At a glance
The joist frame is the hidden structure that determines whether a deck performs well for decades or starts to bounce, flex and deteriorate within a few years. Most visible deck failures – boards that spring underfoot, surfaces that become uneven, screws that start backing out – originate not in the boards themselves but in a sub-frame that was under-specified, incorrectly spaced or poorly detailed at the fixings. Getting the joist sizing and spacing right adds no cost to a deck build when planned from the start, but correcting a frame that is already boarded over is disruptive and expensive, typically requiring the entire board surface to be lifted before any remediation work can begin. The decisions made at frame stage are the ones that determine the long-term quality of the deck, and they are made before a single board is placed.
This guide covers the correct timber sizes and treatment specifications for domestic garden decking joists in the UK, the spacing rules for different board types and laying patterns, and the fitting details – joist hangers, noggins and bearer connections – that determine the long-term stability of the frame. The focus is on standard ground-level domestic decking up to 30cm above ground level. Raised decks above this height involve additional structural considerations and may require building regulations approval depending on the specifics of the project. The principles for joists and their connections are the same regardless of height, but the bearer sizes and post section requirements increase with height and load, and for any elevated structure above 30cm it is worth consulting a structural engineer or experienced contractor to confirm the frame design is adequate. Getting professional input at design stage is considerably cheaper than correcting an under-specified frame after the deck is built.
What You’ll Need
Joist Sizes and Timber Specification
The standard joist size for domestic garden decking in the UK is 47x150mm C16 or C24 graded pressure-treated softwood. This section size provides adequate rigidity for normal residential loading at joist spacings up to 400mm centres, which is the maximum recommended spacing for most standard 145mm decking boards. For spans over 2.4 metres between bearer supports, or for decks intended to carry heavier loads such as hot tubs, outdoor furniture clusters or groups of people, upsize to 47x200mm joists and reduce the bearer spacing accordingly. The depth of the joist section – 150mm vs 200mm – is what determines deflection under load across the span. Width (47mm) stays constant because it provides the bearing surface for fixings and the face for noggins, and reducing it to save timber is not recommended regardless of the loading situation.
Timber specification matters as much as size. All decking joists must be pressure-treated to UC3b or UC4 standard – the treatment rating indicates the level of protection against fungal decay and insect attack, and the UC4 rating is appropriate for any timber in ground contact or in positions where drainage is restricted. C16 or C24 strength grading ensures the timber meets minimum structural standards – never use ungraded timber for structural joists regardless of how sound it looks, as ungraded timber can have hidden defects that reduce its effective strength significantly below what the section size suggests. When cutting treated timber on site, coat all cut ends immediately with an end-grain timber preservative – the pressure treatment does not penetrate all the way through a 150mm joist, and fresh-cut ends expose untreated wood that will decay significantly faster than the treated surface.
Spacing Rules for Different Board Types
The joist spacing you set is determined by the board laying direction, not just the board type. Standard 145mm decking boards running perpendicular to the joists can span 400mm centres. The same boards running at 45 degrees diagonally require joists at 300mm centres because the diagonal span between joist contact points is longer. If you are planning a herringbone or diagonal board pattern, plan the joist spacing first – it is a structural decision, not an aesthetic one. A diagonal board pattern on a frame spaced for perpendicular boards will produce a bouncy, under-supported surface regardless of board thickness or quality, and no amount of screwing the boards down more tightly will compensate for inadequate joist spacing beneath them.
Always verify the maximum joist spacing with the specific board manufacturer’s installation guidelines before setting out the frame – the figures above are general guidance for typical products and some composite ranges specify tighter or looser spacings based on the engineering properties of that particular board. Using a wider joist spacing than the manufacturer specifies voids any warranty and is a common source of board deflection and springiness that makes the deck feel bouncy underfoot. When in doubt, go closer – 350mm centres rather than 400mm costs a little more timber but produces a noticeably stiffer and higher-quality feel underfoot. The additional cost of the extra joist per 2.4 metre run is a modest investment that pays dividends in the quality of the finished surface for the lifetime of the deck. Boards that feel solid and unyielding underfoot are one of the clearest quality signals to anyone using a well-built deck, and it costs very little to achieve.
How to Fit Joists Correctly
The correct method for connecting joists to the main bearer is with galvanised or stainless steel joist hangers, not notched or birdsmouth joints cut into the joist or bearer. Notching weakens the timber at its most structurally critical point – the bearing end – and is not appropriate for domestic decking frames. Joist hangers are faster, stronger and allow the joint to be inspected and replaced if needed. Use the correct hanger for the joist section size – a 47x150mm joist requires a hanger sized for exactly that section, not a larger one bent to fit. Fix hangers with the nails or screws specified by the hanger manufacturer: typically 3.75mm or 4mm diameter nails through all the pre-punched holes, not a selection of them. Under-fixing joist hangers is as common as not using them at all – a hanger fixed with three nails out of twelve has a fraction of the designed load capacity and provides false confidence rather than genuine structural security. Take the time to fix all nail holes completely before moving on.
Noggins – short sections of joist timber fitted horizontally between joists at mid-span – are essential for any joist run over 1.8 metres long. They prevent joists from twisting and bowing sideways over time, which causes the deck surface above to become uneven. Space noggins at no more than 1.2 metre intervals along the joist run, staggered slightly to allow easy nailing through the joist face rather than end-nailing. On a standard 3.6 metre joist span, one row of noggins at mid-span is the minimum; two rows at one-third intervals is better practice and costs very little in additional time and timber. Many DIY deck frames omit noggins entirely to save time and material, and the results are typically visible within two to three years as boards begin to follow the line of twisting joists and the surface develops a wave-like unevenness that cannot be corrected without lifting the boards.
Never use zinc-plated or ordinary steel fixings in a decking frame. Standard zinc-plated joist hanger nails and screws corrode rapidly in the wet outdoor environment that a deck frame experiences and begin staining the surrounding timber within two to three years. More seriously, corroding fixings lose structural strength over time. Use only hot-dip galvanised or stainless steel fixings throughout the entire frame, including every nail through the joist hangers and every screw into the bearer. The cost difference is marginal and the long-term performance difference is significant.
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