Floor Boarding: Complete Guide for Homeowners

The boards that sit directly on your joists do more than hold you up — they regulate moisture, control deflection, and decide whether your finished floor squeaks in five years or stays silent for thirty. Choosing the right floor boarding is one of the few renovation decisions that affects every single room once the house is finished, yet most homeowners leave it entirely to the builder. This guide covers the real material options, sensible joist spacing, and the fixings that separate a professional install from a creaky disappointment.

What the Deck Actually Does

Structurally, the deck spans between joists and transfers point loads back to those joists without sagging. It also stiffens the floor diaphragm, which keeps partition walls plumb and stops doorways from racking. Functionally, it is the substrate that your underlay, adhesive, or final floor covering clips or glues to.

Get the boarding wrong and you will see it in every finish that follows. Tile cracks along a line where two boards meet. Laminate clicks pop open when the joists flex. Engineered wood cups at the seams because moisture is wicking up through a poorly chosen sheet material.

The Main Material Choices

• P5 moisture-resistant chipboard: 18mm or 22mm tongue-and-groove sheets. Cheapest mainstream option at $0.85 to $1.30 per sq ft. Standard in UK new-builds.
• OSB/3: Oriented strand board rated for humid interior use. 18mm boards at $1.00 to $1.45 per sq ft. Very popular in North American construction.
• Plywood: Softwood or hardwood veneer ply at 18mm or 22mm. Most expensive at $2.20 to $3.80 per sq ft, but the dimensional stability justifies it on tile floors.
• Softwood floorboards: Tongue-and-groove pine or fir, 22mm thick. $3.50 to $5.80 per sq ft. Traditional and still used in restoration work.
• Filmed chipboard (Caberdek, Egger Protect): P5 with a peelable protective film. $1.30 to $1.70 per sq ft. Great for long builds.

Joist Spacing and Thickness

The classic rule is 18mm board for 400mm (16-inch) joist centers and 22mm board for 600mm (24-inch) centers. Go any wider and you start to feel noticeable deflection between the joists, which telegraphs through tile and hardwood alike.

If you are tiling over a timber floor, add a second layer. Most manufacturers require a minimum of 28mm total deck thickness plus a cementitious backer like Hardiebacker or No More Ply before the tile adhesive goes down. Skipping this layer is the single most common reason DIY tile jobs crack.

Fixings That Matter

Screws beat nails every time on modern subfloor panels. Use 4.5mm diameter by 65mm long coarse-thread screws for 22mm boards, at 250mm centers along every joist and 150mm centers on the perimeter. Pre-drilling is rarely needed with modern self-countersinking screws.

Adhesive in the joints matters just as much. D4 PVA along every tongue-and-groove edge pulls the system together into a genuine diaphragm. Skipping the glue is why you hear squeaks years later — the boards are microscopically rubbing against each other.

Pros and Cons by Material

Chipboard

Cheap, consistent, and easy to cut. It hates long-term water exposure and will swell irreversibly at the edges. Fine as a subfloor under LVP or laminate but a liability in bathrooms or under tile without perfect waterproofing.

OSB/3

Slightly stronger than chipboard and marginally more water-tolerant. The rough surface texture can telegraph through very thin LVT, so some fitters prefer to overlay with a 6mm ply before the final floor.

Plywood

The gold standard for dimensional stability. If you are installing solid hardwood, natural stone, or large-format porcelain, budget for 18mm WBP ply and never look back. Expensive, but the finished floor will not move.

Common Mistakes to Avoid

1. Running end joints over the same joist for multiple boards in a row. Always stagger by at least one bay.
2. Skipping the 10mm expansion gap at every wall, which causes peaking as the boards absorb moisture.
3. Using internal-grade ply for a bathroom subfloor. Always specify exterior-grade WBP or marine ply near wet zones.
4. Nailing instead of screwing. Nails work loose within 3 to 5 years and become the squeak origin point.
5. Ignoring the underside. If the joists sit over an unheated cellar or a ventilated crawl space, insulate before you deck over or you will fight cold floors forever.

Budget Planning

For a typical 1,500 sq ft first floor with 400mm joist centers, expect material costs of roughly $1,300 for 18mm P5 chipboard, $1,750 for 18mm OSB/3, or $3,400 for 18mm WBP plywood. Installation labor adds $0.80 to $1.40 per sq ft on top if you hire a trade rather than DIY.

Final Thoughts

Choosing the right floor boarding is the cheapest long-term insurance policy in any renovation. Spend an extra $500 on ply instead of chipboard and you eliminate most of the callbacks that plague tile and hardwood installs. Glue and screw every joint, stagger your end butts, and respect the expansion gap. Do those three things and the finished floor above it will stay quiet, flat, and square for decades.