Douglas fir: the wood of the future

Materials

Douglas fir: the wood of the future

As UK and European stocks of Douglas fir begin to reach maturity following increased planting over the past 75 years, Sabrina Pedrono of France Douglas and Elisabeth Piveteau-Boley of Piveteaubois look at the wealth of application of this versatile species.

In 2020, Douglas fir represented less than 3% of the French forest area but 18% of French sawnwood. In 15 years, it could represent a third of the total national softwood production.

Photo: ®Yoann Portejoie, France Douglas

Timber for construction

Since its introduction to Europe from the American West, Douglas fir has become one of the mainstays of the timber sector with the forest area constantly expanding. Strong, hard, workable and abrasion resistant, Douglas fir is a highly versatile structural timber as well as being useful for cladding and external landscaping.

Douglas fir: the species

Introduced into the UK in 1827 by botanist David Douglas, Douglas fir is, in fact, a conifer that grows well in open forest and thrives in damp conditions. ‘Douglasfir’ is traditionally written as one word, without a hyphen, to indicate that it is not a true fir but modern language has adopted the use of two words.

The species is perfectly adapted to mid-mountain areas and grows well in hardiness zones 4–6. An evergreen with resinfilled blisters on its red-brown, corky trunk, three-pointed cones with scales, and flat, soft, flexible needle-like leaves, Douglas fir grows at a medium rate of up to one metre per annum and can live for up to 500 years. Carrying both male and female cones on the same tree, Douglas fir is wind pollinated. In ideal conditions, mature trees will grow to heights of 12–21 metres with spreads of 4–6 metres when mature.

There are two main varieties of Douglas fir: the coast Douglas fir (Pseudotsuga menziesii var. menziesii) and the Rocky Mountain Douglas fir (Pseudotsuga menziesii var. glauca). The first has dark, yellow-green or bluish-green needles, grows faster and lives longer. The second has shorter, bluish-green needles and is slower growing and hardier, but rarely grows over 39 metres tall.

Douglas fir: in UK and Europe

Douglas fir covers almost 20 million hectares of western North America. When imported from North America it is sold as ‘Oregon pine’. Initially brought to Europe as an ornamental tree in 1827, it was gradually introduced to the forests of Europe after the Second World War. As a result of the reforestation programmes set up in France, Douglas fir now covers 426,000 hectares of the country. The species also thrives in the western areas of the UK, where rainfall is high >>

“As renewable material with both excellent properties of natural durability and mechanical performance, Douglas fir is perfectly placed for a wide range of applications: structural use, cladding, decking, interior decoration, exterior fittings – even works of art.”

Douglas fir decking at Clifford’s Tower

The free-standing timber viewing deck inside Clifford’s Tower, an English Heritage castle overlooking Old York, was designed by Hugh Broughton Associates and built by Simpson York. Accessed by new walkways and legacy staircases, the deck was opened in April 2022 to give visitors spectacular panoramic views over York’s skyline. Constructed to protect the tower’s historic stonework, the structure includes primary steel flitch beams and secondary glulam joists that make up 14 enormous structural timber frame sections, all manufactured using C24 sapwood-free Douglas fir from Piveteaubois. These include four columns, each 550mm square and 9m high. The platform was then covered in Douglas fir decking sealed with Liberon decking oil. Buckland Timber manufactures glulam from both UK-grown and imported French Douglas fir and occasionally from North American Douglas fir.

As UK and European Douglas fir forests gradually mature, timber harvests increase significantly. In France alone harvests exceeded 3 million cubic metres in 2020. When reaching full potential around 2035, production is expected to deliver 6 million cubic metres of roundwood each year – more than 30% of France’s national softwood production. This makes Douglas fir the top emerging timber resource in France, yielding very straight trees that grow up to 50m tall.

Douglas fir: the structural timber

As renewable material with both excellent properties of natural durability and mechanical performance, Douglas fir is perfectly placed for a wide range of applications: structural use, cladding, decking, interior decoration, exterior fittings – even works of art. It offers great architectural freedom, is easy to work with and boasts a high level of dimensional stability. All these benefits make it appealing to architects, building designers, structural engineers and the joinery sector.

PEFC-certified Douglas fir CLT at the Chelsea Flower Show

Designed by Robert Myers and built by landscapers Bowles & Wyer for the 2021 RHS Chelsea Flower Show, the Florence Nightingale Garden showcased an eco-friendly PEFC-certified CLT timber pergola. This 60ft pergola, made of sapwood-free Douglas fir from Piveteaubois, was constructed off-site and assembled by ConstrucktCLT on site in a matter of days. The complex geometry of each individual timber profile was templated by ConstrucktCLT. The curved structural CLT ribs were then cut to shape by Constructional Timber (Manufacturers) Ltd. The garden was dismantled after the show and transported to its permanent home at Guy’s and St Thomas’ Hospital, London where it will be part of the restorative garden for the recuperation of patients.

Timber in construction must meet certain requirements of structural stability and durability to insect or fungal attack. Douglas fir heartwood is reddish-brown and highly distinct from its cream white sapwood. While specifiers should refer to standards to ensure the desired service life is achieved, when out of ground contact the heartwood can be used in exterior applications. This opens up significant additional areas of exterior architectural use, including cladding, decking, and applications in high moisture atmospheres such as swimming pool halls.

Douglas fir timbers grown in the UK can have less resin than North American wood, offering average graded weights when dry of around 530 kg/m3 Once dry, it becomes very hard. Characteristic strength ranges from about 10 to 25 N/mm2 with mean stiffness ranging between 8 to 13 kN/mm2 The variability in UK growth rates give lower density rates than European grown timbers but characteristic properties are quickly improved by grading, with grades of C24 and C28 readily available. >>

Photo: Dirk Lindner

Photo: Richard Bloom

A softwood with long, straight fibres and a medium texture, Douglas fir offers excellent resistance to traction and bending, with a high weight/resistance ratio. Although the presence of a high number of knots can weaken this mechanical resistance, fewer knots are found in the heartwood and this issue can be minimised further when modifying the wood into cross laminated timber (CLT) or glulam, for example. Its timber characteristics make Douglas fir good for glueing, sawing, nailing and screwing, although a tendency for splitting means pre-drilling is necessary.

Douglas fir: a flexible, technical, economic and environmental option

Timber in construction has undergone many changes in recent years, moving from a demand-based model of delivering individual homes, to huge growth markets such as social housing, retail and commercial buildings, or multi-storey ‘tall timber’ buildings. These markets require highly technical and engineered products such as CLT and glulam – an area where Douglas fir is already common.

Douglas fir glulam at Bristol Old Vic

The £9.3m refurbishment of the Bristol Old Vic by Haworth Tompkins was the result of five years of careful research, design and construction. Framed in PEFCcertified, structural Douglas fir glulam, flitched and with bolts expressed, the new foyer has been designed with a large intake plenum and pre-cooling ‘labyrinth’ with automated, thermosensitive control of low- and highlevel openings with shutters allowing natural light and ventilation to fill the space. The contemporary foyer and roof incorporate 59m3 machined and cut-to-size GL24h Douglas fir glulam designed, manufactured, supplied and installed by Constructional Timber (Manufacturers) Ltd. The timber will naturally bend, flex and darken with age. Designed to span in one direction, the roof beams are short spans linked with RICON® connectors to give tight, angular joints with horizontal plywood bracing. Only the largest spans required steel reinforcement.

Douglas fir: a source of architectural inspiration

Douglas fir products are constantly evolving to meet the needs of the construction market with high-profile landmark projects appearing across the UK and Europe. These projects – through their diversity, building systems, number of components, finishes or aesthetics, all help contribute to the transition towards the circular economy and net zero construction. n

About the authors

Sabrina Pedrono General Manager – France Douglas www.france-douglas.com

Elisabeth Piveteau-Boley Director UK & Ireland – Piveteaubois www.piveteaubois.com/en/

Materials Timber for construction

Photo: Fred Howarth