What Gap Should You Leave Between Decking Boards?

One of the most common questions when installing decking is what gap should you leave between decking boards? It sounds simple, but there isn't one universal answer.

The correct decking gap depends on decking material, manufacturer, board profile, board length, installation temperature, moisture content, fixing system, location and drainage requirements. A Millboard board, conventional WPC composite board and natural hardwood board can all require different spacing.

This is important because decking gaps aren't there purely for appearance. Depending on material they can accommodate drainage, thermal expansion, moisture movement, ventilation and installation tolerances. Using wrong gap can result in boards that are too widely spaced, water and debris problems, or boards that expand into each other or surrounding walls.

Rather than asking “what is the UK regulation for decking gaps?”, the better question is “what gap is required for this particular decking product and installation?”

Is There a UK Regulation for Decking Board Gaps?

There isn't a single UK Building Regulation stating that every decking board must have, for example, a 4mm, 5mm or 6mm gap. Many exact spacing dimensions come from manufacturer installation requirements. These should not be incorrectly described as universal UK regulations.

For example, Millboard publishes specific installation guidance. A conventional composite manufacturer may publish completely different expansion requirements. Hardwood spacing is different again because natural timber responds primarily to changes in moisture content. There can also be separate requirements affecting overall project around fire safety, guarding, drainage and accessibility, but these should not be confused with product-specific board spacing.

Why Do Decking Boards Need Gaps?

Rain needs somewhere to go. Standard decking is normally an open-jointed surface — water falls onto boards and passes through gaps. Materials also move. Natural timber primarily responds to changes in moisture content. Many WPC composite boards experience significant thermal expansion and contraction. Millboard has different movement characteristics again. Suitable spacing also allows some small debris and water to pass and creates professional finished appearance.

Side Gaps vs End Gaps

Before discussing measurements, distinguish between two different gaps. Side-to-side gap is gap between long edges of adjacent boards — line you normally see running along entire deck. End-to-end gap is gap where ends of two boards meet, sometimes called butt-joint gap. These dimensions may be completely different. For some composite products, end-to-end expansion is particularly important. Never assume that because manufacturer requires 5mm side gap it also requires 5mm end gap.

What Gap Should You Leave for Millboard Decking?

Millboard has its own product-specific installation requirements. Current Millboard Enhanced Grain guidance specifies 4mm between the sides of boards and 1mm between board ends, with 10mm gap between boards/subframe and any abutting structure to assist drainage. Working specification also notes 4mm between sides, or 6mm if using DuoFix side-fixing guide, and 1mm gap at ends on butt joints, with 10mm gap to solid surfaces.

These measurements relate to Millboard's installation requirements, not universal dimensions for every composite product. Always check latest Millboard installation documentation for specific product before construction.

Millboard isn't conventional WPC. Its boards use wood-free resin-mineral composite construction with Lastane surface, so material behaves differently from typical WPC. Millboard states relatively low dimensional movement, which is why you should not take expansion rules from WPC board and apply them to Millboard.

Clearance between decking and solid objects — house walls, rendered walls, planters, garden rooms and steps — is particularly important to prevent water and debris becoming trapped.

What Gap Should You Leave for Composite Decking?

There is no single answer for all composite decking. Conventional WPC manufacturers publish their own spacing requirements. Side gap may be established automatically by hidden clips, while end gaps may depend on board length, temperature during installation, manufacturer and fixing method.

Saying “composite decking always needs a 5mm gap” isn't reliable advice.

Most materials change dimensions as temperature changes. With many WPC products, thermal movement can become important because boards are relatively long. Small percentage of movement across 4-metre board can translate into several millimetres at ends. This is why manufacturers may specify different end gaps according to installation temperature.

Imagine composite board installed when very cold — board may be near lower end of normal dimensional range. When summer arrives it expands. If installed tightly between two walls with almost no allowance, it may have nowhere to go — pushing against walls, distortion and pressure around fixings. Reverse can happen — board installed when extremely warm may contract as temperatures fall. If installer leaves unnecessarily large expansion gap without considering temperature guidance, gap may become even more visually noticeable in colder conditions.

Correct spacing is a balance, not “bigger gap = safer installation.”

Many composite systems use hidden clips securing board and setting side-to-side gap, but clip usually doesn't automatically solve every expansion requirement. Board ends, walls and fixed objects may still require separate clearances. Read complete installation guide rather than relying solely on clip thickness.

What Gap Should You Leave for Hardwood Decking?

Hardwood is different from composite. Natural timber responds to changes in moisture content — absorbing moisture it tends to expand, drying it tends to shrink. Movement across width of timber board is much more significant than movement along length.

There isn't one universal gap for all hardwood decking. For many hardwood installations, gaps in region of approximately 5–8mm between boards may be encountered, but this should be treated as general range sometimes encountered rather than universal installation instruction. Correct gap depends heavily on condition of timber when installed. Relatively wet timber may shrink as it dries, very dry timber may expand when exposed to wetter conditions.

For example, 145mm-wide board stored in very dry conditions may absorb moisture and increase in width once installed outdoors in UK. If boards were fitted almost touching, gaps could close. If timber delivered at relatively high moisture content, as it dries gaps may become larger. This is why experienced installer considers condition of material rather than using same spacer automatically.

Premium species such as Ipé, Iroko and Balau are dense and relatively stable but remain natural wood — board width, moisture content, installation conditions and supplier guidance still matter. They should not be installed using WPC manufacturer's expansion chart. Movement across timber occurs proportionally across width, so wider board can experience more absolute change than narrow board under same conditions. Generally boards shouldn't simply be installed tightly together without consideration of future movement and drainage.

Decking Gap for Drainage and Detailing

Gap between boards helps rainwater pass through surface, but board spacing is only one part of drainage. Perfectly spaced deck can still have poor drainage if area underneath contains blocked outlets, incorrect falls or standing water. Good drainage requires correct board spacing plus suitable sub-deck drainage. See Decking Drainage: How to Prevent Water Problems.

Once adequate open area exists, making gaps much larger may provide little benefit while creating other problems. Goal is correct gap, not largest possible gap.

Decking around doors, thresholds and rendered walls requires careful detailing. Board shouldn't simply be pushed tightly against door frames, threshold profiles or render where water and debris can become trapped. Consider manufacturer clearance, water drainage, door operation and waterproofing. Where deck is designed to align closely with internal floor, complete threshold detail should be planned before installation.

Where decking is cut around pergola posts, balustrade posts or structural columns, appropriate clearance should be considered. Don't assume board should be cut so tightly it grips post — decking may move and post may need drainage around base. Neat shadow gap can often look better than excessively tight cut. Trees require even more allowance — living tree will continue to grow, so decking shouldn't be fitted tightly around trunk. Adequate space needs to be allowed for future growth, movement and drainage.

Balustrade posts should normally be structurally connected to supporting frame rather than relying on decking board, with decking neatly cut around post with appropriate clearance. Planning balustrade before installing decking usually produces cleaner result.

Butt Joints, Breaker Boards and Picture Frames

Butt joint occurs where ends of two boards meet — often one of most visually noticeable parts of deck. Poorly planned butt joints can create inconsistent gaps, unsupported board ends and movement. Board ends should have appropriate structural support — double joists, additional blocking or proprietary clips. Trying to fix two board ends into extreme edges of one narrow joist can create poor fixing positions.

Board layout and subframe should be designed together. That links directly to steel-frame work.

Breaker board is board installed in different direction to divide sections — for example central board running perpendicular to main boards. This can avoid long end-to-end butt joints, create deliberate design feature, divide large areas visually, help manage board lengths and simplify some expansion details. On large composite decks, breaker boards can be both technically and visually useful.

Picture-frame border runs around perimeter, creating premium finished appearance and hiding cut board ends. However it requires careful structural planning — additional joists or blocking may be needed beneath perimeter boards, and expansion and clearances also need to be maintained. Picture frame shouldn't physically trap main boards and prevent intended movement.

Frame needs to support board ends, breaker boards, picture frames, fascia, steps and balustrades. If frame built first without knowing board layout, important joints may have no suitable support underneath. Good decking starts below surface. Gaps on steps still need appropriate spacing, and fascia boards covering visible edges shouldn't automatically be installed in way that prevents main boards from moving.

What Happens if Gaps Are Wrong and Planning Layout

Potential problems if composite decking has no expansion gap include boards pushing against each other, buckling and pressure against walls. If dry timber absorbs moisture and expands, tightly installed boards can press against each other and close drainage routes. If wet boards shrink, large gaps may develop. Correcting spacing may require releasing multiple rows with clip-fixed composite.

Professional installation isn't about placing one spacer at beginning and assuming rest will remain perfect. Small variations can accumulate — even 1mm error per board repeated across 30 boards can produce 30mm difference at opposite side. Installers should periodically check board alignment, overall dimensions and distance to fixed edges.

Before fixing first board, consider board direction, full deck width, expected number of boards, side gaps, perimeter borders, breaker boards, final board width, steps and doors. This can prevent ending with unattractive narrow strip along one edge. Board direction is determined by design, subframe direction, drainage, sightlines and architecture — boards running away from large doors can visually extend interior into garden, boards running parallel can make terrace appear wider.

Some leaves and small debris will inevitably pass through an open-jointed deck. Making gaps unnecessarily small doesn't eliminate this problem and can restrict drainage as debris accumulates. On roof terraces, strategically positioned removable boards or access panels can make drainage outlets easier to inspect and clean.

Integrated lights should be planned around board layout — if recessed lights positioned randomly before board locations established, they may end up too close to board edge or directly over joist. Determine decking grid first and then position lights relative to that layout. See Decking Lighting Ideas: Where Should You Position Your Lights?

Gaps and Regulations

On certain balconies and roof terraces, fire requirements may affect decking system that can be used, but fire requirements should not be confused with board-spacing instructions. Manufacturer's specified 4mm board gap does not become Building Regulation simply because deck is on balcony.

Planning permission can affect deck height, location, scale, privacy and roof terraces, but generally doesn't tell you whether Millboard boards should have 4mm or 6mm side gap — that information comes from product installation specification.

Manufacturer installation requirements may specify board gaps, joist centres, fixings, clearances and expansion allowances. Building Regulations may address relevant matters such as fire safety, guarding, access and structural safety where applicable. Planning rules may address raised-platform height, development, appearance and overlooking. These are not interchangeable.

Professional Decking Installation

At Decking Gurus, we install Millboard, composite and hardwood decking, with the board layout and supporting structure planned together rather than treating them as separate parts of the project. As Approved Millboard Installers, we follow the appropriate product-specific installation details for board spacing, support, fixings, perimeter clearances and drainage rather than applying generic composite-decking rules to every installation.

The same principle applies to conventional composite and premium hardwood decking. Different materials move differently, which means correct board spacing, end gaps and clearances need to suit actual product and conditions rather than simply copying last deck that was installed.

On raised, curved and multi-level decking, the subframe becomes particularly important. Our bespoke galvanised-steel frames can be measured, cut, welded and assembled on site around required board layout, steps, picture-frame borders and structural details. Structural timber and recycled-plastic systems can also be used where they are more appropriate for the project.

Board spacing also needs to work with everything around deck. Glass or wire balustrades, pergolas, outdoor kitchens, seating, planters and integrated lighting are much easier to coordinate when their positions are considered before decking boards are installed. On roof terraces and balconies, access to drainage outlets and waterproofing below deck also needs to remain part of design.

The correct decking gap isn't simply the gap we've always used. It's the gap required by the material, manufacturer and installation in front of us.

Useful Guides

If you want to understand how the structure underneath affects the finished deck, Timber vs Steel Decking Frames: Which Is Best? explains why different projects need different subframe systems, while Decking Drainage: How to Prevent Water Problems looks at falls, ventilation, drainage routes and the problems caused by standing water beneath decking.

For board choice and long-term value, Millboard vs Composite Decking: What's the Difference? and Hardwood vs Composite Decking: Which Should You Choose? compare appearance, maintenance and lifespan, while How Long Does Composite Decking Last? and Is Millboard Decking Worth the Money? look at why board warranty alone doesn't tell you how long the whole deck will last.

And before you commit to design details, 10 Decking Mistakes to Avoid Before Starting Your Project, Glass Balustrades for Decking: Pros, Cons & Costs and Decking Lighting Ideas help you plan steps, balustrades and lighting upfront — so cables, supports and structural reinforcement can be coordinated from the start rather than added as expensive afterthoughts.