Two or Three Anti-Slip Inserts? Choosing the Right Decking Configuration
Two anti-slip inserts are suitable for most decking applications, but three may be preferred where people mainly walk along the length of the boards. The correct configuration depends on board width, direction of travel, gradient, users, exposure and the consequences of a slip.
Why Does Anti-Slip Insert Layout Matter?
GripDeck® anti-slip boards contain narrow resin-and-aggregate inserts set into the timber. Their purpose is to provide durable areas of enhanced grip within a complete decking board. The number and position of those inserts affect how frequently a shoe is likely to contact an anti-slip zone as someone moves across the deck.
That does not mean the board with the greatest number of inserts must always be specified. An effective layout balances slip performance, board width, appearance, timber content, direction of travel and cost. The decision should be based on the intended route rather than a simple assumption that more is always better.
On a typical 145mm two-insert board, the insert centres are approximately 70mm apart. This is generally less than the width of an adult shoe and distributes the two anti-slip zones across the face. Three inserts reduce the distance between the anti-slip zones and may be valuable where footsteps repeatedly follow the board length.
Specify the whole board—not an insert in isolation
Pendulum test values apply to the tested board, insert arrangement, profile and test direction. They should not be presented as a performance value for the aggregate strip alone or automatically transferred to a materially different configuration.
Two Inserts Versus Three: Quick Comparison
| Consideration | Two inserts | Three inserts |
|---|---|---|
| Usual application | Suitable for most domestic, commercial and general-access decking applications. | Often considered for long linear routes and projects with predominantly lengthwise travel. |
| Typical 145mm board | Two anti-slip zones distributed across the board, approximately 70mm between centres. | Three anti-slip zones with closer spacing across the board face. |
| Direction of travel | Commonly selected where users cross the boards or movement is mixed. | May be preferred where users mainly move along the board length. |
| Examples | Terraces, viewing areas, garden decks, hospitality spaces and many general walkways. | Bridges, ramps, boardwalks, long walkways and some public-access routes. |
| Specification basis | Project risk, board width, users, gradient, exposure and relevant test evidence. | The same factors; three should not be chosen solely because it sounds safer. |
These are specification principles rather than universal rules. GripDeck can advise on the appropriate layout after reviewing drawings, board orientation and how the route will be used.
When Are Two Anti-Slip Inserts Normally Suitable?
Two inserts are GripDeck’s usual recommendation for most applications using a standard-width board. They offer good coverage while retaining the natural timber surface and have been independently tested as part of complete GripDeck® board configurations.
People cross the boards
Where boards run across a path or terrace, each step naturally crosses both inserts. This is a common and effective orientation for pedestrian routes.
Movement is mixed
Patios, viewing platforms and hospitality areas rarely have one tightly controlled walking line. Two inserts provide distributed coverage for varied movement.
The route is level or gently sloping
Where gradients and consequences are not unusually high, the standard two-insert configuration is often the proportionate choice.
A tested standard configuration is wanted
Current GripDeck evidence includes 145mm smooth and grooved boards with two inserts, allowing the specified board to be matched to published results.
Two inserts are available across suitable softwood anti-slip decking, hardwood anti-slip decking and thermally modified products. Timber species and board thickness should still be chosen for the required durability, span and use.
When Should Three Anti-Slip Inserts Be Considered?
Three inserts may be appropriate where the board orientation creates long, uninterrupted lines of travel parallel with the inserts. In that situation, closer insert spacing can increase the likelihood that each foot contacts an anti-slip zone throughout the route.
Bridges and boardwalks
Users often travel in one direction for the full length of the deck, particularly where the route is narrow or bounded by handrails.
Ramps and gradients
A gradient increases the importance of deliberate board orientation, drainage, tested wet grip and a configuration suited to the direction of movement.
Higher-consequence areas
Public access, vulnerable users, exposed wet locations or a serious consequence of falling may justify a more cautious project-specific specification.
Three inserts are not a substitute for correct design. Drainage, board gaps, falls, ventilation, suitable joist centres, corrosion-resistant fixings and planned maintenance all contribute to a safe and durable route. An extra insert cannot correct ponding water, structural movement or contaminated decking.
A useful real-world example is a riverside platform or a long access route where boards and pedestrian flow run in the same direction. GripDeck has supplied three-insert softwood boards for this type of application. By contrast, a social terrace with varied movement may gain little practical benefit from changing the standard two-insert arrangement.
Three is an option, not a universal upgrade
Describe the route, gradient, board direction and users when requesting a quotation. GripDeck can then recommend two or three inserts and confirm whether the proposed layout is represented by current test evidence.
Why Does Direction of Travel Matter?
Imagine a shoe moving across a 145mm board. When the person walks perpendicular to the board length, the shoe passes across both anti-slip inserts in sequence. When the person walks parallel with the board, the foot may follow a narrower line between inserts. Adding a third insert reduces the space between the anti-slip zones.
Board orientation should therefore be resolved early, particularly on bridges, ramps and paths. It is harder and more expensive to correct the relationship between board direction and pedestrian movement after the substructure has been designed or installed.
Direction of travel is only one part of the assessment. Designers should also consider:
- whether movement is controlled, mixed or bidirectional;
- the gradient and any change of level;
- expected footfall, footwear and mobility needs;
- shade, rainfall, nearby vegetation and other contamination sources;
- handrails, edge protection and the consequence of a fall;
- cleaning access and the planned inspection regime; and
- whether the specification requires documented wet-condition test evidence.
For public and commercial work, the broader duties and guidance are explained in our guide to UK anti-slip decking regulations.
Board Width, Profile and Timber Selection
The correct insert count cannot be separated from the board itself. A 115mm board and a 145mm board do not present the same walking surface, so the layout must be assessed and tested as a complete configuration.
GripDeck’s published test programme includes 115mm and 145mm boards, smooth and grooved faces, and two- and three-insert layouts. When comparing evidence with a proposed product, check the board width, face profile, insert quantity and test direction rather than relying on a headline number.
| Design choice | What to check | Why it matters |
|---|---|---|
| Board width | Confirm the exact finished width and insert positions. | Spacing and shoe contact differ between narrow and wide boards. |
| Face profile | Identify whether the walking face is smooth or grooved. | Test evidence must relate to the complete tested surface. |
| Board thickness | Choose for span, loading and joist spacing. | Insert count does not determine structural capacity. |
| Timber | Select softwood, hardwood or thermally modified timber for the project. | Durability, appearance, movement and maintenance requirements vary. |
| Insert colour | Choose a contrasting or sympathetic aggregate colour. | Visual contrast and design intent may matter, but colour does not replace test evidence. |
GripDeck’s standard softwood sizes include 28mm, 34mm and 45mm thick boards at approximately 145mm wide. The thickness should be selected alongside the substructure design and joist centres; it is not simply a visual preference.
What Do GripDeck’s Anti-Slip Test Results Show?
GripDeck’s current independent testing was completed to BS EN 16165:2021 Annex C and UK Slip Resistance Group guidance. Every tested GripDeck® anti-slip configuration achieved a Low Risk classification in both dry and wet conditions.
| Tested 145mm board | Mean dry PTV | Mean wet PTV | Wet classification |
|---|---|---|---|
| Smooth with two inserts | 82 | 72 | Low Risk |
| Smooth with three inserts | 84 | 73 | Low Risk |
| Grooved with two inserts | 79 | 69 | Low Risk |
| Grooved with three inserts | 91 | 73 | Low Risk |
| Standard grooved board without inserts | 69 | 30 | Moderate Risk |
These results apply to the tested samples and configurations. Review the complete reports and current product specification before relying on a value for a project.
The important conclusion is not that three inserts are essential. Both the tested two- and three-insert GripDeck configurations achieved Low Risk wet classifications. Insert quantity should therefore be chosen to suit the layout and intended use, supported by relevant evidence.
A Pendulum Test Value of 36 or above is commonly classified as Low Risk under UK guidance, but it is not a universal statutory pass mark for every deck. Read our PTV rating for decking guide and view the complete GripDeck anti-slip test results.
How to Specify the Correct Insert Configuration
Include these details in the enquiry or specification
- Route and use: explain whether the deck is a terrace, ramp, bridge, boardwalk, balcony, platform or general walkway.
- Direction of travel: show how pedestrians will move in relation to the proposed board direction.
- Board details: state timber, finished width, thickness, length, face profile and required treatment or durability.
- Insert layout: specify two or three factory-applied inserts only after assessing the complete design.
- Environment: record exposure, shade, water, vegetation, contamination and drainage conditions.
- Users and risk: identify public access, mobility needs, heavy footfall, gradient and the consequence of a slip.
- Evidence: request independent wet-condition test reports for the relevant board configuration.
- Installation: coordinate joist centres, board gaps, fixings, end support, ventilation, falls and access for maintenance.
The TDCA timber decking specification guidance provides useful independent advice on board selection, treatment, substructure, drainage and fixings. GripDeck is a TDCA member and recommends following its guidance.
For an unusual route or formal specification, send drawings to GripDeck’s product advice and specification support team. Samples can also help the designer and client confirm timber, profile, insert appearance and colour before ordering.
GripDeck Softwood Anti-Slip Decking Options
28 × 145mm GripDeck® Softwood
A popular European Redwood board for suitable domestic and commercial projects, supplied with factory-applied anti-slip inserts.
Buy Now34 × 145mm GripDeck® Softwood
A thicker treated softwood option for projects whose span, loading and specification call for a 34mm board.
Buy Now45 × 145mm GripDeck® Softwood
A substantial treated board for appropriate commercial and heavy-duty applications, subject to complete structural design.
Buy NowNeed a Three-Insert or Bespoke Decking Specification?
Send GripDeck your drawings, board direction, route dimensions and project requirements. We can advise on insert quantity, timber, board thickness, test evidence and a project quotation.
Frequently Asked Questions About Two and Three Anti-Slip Inserts
Do I need two or three anti-slip inserts in each decking board?
Two inserts are suitable for most standard-width decking applications. Three may be considered where people predominantly walk along the board length, particularly on bridges, ramps, boardwalks and long linear routes. The final choice should reflect the complete project design.
Are three anti-slip inserts always safer than two?
No. Both tested two- and three-insert GripDeck configurations achieved Low Risk wet classifications. Three inserts provide closer spacing, but the appropriate choice depends on direction of travel, board width, gradient, users, exposure and the relevant test evidence.
Why does the direction of travel affect insert quantity?
A person crossing the boards naturally steps across the anti-slip inserts. When walking parallel with the boards, a foot may follow a line between them. A three-insert layout reduces the spacing between anti-slip zones on long lengthwise routes.
Are two inserts enough on a 145mm decking board?
Two inserts are the usual recommendation for most 145mm GripDeck applications. Their centres are approximately 70mm apart on a typical board, generally less than the width of an adult shoe, and tested two-insert configurations achieved Low Risk wet classifications.
Should every timber access ramp have three inserts?
Not automatically. A ramp requires project-specific consideration of gradient, board orientation, users, drainage, exposure and consequences. Three inserts may be preferred where travel runs along the board length, but the complete ramp design matters more than insert count alone.
Does a smooth or grooved board change the decision?
The face profile forms part of the tested configuration and should be stated in the specification. Current GripDeck evidence includes smooth and grooved boards with two and three inserts, but the report matching the proposed board should be reviewed.
Are two- and three-insert GripDeck boards independently tested?
Yes. GripDeck publishes independent pendulum test reports for current smooth and grooved board configurations with two and three inserts. All tested GripDeck anti-slip configurations achieved Low Risk classifications in wet and dry conditions.
Does the timber species determine how many inserts I need?
No. Timber species affects durability, appearance, movement and maintenance, but insert quantity is mainly influenced by board width, route design, direction of travel and risk. Choose the timber and anti-slip configuration together as a complete product specification.
Can GripDeck manufacture a bespoke three-insert board?
GripDeck can discuss three-insert and other project-specific configurations subject to timber, profile, quantity, manufacturing and testing requirements. Send drawings and project details for specification advice and a quotation.
Are factory-applied inserts the same as RetroGrip inserts?
No. This guide concerns factory-applied GripDeck inserts in new decking boards. RetroGrip is a retrofit system for suitable existing grooved timber decking. The product, installation method and supporting test evidence should be considered separately.
Choose the Layout That Matches the Route
For most projects, two inserts in a 145mm GripDeck board provide an appropriate, tested anti-slip configuration. Three inserts become worth considering when the route is long and linear, users move mainly along the board length or the project risk assessment calls for closer insert spacing.
Make the decision during design, not after installation. Confirm the board direction, width, profile, thickness, timber, gradient and user needs, then match the proposed product to relevant wet-condition test evidence.