Secret nailing is a popular way of showcasing the beauty of timber flooring by creating a floor with no visible nailheads or fixtures. Visual appeal is the main drawcard: all you see is the beauty of the wood. Without visible fixtures, its texture and grain is emphasised, and the floor has a smoother appearance. There are no nailhead cavities filled with wood putty, and finishing procedures such as sanding and coating will not be affected.

The secret nailing process typically involves fixing the planks at the edge with the nails driven at a 45-degree angle. The planks are also glued to the joists or substrate for added strength, and several companies manufacture timber flooring with proprietary fixing systems.

Australian tradesmen have known about secret nailing timber flooring since at least the 1880s, but it was not common practice until nail guns and other modern fixing methods became widespread. Imagine trying to manually drill holes and drive hundreds of nails at a 45-degree angle through the edge of timber flooring planks. Even with a nail gun, it is a job for skilled installers.

While secret nailing is not new, there is a growing trend towards secret nailing wider flooring boards, which can help provide a sense of openness to a room. Dave Angus, marketing manager at Boral Timber, says that while the previously accepted practice in Australia was that you could not secret nail a board wider than 80 mm, Boral Timber’s 108 mm and 130 mm Uni-nail hardwood flooring is purposely designed for secret nailing, with a range of popular species sourced from Australian Forestry Standard (AFS) certified forests.

“We have had independent tests done demonstrating that with correct acclimatisation and installation, Uni-nail hardwood flooring widths up to 130 mm can be secret nailed. Although with commercial applications you can expect heavier foot traffic and heavier usage, this shouldn’t be an issue,” Angus says.

Installation for Boral Timber’s 108 mm and 130 mm Uni-nail hardwood flooring involves a combination of cleats and full trowel adhesive to attach the boards to a ‘solid’ sheet subfloor. Suitable subfloors to which wide boards can be secret nailed include 15 mm+ Boral plywood and 19 mm+ particleboard flooring. Each must have a moisture content within 2 per cent of the acclimatised timber flooring being installed.

Some more brittle timber species may occasionally split, especially if the board is overly dry. When Roger Johnson, associate at Suters Archietcts, specified secret nailing for the Sydney Bluegum flooring in his own residence, there were some early problems with splitting occurring along the tongue, which the builder solved by changing the nail gauge.

“Installers need to be skilful with the nail gun. As with visible nailing, they need to be competent with cramping pressure and have to move quickly, so as to avoid cupping caused by excess pressure,” Johnson says.

Johnson specifies secret nailing for most jobs with tongue and groove flooring to maximise its natural beauty. He believes that if you find the right contractor, it is less work without the need to go back and plug all the nail holes.

Sydney Flooring uses secret nailing in both commercial and domestic projects. Managing director and founder, Bill Durkowyak, says that while the company has been secret nailing timber flooring for many years on thousands of projects, secret nailing wider boards is only a new practice. There is now a secret nailed 130 mm wide board display at Sydney Flooring’s Surry Hills showroom, and the company has already undertaken several successful residential jobs.

“The widest size boards are 133 mm wide. For any boards wider than this, they must be top-nailed,” Durkowyak says. “When secret nailing the wider boards, we need to fully adhere the back of the boards to the substrate as well as nail. The wide boards can be secret nailed onto 15 mm (or thicker) plywood and 19 mm (or thicker) particleboard flooring.”

Basically, all hardwood flooring is suitable for secret nailing. Common hardwood species include Blackbutt, Blue Gum, Brushbox, Spotted Gum and Stringybark, although the choice is more extensive than this. While definitely not recommended for DIY installs, apart from fixing each board individually, secret nailing is a fairly straightforward process. However, careful preparation is necessary. The substrate needs to be flat and dry prior to work commencing. It is also important to ensure the sub floor has a moisture content within 2 per cent of the timber flooring that is being installed, according to Durkowyak.

Russell Grainger, manager at CAA Timbers, says that secret nailing suits any application where top nailing is specified, including commercial projects. “I believe that secret nailing can’t be used in the key of a basketball court, but I’m not aware of any other limitations in regular or commercial installations.”

Secret nailing is slower than top nailing when dry fitting over joists, as each board has to be fixed through its edge, whereas with top nailing, groups of boards can be cramped together and quickly fixed through the top of the boards. But a skilled installer can save time at the other end of the job because there are no nails to punch and fill.

While CAA Timbers only facilitates installation, Grainger says all the installer crews use secret nail profiles, as top nail profiles have undesirable exposed fixings on the surface of the boards. “All popular flooring species in 60 mm and 80 mm cover versions are produced as secret nail profiles. Anything over 80 mm cover or wider is produced as a top nail pattern,” Grainger says.

If timber floors are to be dry fitted over joists, then one nail through the edge of the board is fine for 80 mm or narrower boards, but anything wider needs to be top nailed, unless it is glued down as an overlay on existing plywood, timber floorboards, particleboard or concrete substrates, according to Grainger.

“With the addition of polyurethane glues, any width or thickness boards can be secret nailed, as the nails just hold the boards down until the glues cure. Any width or thickness can be fixed this way with the correct glues if they are used as specified by their various manufacturers,” Grainger says.

While sticking to the manufacturers’ instructions is common sense for any building product, Angus emphasises that it is particularly important for secret nailing: “Installation is not a complex task if you follow instructions, but we strongly recommend that you use a skilled professional installer,” he says.

With a growing range of quality timber flooring products designed to allow safe installation of wider boards, timber flooring is looking better than ever.