How much space to leave between decking boards
November 12, 2025
Getting the gap right between decking boards seems like small details, but it's one of those things that separates a final piece that lasts decades from one that starts causing problems within a few years. And here at UK Oak, we do get this question commonly… and there's more to the answer than you might expect.
The Standard Answer (and Why It's Not Always Right)
Most installation guides will tell you that 5-8mm spacing for hardwood decking boards is the standard - and whilst that's not wrong—it's incomplete. Oak reacts differently depending on its moisture content when installed, and British weather throws another spanner in the works entirely!
Fresh-cut green oak can have moisture content above 30%. As it seasons down to usually about 12-18% in UK conditions - it'll shrink further. Install green oak at 5mm spacing and you may well end up with 12mm gaps after the first summer. Not ideal if you're wearing heels or have young kids running about.
Kiln-dried oak at 8-10% might actually expand slightly when exposed to damp conditions. Install it with 8mm gaps, and you could get boards touching in wet winters, which defeats the entire purpose of spacing.
What the Building Regs Actually Say
BS 8417, the British Standard for the preservative treatment of timber, does not specify the gap requirements; instead, adequate ventilation and drainage are emphasised. The Building Research Establishment recommends a minimum 6 mm gaps for hardwood decking, with adjustments on board width and installation conditions.
For decking that is attached to buildings, Part M of the Building Regulations has something to say too: gaps can't exceed 13mm in areas that require accessible routes. Worth knowing if you're decking near doorways or creating level-access spaces.
Seasonal Timing Matters
Installing oak decking in February for example is different from doing it in August. Winter-installed timbers are usually higher in moisture content and will shrink as we go further into the spring and summer. Summer installations may be at lower moisture content and could swell when autumn rain arrives.
Professional decking contractors often vary the spacing by 1-2mm according to the month of installation. Spring installation-March to May? They'll be toward the narrower option of the range. Autumn fitting-September to November? Slightly wider gaps make more sense.
This isn't an exact science-you're not going to ruin a deck by getting it slightly wrong-but it's the difference between good practice and best practice.
Common Mistakes We See
The most common mistake on an install is installing them too tight, usually because people are fearful of debris falling through. Boards touching even occasionally means trapped moisture, staining where they contact, and accelerated wear.
Irregular spacing looks worse than slightly wider gaps. Human eyes pick up inconsistency more than absolute gap width. A deck with consistent 8mm gaps looks professional; one varying between 5mm and 9mm appears rushed.
Not considering the condition of the timber causes either problems or wasted effort. Installing green oak at 8mm means unnecessary spacing work for gaps that will open up anyway.
When Seeking Professional Help Makes Sense
Self-installation of decking is certainly within the capacity of competent home improvers, but there's no shame in engaging professional installers for the actual laying of the boards. The difference between adequate and excellent decking often lies in consistent spacing and proper fixing, both of which professionals do hundreds of times.
In cases where you are working with highly expensive timber, complex patterns, or vast areas, professional installation often pays for itself through reduced waste and fewer errors.
The Realistic Summary
For most oak decking installations in the UK, 6-7mm is a good spacing that works. Make it slightly wider for large boards or green timber; slightly narrower for kiln-dried material or covered areas. Just keep it consistent, make sure there's good drainage and airflow, and don't overthink it beyond that.
Oak's a forgiving material-it'll tolerate imperfect spacing way better than most other materials. The important thing is leaving enough space, not getting it perfect to the millimetre.
Planning an oak deck and need advice specific to your situation? Give us a ring on 01536 267107. We'll discuss your chosen timber, its moisture content, and the conditions under which it will be installed, in order to give you some real-life spacing recommendations which will actually work for you. Sometimes a five-minute conversation saves hours of head-scratching on site.