Many moisture problems in mass timber buildings are associated with exposure during construction, limiting exposure to moisture is imperative


Why Moisture Management is Important

› Expansion. Timber naturally absorbs water and expands when wet. Particularly with large mass timber components this movement can create headaches.
Woodspan timber is kiln dried to 8-16% MC when it is machined to size. For every percentage point that this moisture content increases, width and depth of mass timber members could be expected to expand by (0.5%).

› Distortion. Although less prone to this than solid timber, engineered wood products can still change shape as the timber reacts to moisture. Bow, crook and cupping can all occur with variation in the moisture content within a piece of engineered wood.

› Checking and Cracking. Surface checking can occur as the fibres on the face of the timber contract as they dry out faster than the timber deeper within the component. Additionally, the ends of components are susceptible to cracks and splits as the end grain will absorb and release moisture faster than the rest of the timber.

› Mould Growth. Mould thrives in damp humid conditions. Although timber is treated with fungicide, surface dust and dirt can still provide compatible growing conditions for mould. It is important not to enclose wet timber with linings as the moisture will struggle to escape and continue to provide a damp environment for mould growth.

› Surface Staining. Water can leave marks on any timber product. Usually watermarks are only on the surface and a light sand will rectify.

Planning for moisture management

› Build Methodology. One of the key benefits of utilising mass timber on a project is speed of construction. This thinking needs to be applied to all aspects of the project as the best possible way to mitigate the effects of moisture on mass timber is to get the project closed in as soon as possible. Or for exterior applications, get the finished coats of paint or stain on as soon as practicable.
Plan to close in; with any mass timber building the key to moisture management is to close the building in ASAP. Avoid prolonged direct exposure to rain for more than 7 days. Any longer exposure should be dealt with by covering with breathable membrane of sd<0.4m to avoid sweating. PLT should only be enclosed or lined once EMC is 18% or lower (NZBC E2/AS1 Clause 10.2)

› Site Storage. Ideally components should be installed as soon as they are delivered. If products must be stored onsite ensure they are up on dunnage and covered. Product left on the ground will absorb moisture.

› During Construction. Weekly inspections are necessary both visually and with moisture metre readings. Be sure to check that your moisture metre is calibrated and allow for the offset that the manufacturer recommends when testing treated timber. Typically the moisture metre will give a reading that is slightly too high for MCA treated timber and slightly too low for LOSP treated timber. It is always good practice to take multiple readings at different locations. Keep the PLT dry and clean until and throughout installation.

› Weather Protection. We recommend covering exposed components with waterproof tarps, wraps or membranes to prevent rainwater infiltration.

› Waterproof Sealer. Woodspan products are generally coated with a temporary waterproof sealant before they leave the factory. This provides short term protection for around 4-6 weeks and does not interfere with stains or polyurethanes. Contact us for more information regarding this sealant. Additional coats of sealant may be applied onsite for increased protection. Once the PLT is cut or drilled, seal the open timber immediately.

What to do if Mass Timber Gets Wet

› Swift Action. Address any moisture issues promptly to prevent further damage. Sweep away pooling water and ideally get the timber covered.

› Drying Out. Allow wet components to dry down to 14% MC before enclosing them. It may be necessary to utilise dehumidifiers – particularly if floor panels have become very wet.

› It is recommended to avoid installing a high density of dowel connections or concentrated screw patterns with multiple washer heads (or similar) until the in service equilibrium moisture content (EMC) stabilizes within ±5% of its expected value. Exceeding this threshold can lead to increased stress on the screws, potentially causing unnoticed ruptures.

PLT Panels

To maintain the look and functionality of Woodspan PLT panels, exposure to moisture must be controlled.
During all stages of supply, storage and installation, climatic conditions shall be as close as possible to in-service conditions.
Where changes to EMC cannot be avoided, the coefficients for shrinkage and expansion of wood must be taken into consideration.

Managing Moisture Before a Building is Weathertight
› Ensure water does not pond on the surface, Sweep ponded water from the surface,

› Cover the construction area where possible.

› Woodspan PLT should only be enclosed or lined once EMC is 14% or lower,

› 5mm expansion gaps perpendicular to panel edge must be allowed for,

› Ceiling linings susceptible to cracking (e.g. stopped plasterboard) should not be fixed directly to Woodspan® PLT. Instead a layer of battens/strapping should be installed (refer Woodspan PLT standard details).


Key points are:
› Keep the glulam dry and clean until installation.

› Plan to close in; with any mass timber building the key to moisture management is to close the building in ASAP.

› Glulam should only be enclosed or lined once EMC is 18% or lower (NZBC E2/AS1 Clause 10.2)

› As standard practice we apply a temporary moisture sealant in the factory (that lasts 6 weeks) to help withstand moisture while the building is closed in, if it is unavoidable to have the timber exposed for longer – reapplying a moisture sealant is important.

› Once the glulam is cut or drilled, seal the open timber immediately.

› For any further questions please contact the Woodspan team.



Jared Wilson

Market Manager
Richard Peach

Business Development Manager


More information on our full product range, PLT; Floor Panels, Roof Panels, GluLam beams and Long-span Decking. Including links to all the resources & downloads.