Glulam in corrosive farm environments. Specifications, compliance and four New Zealand builds

Ammonia, effluent and daily wash-down corrode steel. In a composting barn the air sits warm and humid for 12 months of the year, loaded with ammonia off the pack. On a wintering pad it is effluent and wash-down, over and over. Add coastal air on the West Coast or Banks Peninsula and the exposure stacks.

Timber does not rust. That single fact removes the main failure mode of a steel farm building, and it is why Numat specifies glulam for composting barns and wintering pads.

This page sets out what the material is, what it is certified to, how it performs, and where we have built with it. The structural data is Prolam’s published engineering data. The farms are ours.

What NuSpan glulam is

NuSpan is the Numat name for the glulam we build with. It is Prolam PL8 glulam, designed and manufactured in New Zealand by Prolam at two sites near Motueka in the Tasman district.

Glulam is glued laminated structural timber. Strength-graded, kiln-dried New Zealand radiata pine is finger-jointed and laminated into a single structural member, with laminations between 28.5 mm and 50 mm thick. Natural defects such as knots and poor grain orientation are removed during manufacture. The result has uniform, reliable properties with higher strength and stiffness than sawn timber of the same section, and far less tendency to twist, split or shrink.

Resorcinol and melamine based structural adhesives are used for finger-jointing, and resorcinol based structural adhesives for laminating, to AS/NZS 1328 Parts 1 and 2 and AS/NZS 1491. Moisture content leaving the factory is 12 to 18 per cent.

NuSpan is supplied only as part of a Numat Agri Construction build. We do not sell the beams separately, because the structure performs as designed only when the engineering, the treatment, the fixings and the coating are specified together.

Treatment, and why it goes all the way through

Prolam PL8 beams for exterior and weather-exposed use are H3.2 treated with MCA. Posts are MCA H5 treated for in-ground and weather-exposed applications.

The detail that matters on a farm build is when the treatment is applied. H3.2 and H5 treatments are applied before the timber is laminated into the final member, so treatment level is consistent through the whole section, unlike large sawn members and poles where treatment may not reach the core.

In a composting barn, where humidity is constant and the structure is never fully dry, that consistency is the difference between a treated surface and a treated beam.

Treatment is carried out to NZS 3640:2003, AS 1604.1:2012 and AS/NZS 1604.5:2012. Prolam treats its own timber and is third party audited for compliance with the New Zealand treatment standard.

Corrosion resistance

Prolam structural laminated timber is already specified in genuinely hostile environments. Corrosive chemical stores. Fell-monger plants. Acid plants. Salt-water environments. Swimming pools.

It performs in those settings for two reasons. It does not rust, and the adhesives used are certified to service class 3, the wettest and most humid service condition in the standard.

A composting barn is a service class 3 environment. So is a washed-down wintering pad.

Durability and building code compliance

Prolam products are manufactured under an approved quality system based on the ISO 9000 series, with the production process certified by Bureau Veritas.

Used in accordance with Prolam product literature and NZS 3602, Prolam glulam meets the requirements of clause B2 of the New Zealand Building Code for 50 year durability. That is a compliance position, not a marketing estimate.

Compliance summary

Code clause

Position

B1 Structure

Designed to NZS AS 1720.1:2022, currently an alternative solution for B1. Design actions to AS/NZS 1170 Parts 0, 1, 2 and 3 and NZS 1170.5. Also covered by B1/AS1 via AS1170:2002 span tables, AS/NZS 1328:1998 and AS5068

B2 Durability

Meets B2.3.1(a) and B2.3.2, and 50 year durability when used to NZS 3602. Service class 3 permitted with H3.2 treatment or higher

C Protection from Fire

Meets C6.3 and C6.4. APA evaluation of char rate to ASTM E119, TT-118, January 2015. FRR design to TT-118 or NZS 3603:1993 section 9

F2 Hazardous Building Materials

Meets F2.3.1. Treatment suppliers meet NZS 3640 including flash-off requirements

Manufacturing standards: AS/NZS 1328.1:1998, NZS 3640:2003, AS 1604.1:2012, AS/NZS 1604.5:2012.

Structural properties

Characteristic strengths for Prolam PL8 at moisture content of 15 per cent or less. These values apply to beams and posts, including bending about the weak axis.

Property

PL8

Modulus of elasticity, MoE

8,000 MPa

Modulus of rigidity, G

530 MPa

Bending, f'b

19 MPa

Shear in beams, f's

3.7 MPa

Tension parallel to grain, f't

9.0 MPa

Compression parallel to grain, f'c

21.6 MPa

Bearing perpendicular to grain, f'p

6.9 MPa

Design density

450 kg/m³

Characteristic density

375 kg/m³

Joint group

JD5

Material constant ρb

0.88

Design density is used to calculate the self-weight of the member. Characteristic density is used for connection design using the detailed method.

A capacity factor of 0.8 applies to design capacities. The moisture content factor k4 is 1.0 for members below 15 per cent moisture content. For PL8 and PL12 beams at that moisture content, a creep factor j2 of 1.5 may be used for members 90 mm wide or more. No modification for temperature is needed at New Zealand ambient temperatures, which means a barn roof structure is not being designed around thermal movement the way a steel one is.

Sizes, lengths and achievable spans

Non-visual PL8 beams are produced in depths from 88 mm to 595 mm and widths from 42 mm to 300 mm. Posts run from 88 by 88 mm to 300 by 300 mm.

H3.2 and H5 treated beams are available in standard lengths of 3.6 m, 4.2 m, 4.8 m and then up to 18 m in 600 mm increments, which is what makes long clear spans possible without a post line down the middle of a barn.

Post lengths run from 2.4 m to 12.0 m in standard increments. Non-standard sizes are available on request, subject to confirmation before final specification.

At Willesden Farms we specified 64 Prolam PL8 H3.2 beams at 595 mm by 140 mm, up to 16.2 m long, used as rafters spanning 12 m with a 4 m overhang. It gave us 12 m of clear floor with nothing in the way of a tractor, a bucket or a mixer wagon.

Treated PL8 and PL12 members weigh approximately 550 kg per cubic metre, which governs craneage and handling on site. On the Willesden section that works out at roughly 750 kg per rafter, so every one of those 64 beams went up on a machine.

Fire performance

Timber chars at a known and predictable rate, and retains structural capacity behind the char layer. Charring rates assigned by BRANZ appraisal on the basis of density:

Density

Charring rate

400 kg/m³

0.75 mm/min

500 kg/m³

0.70 mm/min

600 kg/m³

0.65 mm/min

PL8 has a design density of 450 kg/m³, so a large section retains load-carrying capacity for a measurable and calculable period in a fire, which is more than can be said for an unprotected steel portal. Where a fire resistance rating is required, design is to TT-118 or NZS 3603:1993 section 9, in conjunction with a fire engineer.

Dimensional stability, precamber and tolerances

Non-visual beams are supplied with a default precamber radius of 600 m. Visual beams have no precamber by default. Custom precamber is available on request.

Manufacturing tolerances at time of manufacture, before any dead or live load:

  • Cross-sectional dimensions: plus or minus 2 mm
  • Crook, camber and straightness: 1 mm per 1.0 m to a maximum of 6 mm over 6.0 m. Beyond 6.0 m the tolerance increases by 3 mm per additional 6.0 m or part thereof, to a maximum of 19 mm
  • Squareness: plus or minus 2 mm per 315 mm of specified depth

On a 16 m rafter, a maximum out-of-straight tolerance of 19 mm is what allows a 64 beam roof to go up without the fitting problems that eat days on a big farm build.

What it is like to be in one

Farmers do not usually raise this one first. It comes up on the second visit, standing in the middle of a finished barn, and it is nearly always some version of the same thing. It is a good place to be.

Part of that is light. Timber does not reflect light the hard way a bright metal surface does, so a barn with a timber roof structure sits in a softer, warmer light through the middle of the day. You notice it most in the shoulder seasons, when the sun is low and coming in under the eaves.

Part of it is scale. A 16 m rafter is a substantial piece of timber, and a line of them running the length of a barn reads as structure you can see and understand. Nothing is hidden. A farmer walking their own barn can look up and see exactly what is holding the roof on, which matters more on a building they will be under every day for the next 50 years.

And part of it is that it sits on a farm without shouting. Timber weathers rather than degrades. It sits alongside shelter belts and older farm buildings rather than against them, and it does not need to be hidden behind planting to look like it belongs. On a farm where the family lives on site and looks at the building from the house every day, that is not a small consideration.

Noise, and why cows notice it before you do

Cattle hear a wider range than people, and hear it better. Cattle hearing runs from 23 Hz to 35 kHz against a human range of roughly 20 Hz to 20 kHz, with best sensitivity at 8 kHz and a lowest threshold measured at minus 21 dB. A sound that registers as a whisper to the person walking the barn is plainly audible to every cow in it.

The threshold that matters is 70 dB. Above that, published dairy research associates barn noise with measurable welfare effects: raised heart and respiratory rate, raised body temperature, disturbed rumination, reduced milk production, and higher somatic cell counts. Sudden and intermittent noise is harder on stock than constant noise, which is the reason a hailstorm unsettles a shed full of cows in a way that a running fan does not.

A bare steel panel fixed to open purlins with nothing behind it produces 80 to 90 dB inside during rain and hail. That is a roofing assembly figure rather than a structural one, and it applies to any barn clad that way regardless of what the rafters are made of. We are not going to tell you a timber barn is silent in the rain. It is not.

What the structure does change is what happens to that energy once it is in the frame. Steel has a loss factor of about 0.001, which is effectively no material damping, while measured damping ratios for glulam beams run from about 0.5 per cent for slender sections to about 1.0 per cent for stockier ones. A steel portal carries impact energy along the frame and re-radiates it. A glulam frame absorbs a far greater share of it as heat before it ever reaches the animals below.

What actually quietens a barn

Honest answer first. The structural frame is the smaller lever. The bigger ones are the roofing assembly and the internal surfaces, and they are specification decisions rather than material ones.

Profile and gauge. Heavier gauge steel with a ribbed or trapezoidal profile is stiffer and drums less than thin flat sheet. This is the single largest change available on a farm roof.

Fixing detail. Every hard fixing is a path for impact energy into the frame. Fixing into timber rather than steel breaks the metal-to-metal path at every purlin.

The compost bed itself. A 800mm deep woodchip and sawdust pack is a substantial absorptive surface across the whole floor of the barn. On a composting barn the bedding is doing more acoustic work than any structural decision, which is part of why a composting barn is a calmer building than a bare concrete feed pad.

Curtains and side cladding. Mesh curtains, as used at Creaming It, are acoustically open rather than reflective, so they do not create the hard parallel surfaces that build reverberation in a long building.

Where we have built with NuSpan glulam

Four builds from the last two seasons, chosen because they cover the range. A 185 m composting barn on Banks Peninsula, a mono-pitch hybrid in an extreme wind zone, a flood-washed wintering barn on the West Coast, and a centre pole barn in Southland. Different layouts, different climates, same structural material.

Willesden Farms, Banks Peninsula, Canterbury. Composting barn.

185 m by 31 m, part of a 300 hectare dairy conversion. Winters 500 cows and feeds up to 1,000. Designed by Numat, engineered with DE Consulting and Engineers, built by Mitch Frost. 64 Prolam PL8 H3.2 rafters at 595 mm by 140 mm, up to 16.2 m long, 12 m clear span with a 4 m overhang.

Matt Iremonger, General Manager at Willesden Farms: “This eco-friendly alternative to steel-framed barns is up to 20% more cost-effective than traditional builds. We really like the timber construction. We think it’s a really strong design, a durable design, and one that works really well in the environment.”

Creaming It, Carterton, Wairarapa. Hybrid composting barn.

Willie and Sally Bosch, 250 cows. 82.8 m long by 14 m wide, with a 12 m central loafing and compost area and feed faces on both sides. Mono-pitch roof at a 5 degree slope. H5 treated timber poles with Prolam rafters, mesh curtains, and a compost bed close to one metre deep. Built mid-May to end of July, three months start to finish, for $350,000 including earthworks, concrete, curtains and feed face.

The Carterton site sits in an extreme wind zone, which is the reason the structure was specified the way it was. Willie wanted a building that would, in his words, “still be standing here in 50 years time”.

First full season: 30 per cent less pasture damage, milk solids lifted from about 1.9 to just over 2.0 per cow, and no mastitis case picked up since mid-August. On the first morning with cows inside, not a single one was standing.

Kaitiaki Lands, Greymouth, West Coast. Wintering barn with flood wash.

44 m long by 30.5 m wide with a 1 m overhang each side, 1,342 m². Houses and feeds 250 cows through 10 plus weeks of a wet West Coast winter. Centre feed lane, Siesta rubber mats on the loafing pad, NuSpan timber roof. Circa $900,000.

A full flood wash system runs off 60,000 litres of greenwash storage, which means this structure is washed down with recycled dairy shed effluent as a matter of routine. That is the most aggressive service condition of the four builds, and it is exactly the case glulam is specified for.

Glenfruin Farm, Gore, Southland. Centre pole composting barn.

24 m by 94 m with a 1 m overhang, 2,256 m². Built to winter 250 cows on farm, off the wet paddocks and calving down dry. NuSpan timber roof, red colour steel, barn liner, full drainage and stormwater, feed face retaining walls. Circa $2,500 per cow plus earthworks.

Centre pole design, so the roof structure works off a centre line rather than one long clear span, which brings the cost per square metre down on a barn this size.

Specification and installation requirements

Timber performs when it is specified and handled correctly. These are conditions of the material, and they are managed as part of a Numat build rather than left to chance on site.

Coating. H3.2 and H5 members used in exterior situations must be finished with a paint or stain coating within 14 days of installation. Exposed end grain and all cut ends must be sealed with a good quality stain or alkyd primer. Coatings must have a light reflectance value above 45 per cent, because darker colours absorb heat and can cause distortion and cracking in structural timber.

Fixings. MCA treated timber from H3.2 to H5 can accelerate corrosion in standard steel fasteners. Nails and screws are specified for exterior and exposed structural timber. Getting this wrong is how a timber building fails, and it fails at the connections rather than in the beam.

Holes and notches. Drilling or notching is permitted only where detailed by the engineer or the Prolam Specifier. Holes must be small, in the middle third of the span, centred vertically, and never at beam ends or where supports and fixings are required. Unapproved holes can void compliance.

Storage and handling. Stored under cover and dry until installation, stacked on closely and evenly spaced bearers, elevated off the ground for ventilation, with midspan support on longer beams to prevent creep deflection during construction until the building is enclosed.

The cost position, stated plainly

Glulam is not the cheapest structure on day one. At Willesden it was not cheaper than steel upfront. It was cost-competitive, efficient to construct, and it carries lower lifecycle and environmental cost over the life of the building. If you are choosing on the lowest tender you will not choose timber, and if you are choosing on 50 years of ammonia you will.

There is no corrosion maintenance programme on a glulam structure. There is a recoating cycle on exposed members, carried out to the coating supplier’s instructions.

Built with Prolam

Prolam has been designing, manufacturing and supplying glue laminated timber in New Zealand for more than 20 years, from two sites near Motueka. They are certified to produce PL8, PL12 and PL17 grades under AS/NZS 1328.

Prolam’s structural design input shaped the Willesden barn, with the building developed around what the timber could actually do rather than the timber being substituted into a steel design. Prolam have published the Willesden Farms barn as a case study on their own site.

Prolam PL8 also uses 14 times less energy to produce than the equivalent steel beam, and FSC certified timber options are available.

Numat designs the farm system. Prolam engineers the timber. Your engineer signs off the structure.

Willesden composting barn

What farmers ask before they commit

Does glulam rust in a composting barn?

No. Timber does not rust. Prolam structural laminated timber is specified in corrosive chemical stores, acid plants and salt-water environments, and the adhesives are certified to service class 3, the wettest and most humid service condition in the standard.

Used in accordance with Prolam product literature and NZS 3602, Prolam glulam meets clause B2 of the New Zealand Building Code for 50 year durability. The Prolam glulam structure at Willesden Farms carries a projected lifespan of 50 plus years.

Yes. Exposed timber greys under UV, and MCA treatment gives new members a green cast that fades as the building settles in. Neither is a structural issue. The colour change happens at the surface. Strength, stiffness and durability are unaffected, and the 50 year durability position under NZBC B2 is not a function of appearance.

H3.2 MCA for exterior and weather-exposed beams, H5 MCA for in-ground posts. Treatment is applied before lamination, so the treatment level is consistent through the full section rather than sitting on the surface.

H3.2 and H5 treated beams are produced in standard lengths up to 18 m in 600 mm increments. At Willesden Farms, 16.2 m rafters gave a 12 m clear span with a 4 m overhang.

Under the roof, the difference is small. Rain noise is driven by the cladding and its fixing, not the frame, and a steel panel fixed to open purlins produces 80 to 90 dB inside during heavy rain either way. The difference glulam makes is in the frame itself. Steel has a loss factor of about 0.001, effectively no material damping, while glulam damping ratios are measured at 0.5 to 1.0 per cent, so a timber frame absorbs impact and vibration energy that a steel one passes along and re-radiates.

Published research puts the working threshold at 70 dB. Above that, barn noise is associated with raised heart and respiratory rate, disturbed rumination, reduced milk production and higher somatic cell counts. Cattle hear from 23 Hz to 35 kHz with best sensitivity around 8 kHz, so they register sounds a person standing next to them does not.

Not upfront. At Willesden Farms the glulam solution was cost-competitive with steel rather than cheaper, and the owner has since described the build as up to 20% more cost-effective than traditional builds. The material saving is in lifecycle cost, because there is no corrosion to repair or replace.

Nails and screws are specified for exterior and exposed structural timber. MCA treatment accelerates corrosion in standard steel fasteners, so fixing specification is not optional detail.

Yes. Prolam glulam adhesives are certified to service class 3, the wettest and most humid service condition in the standard, and the timber does not rust. The Kaitiaki Lands wintering barn in Greymouth runs a full flood wash system off 60,000 litres of greenwash storage, so the structure is washed down with recycled dairy shed effluent as part of normal operation.

Timber chars at a known rate and retains capacity behind the char layer. BRANZ assigned charring rates run from 0.65 to 0.75 mm per minute depending on density. Where a fire resistance rating is required, design is carried out with a fire engineer to TT-118 or NZS 3603:1993.

No. NuSpan is supplied as part of a Numat Agri Construction build, because the beam performs as designed only when engineering, treatment, fixings and coating are specified together.

References

Product and engineering data
Prolam, Specific Engineering Design Guide. Structural design properties, densities, treatment, charring rates, sizes and lengths, precamber, manufacturing tolerances, coating and handling requirements.
Prolam, PL8 Glulam Beams product data. prolamnz.com/products/beams/prolam-pl8-glulam-beams
Prolam, Willesden Farms Composting Barn case study. prolamnz.com/inspiration/case-studies/willesden-farms-composting-barn

Standards and code
AS/NZS 1328.1:1998, Glued laminated structural timber.
AS/NZS 1491, Finger jointed structural timber.
NZS 3640:2003, Chemical preservation of round and sawn timber.
AS 1604.1:2012 and AS/NZS 1604.5:2012, Timber preservative-treated.
NZS AS 1720.1:2022, Timber structures, design methods.
AS/NZS 1170 Parts 0, 1, 2 and 3, and NZS 1170.5, Structural design actions.
NZS 3602, Timber and wood-based products for use in building.
NZS 3603:1993, Timber structures standard.
New Zealand Building Code clauses B1, B2, C and F2.

Animal welfare and acoustics
Heffner and Heffner, behavioural audiograms for cattle and horses.
Dimov, Penev and Marinov, “Importance of Noise Hygiene in Dairy Cattle Farming: A Review”, Acoustics, 2023.
Lanier, Grandin, Green, Avery and McGee, Journal of Animal Science 78, 2000, citing Talling et al. on reactivity to intermittent sound.
Labonnote, experimental modal analysis of glulam and solid timber beams.
Standard structural damping data for steel loss factor.
Roofing industry measurement of rain noise on metal cladding fixed to open purlins.

Numat project records
Willesden Farms, Banks Peninsula. Creaming It, Carterton. Kaitiaki Lands, Greymouth. Glenfruin Farm, Gore.

Go and stand in one

Specifications tell you what the material does. They do not tell you what the building feels like at six in the morning in July, or what the owner would change if they built it again.

Numat can arrange for you to visit any of the four builds on this page and talk to the farmer running it. Not a sales visit with us doing the talking. You walk the barn, you ask your own questions, and you get the answers from the person who signed the cheque.

Farmers who have been through it are the most honest source of information you will find on a build like this. Over 60 came through Creaming It on the open day and put Willie on the spot for two hours. That is the conversation worth having before you commit.

Call Milan Renton on 021 571 203 and tell him which build you want to see.

Let’s Price It Right

Talk to Milan Renton about a glulam barn or wintering pad for your farm. He will walk your site, work through span and layout, and tell you straight whether glulam is the right call for what you are building.

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