What wood is best for boat trailer bunks, and how should it be protected?

For most fibreglass boat trailers, choose sound pressure-treated pine with straight grain, sized to match the original bunks and covered with purpose-made bunk carpet. Treat fresh cuts and drilled holes with a compatible end-grain preservative, provide drainage, and keep copper-treated timber away from aluminium hulls and components unless an approved isolation system is fitted.

Check the original bunk dimensions

Check the trailer maker's bunk dimensions, bracket spacing and hull support positions before buying timber. Strength and flexibility matter as much as rot resistance. A board that is too stiff can leave the hull resting on a few pressure points; one that is too thin can sag between brackets.

Remove the boat before replacing its supports, or have a yard support it properly on rated equipment. Never work beneath a hull held only by a jack.

• Measure the actual board thickness, width and length, and note whether it sits flat or on edge.
• Photograph the brackets and mark their positions before dismantling.
• Check for cracked brackets, elongated bolt holes and corrosion hidden beneath wet carpet.
• Look for matching pressure marks on the hull, which can indicate poor bunk alignment.

Common American nominal sizes are 2 × 4in and 2 × 6in, typically about 38 × 89mm and 38 × 140mm finished. UK timber sold under similar descriptions may measure differently. Match the actual section rather than relying on the name.

Choose timber to suit the hull

Pressure-treated pine is usually the economical choice for a fibreglass hull. Choose structural-grade boards with straight grain. Reject large knots, splits or excessive twist, particularly around mounting positions. Ask for treatment suitable for repeated wetting. In the UK, UC4-treated timber is a sensible specification where available, but it is not a specification for permanent seawater immersion.

Douglas fir is a useful alternative where specified by the trailer builder, particularly when avoiding copper preservatives. Select sound structural timber, preferably with a high proportion of heartwood. Untreated sapwood remains vulnerable beneath damp carpet. Cypress is another established option where readily available.

Oak and teak are generally unnecessary expenses. Ordinary plywood, MDF, chipboard and decking made from wood-plastic composite are poor substitutes for structural bunk boards. Composite decking can creep under sustained load unless expressly designed for this application.

For aluminium boats, check the hull maker's requirements before choosing treated timber. Many modern preservatives contain copper, which can promote aluminium corrosion when wet. Carpet alone does not provide dependable electrical isolation. Use the maker's specified timber and covering, or an approved plastic bunk system.