a wooden structural frame from the time before the major city fires and the introduction of the 1874 Building Code until 1994, when the ban on building in wood above more than two storeys was finally lifted. The function-based requirements that were introduced provide a choice as to how one meets the fire safety standards. And we are now beginning to build up a bank of knowledge about high-rise wooden buildings – both technically and architecturally. Reference projects can be found in regions of Europe and the USA that have been quicker to develop new techniques.

 

With domestic sprinklers, fireproofing treatments and so on, buildings with a wooden structural frame can now be fitted with any of the usual façade systems. Conversely, buildings with other structural systems can also be clad with wooden façades. Exposed wood on the façade offers enormous opportunities for a varied and exciting look. The options include many different wood species, cladding types with different dimensions, orientations and fitting methods and other wood-based materials, not to mention combining any of these with other façade materials. Surface treatment and colour choices provide a further abundance of variety.

 

Wooden structures offer particular opportunities for good and efficient layouts and for the architectural design of the building. Normal floor structures in wood are able to span around 6 m.

 

The options for stabilising the building against horizontal forces, such as wind, vary somewhat between buildings with a lightweight wooden frame and those with a heavy-duty frame. Wooden structures that use a lightweight construction technique require particular care when planning window openings, window sizes, positioning of load-bearing walls and so on. Solid wood structures offer greater freedom, which allows more flexibility in designing the layout. The decision as to how the wooden structural frame will be stabilised against horizontal wind forces must therefore be taken early on in the process. 

 

Generally speaking, wooden buildings do not require foundations on the same scale as heavy buildings. The wooden building weighs only around a third of an equivalent concrete building. There are several foundation methods that are suitable for different types of buildings, ground conditions and other factors.

 

Floor structures in solid wood are the only structure that can be allowed to protrude (over balconies). This means, for example, that windows can be placed in corners, without any corner post. The relatively good thermal insulation properties of wood create considerably less of a risk of thermal bridges and condensation than steel and concrete structures. This makes it both easier and cheaper to build protruding components such as balcony floors, platforms, bays, canopies, etc. It also makes it possible to expose the wooden structure on the façade. Solid wood components can be as long as 12 metres, giving the architect considerable freedom when designing the building. 

 

Impact sound insulation between apartments was initially seen as a problem. Today, however, there are various different types of floor structure that meet the tighter housing regulations. The thickness of the floor structure for normal spans (approx. 6 m) ranges from just under 400 mm to around 500 mm. Choosing a thicker floor structure can cause problems with too tight a height for the eaves and roof ridge in detailed plans, where the height of the storey is calculated based on the usual floor structure thickness.

 

Wood in the interior is felt to be authentic, pleasant and natural. The fact that it is an organic and renewable material also contributes to the positive experience. Wood indoors is flexible and easy to repaint, restore, hang pictures and shelves on, and so on. It also makes the acoustic environment more pleasant than in equivalent rooms with harder façade materials, since the wooden surface reduces the reverberation time in the room. A structural frame made of wood panels also has a similar effect that is particularly noticeable in stairwells of apartment blocks.

 

Five apartment blocks with a structural frame made from crosslam and a wooden façade of glulam cladding, Sundsvall. Photographer: Svanthe Harström

Trähus 2001, Bo01 Malmö. Architects: Tue Traerup Madsen and Kim Dalgaard. Photo: Åke E:son Lindman.

For certain products under major price pressure, wood construction techniques with a high degree of prefabrication have quickly achieved a dominant position for blocks of up to four or five storeys, and there is also an accelerating trend towards the use of wood in buildings as high as 10 storeys. Student accommodation is a prime example of this. This dominant market position for industrialised construction in wood is due in no small part to apartment dimensions that allow the use of standardised volumetric units.

For over a century, Sweden had a ban on building wooden homes above two storeys. This ban was lifted in 1994. Today, there are many examples of exciting new uses for wood in multi-storey blocks – both technical and architectural. Good solutions have been developed for sound insulation and fire safety, which were initially problem areas for high-rises.