Timber Decking Joist Spacing: Complete UK Guide
Timber decking joist spacing determines how well the boards are supported and directly affects movement, deflection, fixing performance and the long-term feel of the finished deck. This guide explains typical joist centres for GripDeck® softwood, hardwood and thermally modified decking, including diagonal layouts and commercial projects.
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What Is Timber Decking Joist Spacing?
Joist spacing, also called joist centres, is the centre-to-centre distance between adjacent supporting joists. It is not the clear gap between the edges of the joists.
This spacing determines how far each decking board must span before it reaches the next support. If the distance is too great for the board thickness, species or use, the surface may feel springy and the fixings may be subjected to additional movement.
Board spacing and joist span are different calculations
The figures in this article describe the spacing of joists beneath the decking boards. They do not tell you how far the joists themselves can span between beams or supports. Joist size, grade, load and structural span must be designed separately using suitable span tables or structural calculations.
Board thickness
Thicker boards can generally bridge a greater distance between joists than thinner boards.
Timber species
Softwood, hardwood and thermally modified timber have different stiffness, density and movement characteristics.
Expected loading
Public access, concentrated loads, furniture and other uses may justify closer centres or engineering input.
Board direction
Boards laid diagonally travel further between joists and therefore require the joists to be closer together.
Board joints
Every board end and butt joint needs full support, regardless of the general joist spacing.
Manufacturer guidance
The requirements for the actual board and fixing system should take priority over generic rules.
Timber Decking Joist Spacing Table
| Decking type | Typical board size | Typical maximum centres | General application |
|---|---|---|---|
| GripDeck® softwood | 28 × 145mm | 400mm | Domestic, commercial and public-access decking |
| GripDeck® softwood | 34 × 145mm | Up to 500mm | Domestic and selected commercial projects |
| GripDeck® softwood | 45 × 145mm | Up to 500mm | Heavy-duty board profiles and demanding applications |
| Hardwood | 21–22 × 145mm | 350–400mm | Thinner hardwood decking profiles |
| Hardwood | 28 × 145mm | 400–450mm | Many residential and commercial applications |
| Hardwood | 34–45 × 145mm | 450–500mm | Thicker hardwood decking profiles |
| ThermoDeck® | 26 × 115mm | 400mm | Gardens, terraces and design-led external spaces |
These are general product-support guidelines, not universal structural design values. Confirm the final arrangement for the exact board, grade, loading, fixing system and project.
Do not default to 600mm centres
For most commonly supplied timber decking profiles, 600mm centres are too wide and can lead to excessive movement or deflection. A wider spacing should only be used where the board manufacturer and project design specifically support it.
Joist Spacing for Softwood Timber Decking
GripDeck® softwood anti-slip decking is commonly manufactured from pressure-treated European Redwood. The frequently supplied 28 × 145mm board is normally installed at 400mm joist centres.
| Board size | Typical grade | Typical maximum centres | Notes |
|---|---|---|---|
| 28 × 145mm | C16 | 400mm | A widely used arrangement for domestic, commercial and public-access decks. |
| 34 × 145mm | C16 or as specified | 500mm | Closer centres can be chosen for a firmer surface or more demanding use. |
| 45 × 145mm | C16 or as specified | 500mm | Centres of 400–450mm can provide a particularly rigid finished deck. |
Commercial decks, bridges, ramps and boardwalks may use closer spacing even where a board is capable of spanning further. Reducing movement can improve underfoot feel and reduce repeated stress on the board and fixings.
Joist Spacing for Hardwood Timber Decking
Hardwood decking is dense and durable, but it still requires adequate support. The board thickness matters more than the general label “hardwood”, and thinner profiles normally need closer joist centres.
| Typical hardwood size | Typical centres | Installation notes |
|---|---|---|
| 21–22 × 145mm | 350–400mm | Closer spacing helps control movement in thinner boards. |
| 28 × 145mm | 400–450mm | Suitable for many residential and commercial installations. |
| 34–45 × 145mm | 450–500mm | Final spacing depends on species, grade, loading and design. |
Hardwood fixing reminder
Pre-drill hardwood decking and use suitable stainless-steel fixings. Two fixings should normally be used at each joist position, with enough distance from the board ends and edges to reduce splitting.
Joist Spacing for ThermoDeck®
ThermoDeck® is commonly supplied in a 26 × 115mm profile. Thermal modification improves dimensional stability, but the board still needs regular and adequate support.
Follow the installation requirements for the specified concealed clip or fixing system. Ventilation, drainage and board gaps remain essential around thermally modified timber.
Hardwood Decking
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View Hardwood DeckingJoist Spacing for Diagonal Decking Boards
When boards run perpendicular to the joists, the centre-to-centre joist distance is also the distance travelled along the board between supports. Turning the boards diagonally increases that effective distance.
At a 45° angle, the board travels approximately 1.414 times the joist spacing between supports. To retain the same effective support distance as a perpendicular layout, the joist centres need to be multiplied by about 0.707.
| Normal perpendicular spacing | Approximate centres at 45° | Reduction |
|---|---|---|
| 400mm | Approximately 280mm | About 29% |
| 450mm | Approximately 320mm | About 29% |
| 500mm | Approximately 350mm | About 29% |
These figures illustrate the geometry of a 45° layout. The final centres still need to comply with the board manufacturer’s limits and the structural design.
Do not apply a universal 10–15% reduction
The required adjustment depends on the actual board angle. A small angle away from perpendicular needs a smaller correction, while a 45° layout requires a reduction of about 29% to maintain the same effective span along the board.
Joist Grade, Treatment and Structural Span
The decking-board centres are only one part of the substructure design. The joists themselves must have the correct treatment, strength grade, cross-section and support span.
Joist specification points
- Use structural timber suitable for the intended external exposure.
- Deck joists close to or in contact with the ground commonly require Use Class 4 treatment.
- C16 is widely used for domestic structures where the section and span are suitable.
- C18 or C24 may be specified for smaller sections, longer spans or commercial performance requirements.
- Select joist dimensions using recognised span tables or structural calculations.
- Consider concentrated loads such as planters, furniture, hot tubs and public gatherings.
The Timber Decking and Cladding Association’s deck-joist guidance explains the importance of treatment, strength grade, loading and suitable span information.
Raised decks may require Building Regulations input
Elevated decking connected with a building can fall within structural and guarding requirements. The applicability varies with the project and location, so consult Building Control and a competent designer where appropriate.
Commercial and Public-Access Decking Joist Spacing
Commercial, public-access and infrastructure decks can experience higher loads, concentrated traffic and more demanding service conditions than a typical garden deck.
Higher foot traffic
Closer centres can reduce flexing and produce a firmer, more reassuring walking surface.
Greater consequences
Loose or moving boards can create maintenance, accessibility and trip-hazard concerns.
Long-term use
Additional support can reduce repeated stress on boards and fixings over the service life.
Project design
Bridges, platforms, raised decks and heavily loaded areas should be professionally designed.
Centres in the 300–400mm range are often considered for commercial or public-access applications, but this is not a universal rule. The board thickness, grade, load, structural arrangement and project specification must determine the final spacing.
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Decking Installation Details That Work With Joist Spacing
Two fixings per joist
Two suitable fixings at each support help control twisting, cupping and lateral board movement.
Support every end
Every board end and butt joint needs full support. Double joists are often used at joints.
Allow board gaps
Provide suitable gaps for drainage, ventilation and seasonal timber movement.
Use noggins and bracing
Blocking and bracing help prevent joist twisting and lateral movement in the frame.
Provide ventilation
Airflow below the deck reduces trapped moisture and supports timber durability.
Plan drainage
The surface and substructure should allow water to escape rather than collect around timber.
Stainless-steel fixings are recommended for hardwood and other corrosive or exposed environments. Pre-drill hardwood and keep fixings an appropriate distance from board ends.
Common Timber Decking Joist Spacing Mistakes
- Spacing joists too far apart: this can cause deflection, squeaking and fixing fatigue.
- Ignoring board thickness: thinner boards generally need more frequent support.
- Using the same spacing for diagonal boards: the effective span increases as the board angle changes.
- Leaving board joints unsupported: every end must be securely fixed over a joist or suitable support.
- Confusing board centres with joist structural span: these are separate design decisions.
- Using untreated or unsuitable joists: substructure durability is as important as the visible boards.
- Failing to account for loads: furniture, hot tubs, gatherings and commercial use can substantially change the design.
- Relying on one rule for every project: product guidance and project-specific design should always prevail.
Frequently Asked Questions About Timber Decking Joist Spacing
What is the standard joist spacing for timber decking?
For many 28mm timber decking boards, 400mm centres are commonly used. The correct spacing depends on the exact board thickness, species, grade, loading and manufacturer guidance.
Is 600mm joist spacing too wide for decking?
For most common timber decking profiles, 600mm centres are wider than recommended and may lead to excessive movement or deflection. Only use this spacing where the product and structural design expressly allow it.
Can I use 500mm joist centres?
Some 34mm and 45mm GripDeck® boards may be suitable at up to 500mm centres. Closer centres may still be selected for commercial use, high loads or a firmer finished surface.
What joist spacing should be used for 28mm decking?
GripDeck® 28 × 145mm softwood decking is normally supported at 400mm centres. Hardwood profiles must be checked by species and board specification.
Does hardwood decking need closer joist spacing?
Thin hardwood profiles often require centres of around 350–400mm. Thicker hardwood boards may be suitable at 400–500mm centres, subject to the product and project design.
What joist spacing should be used for decking laid at 45 degrees?
To maintain the same effective span along the board, centres need to be reduced to about 70.7% of the normal perpendicular spacing. A 400mm layout would therefore become approximately 280mm at 45 degrees.
Do all decking board ends need support?
Yes. Every board end and butt joint should be fully supported. Double joists or another appropriate detail may be needed so that both boards can be fixed correctly.
Should commercial decking use closer joist centres?
Often, yes. Greater foot traffic, loading and service demands may justify closer centres, but the final arrangement should be project-specific.
What grade of timber should deck joists be?
C16 is commonly used where its section and span are suitable. C18 or C24 may be specified for longer spans, smaller sections or commercial performance requirements. Suitable preservative treatment is also essential.
Should hardwood decking be pre-drilled?
Yes. Hardwood should generally be pre-drilled, particularly close to board ends and edges, and fixed using suitable stainless-steel screws.
Choosing the Correct Joist Spacing for Your Deck
For many GripDeck® installations, the useful starting point is 400mm centres for 28mm softwood and 26mm ThermoDeck®, with up to 500mm for suitable thicker boards. Hardwood spacing varies with the profile and species.
These dimensions only describe support beneath the decking boards. A safe deck also requires correctly specified joists, beams, posts, foundations, bracing, fixings, guarding and drainage. Raised, heavily loaded, commercial and public-access structures should be reviewed by a competent designer or structural engineer.
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