By the time, after the war, when architects Kaija and Heikki Siren were beginning their careers, the research and development of factory-made wood-framed houses in Finland had already been in progress for some years. An important step in the industrial prefabrication and standardization of low-rise houses in Finland was taken when the company Puutalo was founded following the Winter War of 1940. This joint body of the wooden house industry, founded by 21 companies, focused on the design and marketing of industrially-produced standardized wooden houses.
The Sirens were actively interested in the development of new technology and new construction techniques. Their long-term goal was to increase the use of industrially-produced building components in their buildings. The architects began cooperating with Puutalo in the mid-1950s, and this continued well into the 1960s, resulting in several bold experiments, some of which were actually built. A good example are the prefabricated wooden facade elements which, following the Sirens’ designs for terraced houses and apartment buildings, became Puutalo’s most popular product.
Two-storey terraced houses
The first building type in which the Sirens experimented with the use of industrially manufactured wooden building components was the two-storey terraced house. The terraced houses built at Kontiontie 3 in Tapiola in 1954–1955 marked the beginning of a series of experimental houses. They were the first wooden houses built in Finland using large-sized prefabricated elements. The wooden components of the facades were designed in collaboration with the company Puutalo. The elements are exceptionally large: 16 square metres in size. Their use was made possible by the fact that the load-bearing partition walls were built in brick and the supporting longitudinal beams in concrete. In addition to the facade elements, the houses’ wooden roof trusses were also preassembled at the factory and then delivered to the building site.
The terraced houses built at Kimmeltie 11-13 in Tapiola in 1955, north of the Silkkiniitty park, used the same construction methods as on Kontiontie, the only difference being that the load-bearing partition walls are concrete instead of brick. Wood-concrete composite slabs were tested for the intermediate floors, in which wood baulks replaced the need for reinforcement bars in the concrete. This was presumably the first and also the last time the Sirens experimented with this special type of construction.
A couple of years after the completion of the terraced houses in Tapiola, another two Siren-designed two-storey terraced houses were built, the Näätäkallio condominium at Näätätie 19, in Herttoniemi in Helsinki. Puutalo was a partner in the design of this project, too. The wood used in the houses was sawn into suitably sized parts and numbered at the factory, after which the material was transported to the building site. In addition to this so-called pre-cut construction method, the wooden roof trusses, the houses’ exterior and partition wall panels and window frames were pre-assembled at the factory.
Näätäkallio differs, however, from the terraced houses in Tapiola, in that the load-bearing structure on the ground floor consists of wooden frames placed on top of a concrete plinth and with pillars placed next to the stairs, thus leaving the rest of the dwelling as free space. The upper-floor ceiling is made of timber boarding and the roof structure rests on trusses supported at each end. The facades are comprised of prefabricated wooden elements with both vertical and horizontal wooden boarding.
The Siren’s terraced houses at Kanneltie 12 in the Helsinki district of Etelä Kaarela were completed in 1960. The building commission had come directly from Puutalo, because they were interested in developing new construction methods. The company delivered all the wooden components for the houses: panels, doors and windows. The load-bearing frame consists of transverse, piled concrete walls, on top of which were placed prestressed concrete beams – so-called MJ beams – to form the lower floor slab, and braced beams made from multiple pieces of plywood nailed together that supported the roof construction. Chipboard was used for the ceilings and the outer layer of the partition walls, and with a laminate-coated variety for the bathrooms. The wooden facades appear to be prefabricated elements, though this was not mentioned when the project was presented in the journal Arkkitehti.
Prefabricated wooden facade elements in apartment buildings
At the beginning of the 1950s, the Sirens were awarded the commission to design the student dormitories for Helsinki University of Technology, located on Jämeräntaival in the Otaniemi campus’s student village. They were built using traditional construction methods, with the load-bearing structure, as well as the facades, built of fair-faced brick. However, the Sirens used industrially produced building components in the housing in the Retuperä area, south of the student village – designed at the same time as the terraced houses of Kontiontie and Kimmeltie. The complex of four condominiums was built in several stages from 1952 to 1962. To save on costs, as much new construction technology as possible was utilised in the houses, including concrete for the bathroom walls and the floor slabs of the balconies.
Unlike in the terraced houses, large-sized prefabricated wooden elements could not be used in the construction of the apartment buildings. The lighter prefabricated exterior wall elements for the two apartment buildings Otaharju (1955) and Otalaakso (1956) were manufactured by Puutalo, for the use of which special permission had to be obtained from the fire authorities. According to a publication from 1965 marking the 25th anniversary of the founding of Puutalo, such lightweight prefabricated wooden elements were first used in an apartment building designed by architect Matti Lampén in 1954, and though the name of the building is not mentioned, it is probably the Uusi Pajamäki condominium on Poutamäentie in Helsinki. The use of prefabricated elements required that the transverse partition walls separating the dwellings and the concrete intermediate floors acted as the load-bearing structures. A similar construction was also implemented in the Otaharju and Otalaakso apartment buildings. Although the example included in the publication was the apartment building designed by Lampén, the accompanying photo shows the Sirens’ Otalaakso apartment building.
Tapiola Primary School and teachers’ terraced houses
Tapiola Primary School and the nearby teachers’ dwellings were completed in 1957. The school and teachers’ terraced houses were considered pioneering, and the buildings were published in several magazines, also internationally. In the design of the school building the Sirens experimented with such advanced design principles that the implementation required deviations from the prevailing building regulations.
The exterior walls and part of the walls of the classroom corridors of the single-storey, flat-roofed school building were built from prefabricated wooden elements manufactured by Puutalo. Although they were not as large as those in the houses on Kontiontie and Kimmeltie, they probably had a similar construction. The construction of the walls was examined closely in 2017 for the preparation of a building-historical report on the school. The prefabricated wood-framed elements of the exterior walls were fixed on to cast concrete plinths. The composition of the elements, from the outside layer inwards was as follows: vertical wooden panelling, rough-sawn T&G boarding, 22 mm lining felt, 100 mm rock wool, 2 layers of aluminium membrane, 15 mm wooden boarding, and gypsum board. The light-weight internal partition walls were mainly blockboard given a stained finish.
Built close to the school building were three terraced houses for teachers, one of which has two storeys. Typical of the time, the Sirens used composite structures in the houses, and took advantage of new materials as they became available. The long facades consisted of prefabricated wooden elements, the gable walls were heat-insulating lightweight concrete blocks, the partition walls and intermediate floors were reinforced concrete and the roof construction and trusses were wood. The exterior walls of the houses were made from the same materials as those of the school. The facades of the upper floor of the two-storey terraced house were painted in a dark tone, just like the Kontiontie terraced houses.
Volumetric modules of the 1960s
In 1964 the Housing Foundation organised an invited competition among the building industry, in which stakeholders representing its different branches were given the opportunity to present their views on how a low-rise house of the future should be built. From among the proposals, the competition jury found the most interesting to be that submitted by Puutalo, the designers of which were Heikki Siren, Kaarlo Rautkari and Kauko Rastas, and the proposal was awarded first prize. Heikki Siren gave it the name “Polar”. The detached house version, the so-called Polar People’s House, was introduced in 1966. In the Helsinki metropolitan area, the type was first built in 1967 as a two-storey terraced house in Tapiola, and a year later as a single-storey version in Hakunila in Vantaa. Elsewhere in Finland, the type was built at least in Punkaniemi in Punkasalmi, Kaarela in Turku, Koskela in Oulu, and a dozen or so in Mäntylä in Lappeenranta.
Large-sized prefabricated elements were first used in Tapiola, in the Tapionsolu condominium at Aarnivalkeantie 10, designed by the Sirens and completed in 1967. 80% of the building components of the houses were prefabricated at a factory. The “boxes” were dimensioned so that they could be transported by road from the factory to the building site and then hoisted onto lightweight foundations. According to the earlier mentioned 1965 Puutalo anniversary publication, the houses were raised “figuratively speaking, in the midst of rose bushes pre-planted in the garden”. Typical of the time, the author also noted that society was now entering the “space age”.
Tapionsolu consists of seven large-size prefabricated units. The wooden volumetric modules were placed on top of concrete walls cast using gang forms. The house consists of two such “building boxes”, the ground floor one of which comprises all the technical facilities, a garage, a laundry room and a sauna, while the upper one comprises the living room, bedrooms, kitchen and bathroom. The concrete walls extend beyond the house to form 90m2 sheltered gardens between the units.
In addition to the Polar houses, the prefabricated volumetric modules developed by Siren, Rautkari and Rastas were also used for temporary churches in the Helsinki city districts of Myllypuro and Kontula. Temporary lightweight churches were erected in the new city districts that urgently needed congregational facilities. The Myllypuro and Kontula churches were wooden structures with a floor area of around 400m2. The units originally designed for residential buildings were here attached to a main frame, which comprised the nave of the church. These were used, for instance, for church clubrooms. Both the exterior facades and the interior walls were clad with vertical wooden boarding. The two churches were built between 1968 and 1969. Relocation of them was never considered, and both have since been demolished.
Impressive wooden roof structures
In the 1950s, Kaija and Heikki Siren designed two buildings in the Otaniemi campus’s student village which feature impressive wooden roof structures. The first, Servin Mökki, a student self-service restaurant and events space, was completed in 1952. The building’s large restaurant hall comprises V-shaped pillars made of unplaned baulks bolted together, which in turn support braced beams supporting the roof. The braced beams extend beyond the uniform glass facade. No prefabricated parts were used in the structures, because the wood was surplus material from the nearby student dormitories. Interestingly, the construction manager for the student village’s building board was Kauko Rastas, who later became a partner in the major construction company Polar. Also involved in the project was Kaarlo Rautkari from Puutalo. Heikki Siren developed prefabricated wooden structures with both these engineers up until the 1960s. The structure of the roof supports in Servin Mökki are very similar to the Swedish-designed HB braced beams marketed by Puutalo in the early 1950s. A good example of product development collaboration is also the “lighter” versions of the roof beams, the plywood-framed braced beams that were used in the roofs of the terraced houses at Kanneltie 12.
Five years after the completion of Servin Mökki, the Sirens’ Otaniemi Chapel was built nearby. Within the main church space, perpendicular to the south-facing upper window, is a system of wooden trusses that supports the roof structure. Within the trusses are cables connecting the vertical supports to the roof beams. The solution means that the loads from the roof structure are transferred to the exterior walls of the building, thus freeing the space of the need for pillars. Lamps have been inserted into the ends of the roof beams nearest the altar. Just as in Servin Mökki, exposed large-sized bolt heads have been used in the roof construction as an artistic device.
The development of glulam beams, made by gluing together laminations of boards, marked an important new area in the development of roof structures. Thanks to the invention, wood became a competitor to concrete and steel, as it was now possible to create a long-span load-bearing structure using wooden beams. Glulam began to be used in the Finnish construction industry in the late 1950s. For example, Puutalo developed a so-called “yardstick” system for glued wood for two-storey terraced houses. The Sirens utilized glulam beams at least in the central garage belonging to the company Tapiolan Lämpö, built at Jousenkaari 12 in the western suburb in 1961, and which still exists today. The garage’s curved glulam roof beams are elegantly sculptural and create the impression of a new modern building type.
Author Elina Standertskjöld, Lic.Phil., is a researcher in art history, who specialises in 20th century architecture.
 Heikki and Kaja Siren first founded their joint architects’ office in 1946.
 Ab. Puutalo Oy, history (PDF in Finnish), Central Archives for Finnish Business Records (ELKA). See also: “Puutalo”, Wikipedia: https://fi.wikipedia.org/wiki/Puutalo_(yritys); “Jälleenrakennuskausi” (in Finnish), Museum of Finnish Architecture: https://www.mfa.fi/kokoelmat/tietopaketit/jalleenrakennuskausi/organisointi-ja-tarkeimmat-organisaatiot/organisaatiot/#puutalo (accessed 5.1.2021).
 Nikula, Riitta (2014). Suomalainen rivitalo –Työväenasunnosta keskiluokan unelmaksi. Suomalaisen Kirjallisuuden Seura, 112.
 Siren, Kaija & Heikki (1956), “Rivitaloale Kontiontie”, Arkkitehti, 1–2/1956, 22–25. See also: Tuomi, Timo (1995). Tapiola – A History and Architectural Guide. Espoo City Museum, 88–89.
 Tuomi 1995, 90–91.
 Siren, Kaija & Heikki (1959), “Rivitalotyyppi”, Arkkitehti, 1–2/1959, 19–20.
 Siren 1959, 19–20. Kaarlo Rautkari and Paavo Simula were responsible for the structural engineering, while Matti Hakala and Lasse Gestranius from the Sirens’ office were responsible for the design.
 The braced beam (in Finnish: naulapalkki) is formed from boards or planks mounted side by side; they are fixed together with ledges, that is, perpendicular and diagonal boards nailed to the flat sides. So-called MJ beams are 30 cm wide channel-like beams made of reinforced concrete.
 Siren, Kaija & Heikki (1961), “Kaksi rivitaloa”, Arkkitehti 4–5/1961, 67–71. Participating in the work were Kaarlo Rautkari from Puutalo and Paavo Simula, who designed the structures. Heikki Tegelman at the Sirens’ office was project architect.
 It was not until the mid-1960s that legislation regarding these was updated. Kouti, Yrjö (ed.) (1965) 25-vuotta suomalaista puutaloteollisuutta, Myyntiyhdistys Puutalo, 30. The structures of the Retuperä apartment buildings were designed by Paavo Simula. Siren, Kaija & Heikki (1959), “Asuntoalue Otaniemi”, Arkkitehti, 1–2 / 1959, 13–18.
 Kouti (ed.) 1965, 30.
 Museum of Finnish Architecture photo archive. The photos show that the rear walls of the balconies of the buildings are wooden elements. However, it could be concluded from the wording in the publication that the building was situated elsewhere in Finland.
 Kouti (ed.) 1965, 30.
 Saatsi Arkkitehdit (2017), Aarnivalkean koulu. Rakennushistoriaselvitys 29.9.2017 [Aarnivalkea school. Building-historical report]. Espoon kaupunki, 20–22.
 Bruun, Erik & Popvits, Sara (eds.) (1976). Kaija + Heikki Siren: Architects/Architekten/Architectes. Otava, 221.
 The project’s structural engineer was Paavo Simula and the technical expert was Kauko Rastas. Saatsi Arkkitehdit 2017, 24–25.
 Saatsi Arkkitehdit 2017, 30, 201. Siren, Kaija & Heikki (1957), “Tapiolan kansakoulu”, Arkkitehti, 6–7/1957, 94.
 The Housing Foundation (Finnish: Asuntosäätiö) organised the invitational competition, in which the planning of the guidelines for the industrial production of houses were to be within the framework of Puutalo’s manufacturing programmes. Heikki Siren worked together with the engineer Kauko Rastas. They undertook research development under the direction of Kaarlo Rautkari. Rautkari, Kaarlo (1964). “Kevytrakenteinen rivitalo puusta”, Rakennustekniikka 13–14/1964, 834–835. Kouti (ed.) 1965, 5.
 The Polar prefabricated elements were used in the construction of an entire residential area in Hakunila, Vantaa, in 1968. They were designed to be single-storey linked houses. Siren, Heikki (1966). “Polar-kansantalo”, Arkkitehti, 7–8/1966, 114–115.
 Central Archives for Finnish Business Records (ELKA). Puutalo Oy:n arkistoluettelot / Rakenteet [Puutalo Oy archives / structures].
 Puutalo manufactured the elements, and the building contractor was Polar-rakennus.
 Kouti (ed.) 1965, 5.
 Maunula, Jarmo (ed.) (1970). Suomi rakentaa 4. Suomen arkkitehtiliitto, site 35. Bruun & Popvits (eds.) 1976, 212. Tuomi 1995, 132–133.
 Jetsonen, Sirkkaliisa & Viertiö, Sari (2002). Heikki Sirenin haastattelu (Interview with Heikki Siren). Rakennustaiteen seura. ”Elementtirakenteinen siirrettävä kirkko”, Arkkitehti, 2/1969, 56–57.
 Lahti, Juhana (2020). ”Johdanto: Sirenit”, in Lahti, Juhana & Autio, Frans (eds.) Kaikki ja ei mitään – Arkkitehdit Kaija + Heikki Siren. Arkkitehtuurimuseo, 20, 144. Siren, Kaija & Heikki (1956). “Servin mökki”, Arkkitehti, 6/1956. 94–97.
 HB-rakenne. Myyntiyhdistys Puutalo [Puutalo sales publication], no date. This type of roof truss was used, for instance, in the Otaniemi sports hall designed by Alvar Aalto, which was completed the same year as Servin Mökki.
 Siren, Kaija & Heikki (1961). “Kaksi rivitaloa”, Arkkitehti, 4–5/1961, 67–71.
 Siren, Kaija & Heikki (1958). “Teekkarikylän kappeli”, Arkkitehti, 6/ 1958, 87–97.
 Suomen liimapuuyhdistys ry & Puutalo Oy (2014). Liimapuukasikirja osa 1, https://puuinfo.fi/wp-content/uploads/2020/07/Liimapuuk%C3%A4sikirja-Osa-1.pdf (accessed 5.1.2020).
 Kouti (ed.) 1965, 7–8.
 Espoo City Museum photo archives. Asuntosäätiön kokoelma (Housing Foundation collection).