28mm, 34mm or 45mm Decking Boards: Which Thickness Do You Need?
Decking board thickness affects support spacing, rigidity, fixing performance and how substantial the finished deck feels. This guide compares GripDeck® 28mm, 34mm and 45mm European Redwood boards and explains when each size is normally considered.
Why Does Decking Board Thickness Matter?
A decking board spans from one supporting joist to the next. Its thickness influences how much it bends between those supports, how solid the surface feels underfoot and how effectively the fixings can control movement. In general, a thicker board is stiffer than an otherwise equivalent thinner board—but thickness is not the only factor.
Timber species, grade, board width, moisture condition, joist spacing, expected loading and installation quality all affect performance. A correctly supported 28mm board can be a better solution than a 45mm board placed on an unsuitable or poorly constructed frame.
Distance between supports
Thicker boards can generally bridge a greater distance between joists, although the current product guidance and project design must set the actual centres.
Rigidity underfoot
Greater board depth or closer joist centres can reduce noticeable flexing and create a firmer, more reassuring walking surface.
Fixing engagement
A thicker board accepts a longer fixing, but suitable screw diameter, embedment, end distance and two fixings at each joist remain essential.
Project demands
Footfall, concentrated loads, public access, board orientation and the consequences of movement can justify a thicker board or closer support.
Board thickness is not a structural design on its own
Choosing a thicker deck board does not automatically make the joists, beams, posts, foundations, connections or guarding suitable. Raised, commercial, public-access and heavily loaded decks should be designed by a suitably competent person.
28mm, 34mm and 45mm Decking Board Comparison
| Board size | Typical maximum joist centres | Common applications | Main reason to choose it |
|---|---|---|---|
| 28 × 145mm | About 400mm | Garden decks, terraces, general pedestrian areas and many commercial or public-access projects | Versatile, cost-effective and readily available for correctly supported decking |
| 34 × 145mm | Up to 500mm | Commercial walkways, schools, public access and projects requiring a more substantial board | Additional rigidity and potential to accommodate wider support centres where the design allows |
| 45 × 145mm | Up to 500mm | Boardwalks, bridges, ramps and more demanding or heavy-duty applications | A deep, highly substantial board for robust specifications and firmer surfaces |
The figures are general GripDeck product-support guidance, not universal structural span values. Closer centres may be required by loading, grade, diagonal layouts, board joints, access requirements or the project designer.
All three sizes can be manufactured as GripDeck® softwood anti-slip decking using pressure-treated European Redwood with factory-applied resin-and-aggregate inserts. This allows the board thickness to be selected for the project while retaining the required anti-slip configuration.
When Should You Choose 28mm Decking Boards?
The 28 × 145mm European Redwood board is the most widely used GripDeck softwood size. It suits many garden, hospitality, commercial and public-access projects when supported at approximately 400mm centres and installed in accordance with the specification.
Its popularity comes from a useful balance of strength, availability, handling and cost. It is substantial enough for a wide variety of pedestrian decks without adding the extra timber volume and weight of a thicker section.
28mm boards are often suitable when:
- the deck is intended primarily for normal pedestrian use;
- joists can be installed at approximately 400mm centres;
- the boards run perpendicular to the joists;
- every board end and butt joint is fully supported;
- the design does not involve exceptional concentrated loads; and
- a versatile stock product is preferred for straightforward ordering.
Do not assume that “domestic” means structural design is unnecessary. Large gatherings, planters, outdoor kitchens, storage, glazed balustrades and changes of level can introduce loads and detailing requirements beyond a simple walking surface.
When Should You Choose 34mm Decking Boards?
A 34 × 145mm board provides additional depth without moving to the full 45mm heavy-duty profile. It is a useful middle option for commercial paths, schools, public-access areas and projects where a more substantial surface is preferred.
Suitable 34mm GripDeck boards may be supported at up to 500mm centres. However, the ability to use wider centres does not mean they must be used. Centres of 400–450mm can provide a firmer surface, reduce board movement and limit repeated stress on fixings in busier locations.
Choose 34mm for the complete project—not simply to remove joists
Any saving from wider centres must be checked against joist span, load, board joints, frame stability and long-term performance. Adding a few supports can be better value than designing every component at its maximum limit.
The 34mm size may also help where a replacement deck must meet a particular finished level or interface with thresholds and adjoining surfaces. Confirm the full build-up—including joists, ventilation gap and surface drainage—before ordering.
When Should You Choose 45mm Decking Boards?
The 45 × 145mm section is a deep and substantial softwood decking board. It is normally considered for bridges, boardwalks, ramps, high-footfall locations and robust commercial or infrastructure specifications.
A 45mm board can create an exceptionally firm walking surface, particularly when used on closer centres. It also provides generous fixing depth. These advantages are valuable in demanding environments, but the board is heavier to handle and contains more timber, so it should be specified for a clear reason.
Firm underfoot
The deeper section can reduce noticeable deflection between supports and contribute to a solid, reassuring pedestrian surface.
Demanding routes
Heavy-duty boardwalks, bridges and ramps may justify the additional section, subject to grade, loading and professional design.
Weight and handling
Long 45mm boards are considerably heavier. Delivery access, manual handling, cutting and installation sequencing should be planned.
Thicker does not mean indestructible. A 45mm board still needs correct treatment, drainage, ventilation, board gaps, end support, maintenance and compatible corrosion-resistant fixings. It can still move as moisture content changes because it remains a natural timber product.
How Board Thickness Affects Joist Spacing
Joist spacing is measured from the centreline of one joist to the centreline of the next. It describes the unsupported distance beneath the deck boards; it is not the clear gap between joist edges.
GripDeck’s useful starting point is 400mm centres for 28 × 145mm softwood boards and up to 500mm for suitable 34mm and 45mm boards. Read the full timber decking joist-spacing guide before finalising the frame.
| Layout issue | Effect on support | Recommended response |
|---|---|---|
| Boards perpendicular to joists | The joist centres equal the span travelled along the board. | Use the stated product limit or closer centres where the project requires. |
| Boards installed diagonally | The board travels farther between each joist crossing. | Reduce joist centres according to the installation angle. |
| Board ends and butt joints | Each end requires a sound fixing zone and full support. | Use double joists or another properly designed support detail where required. |
| High footfall or demanding use | Repeated loads may make movement and fixing stress more significant. | Consider closer centres even when the board can technically span farther. |
| Wide centres such as 600mm | Most common timber deck boards may deflect excessively. | Do not use unless the exact product and project design expressly support it. |
At a 45-degree angle, a board travels approximately 1.414 times the joist spacing between supports. Maintaining the same effective 400mm board span would reduce joist centres to approximately 280mm. This geometry applies regardless of whether the board is anti-slip.
Do not confuse board support with joist span
The 400mm or 500mm figures describe spacing between joists beneath the boards. They do not state how far each joist may span between beams. Joist section, grade, load and span require a separate calculation.
Match Decking Thickness to Loading and Use
A deck for occasional garden use does not necessarily have the same demands as a school walkway, public boardwalk or commercial hospitality terrace. The expected load and consequences of excessive movement should guide both board selection and support spacing.
Normal pedestrian traffic
Correctly supported 28mm boards are suitable for many normal pedestrian applications. A thicker board may still be selected for feel, layout or project preference.
High footfall
Busy public routes can benefit from thicker boards, closer centres or both, reducing movement and repeated stress on fixings.
Concentrated loads
Hot tubs, large planters, equipment, vehicles and heavy furniture require specific structural design. They cannot be allowed for merely by increasing board thickness.
Raised and public decks
Structures with significant height, guarding or public access require consideration of applicable regulations, loads and professional design responsibilities.
The TDCA advises that structural timber must have a suitable strength class for the loads and service conditions. Its timber decking specification guidance recommends Use Class 4 pressure treatment for external structural support timbers such as joists, beams and posts. GripDeck is a TDCA member and recommends following its guidance.
Installation Requirements for Every Board Thickness
The benefits of a suitable board can be lost through poor installation. Apply the following principles to 28mm, 34mm and 45mm timber decking unless the project specification requires a more demanding detail.
Installation checklist
- Confirm the structural design: verify joists, beams, posts, foundations, bracing and connections separately from the deck-board selection.
- Support every board end: do not leave ends or butt joints floating between joists.
- Use two fixings at each joist: position them in sound timber and never drive a fixing through an anti-slip insert.
- Select suitable fixings: use corrosion-resistant exterior fixings of appropriate diameter and length, with adequate embedment into the joist.
- Respect end and edge distances: pre-drill where necessary to reduce splitting and keep fixings approximately 30mm from board ends where the product guidance requires.
- Allow drainage and movement: provide appropriate gaps between boards and avoid details that trap water.
- Ventilate the substructure: preserve airflow and avoid enclosing damp timber.
- Retreat exposed cuts: apply an appropriate end-grain preservative to site-cut pressure-treated softwood.
- Mix boards before fixing: distribute natural colour and grain variation across the deck.
- Plan maintenance: ensure the surface and substructure can be inspected and cleaned throughout their service life.
Use the GripDeck decking calculator to estimate board quantities, then allow for the selected board width, expansion gaps, layout, cutting and sensible wastage. For budgeting, the anti-slip decking cost guide explains why board price is only one part of the installed cost.
Shop GripDeck Softwood Anti-Slip Decking by Thickness
28 × 145mm GripDeck®
The versatile stock size for garden decking, terraces and many general pedestrian, commercial and public-access projects.
Buy Now34 × 145mm GripDeck®
A more substantial board for commercial paths, schools, public access and projects requiring additional board depth.
Buy Now45 × 145mm GripDeck®
A heavy-duty section for appropriately designed boardwalks, bridges, ramps and other demanding applications.
Buy NowUnsure Which Decking Thickness to Specify?
Send GripDeck your dimensions, board layout, joist centres, project use and delivery postcode. We can help compare 28mm, 34mm and 45mm boards and prepare a project quotation.
Frequently Asked Questions About Decking Board Thickness
What thickness should decking boards be?
For many timber decks, 28mm boards provide a practical starting point when supported at approximately 400mm centres. Thicker 34mm or 45mm boards may be selected for wider support centres, increased rigidity or more demanding project requirements.
Is 28mm decking thick enough?
Yes. A 28 × 145mm GripDeck softwood board is suitable for many domestic, commercial and public-access pedestrian decks when supported at approximately 400mm centres and installed on a correctly designed substructure.
When should I use 34mm decking boards?
Consider 34mm boards where a more substantial and rigid surface is wanted, where the project suits joist centres up to 500mm, or for schools, commercial walkways and public-access areas. The final spacing and design must reflect loading and use.
What are 45mm decking boards used for?
45mm boards are commonly considered for heavy-duty or demanding applications such as boardwalks, bridges, ramps and high-footfall routes. They still require correct grade, support, fixings, treatment, drainage and structural design.
Can 34mm and 45mm decking span 500mm?
Suitable 34mm and 45mm GripDeck softwood boards may be supported at up to 500mm joist centres. Closer centres may be required for high loads, diagonal layouts, public access or a firmer surface, so confirm the exact project specification.
Can I install decking boards over joists at 600mm centres?
For most commonly supplied timber decking profiles, 600mm centres are too wide and may cause excessive movement or deflection. Only use this spacing where the exact product guidance and project-specific structural design expressly permit it.
Does a thicker decking board allow the joists to span farther?
No. Board thickness can affect spacing between joists, but it does not determine how far the joists themselves may span between beams or supports. Joist size, grade, loading and structural span require a separate design calculation.
Do diagonal decking boards need closer joist centres?
Yes. A diagonal board travels farther between adjacent joist crossings. At 45 degrees, maintaining an effective 400mm board span would require joist centres of approximately 280mm rather than 400mm.
Are thicker decking boards always better?
No. A thicker board may provide greater rigidity and fixing depth, but it also costs and weighs more. The best board is the section that meets the loading, support spacing, layout, durability and installation requirements without unnecessary material.
Do 28mm, 34mm and 45mm GripDeck boards have anti-slip inserts?
Yes. GripDeck can supply all three standard softwood sizes with factory-applied resin-and-aggregate anti-slip inserts. Insert quantity and layout should be selected separately according to board width, direction of travel and project risk.
Choose Thickness as Part of the Complete Deck Design
For many projects, 28 × 145mm decking at approximately 400mm centres is the sensible and economical choice. A 34mm board provides a useful intermediate option, while 45mm offers a robust heavy-duty section for appropriately demanding applications.
Do not select thickness in isolation. Confirm the use, loading, board direction, joist centres, grade, support of joints, fixing specification, treatment, drainage and maintenance access before ordering.