Redwood makes an excellent substitute for cedar exterior cladding provided sapwood is excluded and hot-dipped galvanised, silicon-bronze, stainless steel or copper nails are used. In situations where corrosion of fastenings is a problem e.g., areas exposed to salt laden winds, particularly on buildings where the head of the nail is left exposed, annular grooved silicon-bronze or stainless steel nails are a better option. It makes excellent outdoor furniture provided it is used in chunky dimensions. Previous uses in New Zealand for redwood have included exterior finishing work, and panelling.
Californian redwoods are one of the largest-growing tree specimens, and can reach a height of up to 120 metres.
The Redwood Memorial Grove in Rotorua is perhaps the best location to view these magnificent trees, which were planted in 1901.
The heartwood of New Zealand-grown redwood is a warm red which changes to a mellow nut-brown on exposure to sunlight.
It is much faster-growing than redwoods grown in the USA, and therefore much softer.
Redwood is a softwood displaying similar properties to Western red cedar. Larch and redwood are included in MAF statistics which show a total planted area of 26600 ha for softwoods other than radiata, Douglas fir and cypresses. The median age is approximately 15 years.
Botanical name: Sequoia sempervirens
Other common names: Californian redwood
Strength: Not strong.
Durability: Durability is extremely variable. The heartwood of redwood has Class 3 durability (moderately durable, average life 5 to 15 years in ground contact) and is resistant to impregnation with preservatives. The sapwood of redwood is treatable with boron using diffusion processes. Penetration is likely to be variable using CCA preservatives or LOSP and conventional pressure treatment processes.
Finishes: Redwood is easy to machine, glue and paint.
Working properties: Redwood’s working properties are variable. Timber from the best specimens is easily worked, shrinks little, and is very stable in service. Lower quality specimens have lesser stability and/or durability.
Appearance : The heartwood of New Zealand-grown redwood is a warm red which changes to a mellow nut-brown on exposure to sunlight
Redwood can be stable but its stability and durability is variable. Timber from the best specimens is easily worked, shrinks little, and is very stable in service.
The heartwood of redwood is durable (15 to 25 years in ground contact) and is resistant to impregnation with preservatives.
New Zealand-grown redwood timber appears to be less durable than North American sequoia timbers, with an expected life for decking of 10-15 years if left untreated.
The sapwood can be treated by the boron diffusion process if desired.
Redwood is easy to machine, glue and paint. Redwood has an average density of only 360kg/m³ but can vary from 250 to 400kg/m³. It is not a strong timber, but this does not detract from its use as a lightweight panelling and cladding material.
Strength values refer to 20x20mm clearwood specimens.
Please note these comparative measures of strength are “laboratory” values using standardised short lengths of clear timber. These will not be the same strength properties as structural lengths of timber. For the properties of structural length timber please see the tables under the structural design section.
Density at 12% M.C. 360 kg/m3 Modulus of elasticity 6.6 GPa Bending strength 63 MPa Compression strength 36 MPa Hardness 1.9 kN Shrinkage green to OD, tang 2.5% Shrinkage green to OD, rad 1.5%
Refer also: Cown, D J. 2008. Redwood in New Zealand – An end-user perspective. New Zealand Journal of Forestry 52(4): 35-41.
Knowles, F B; Miller, J T; 1993. Introduced forest trees in New Zealand: Recognition, Role and Seed Source. 13. The redwoods. FRI Bulletin No. 124 Part 13. Available from National Forestry library for $14.65
- Case Study
Here redwood is featured in an interior use. The wood was filleted and air dried for 12 – 15 months and the heartwood was picked out for sarking, with no treatment being necessary. The timber is easy to saw and is extremely light and easy to work with. Although dead knots can occur, defects generally add character to feature walls or ceilings.