Quick answer: There is no universal rule that says every decking board must be fitted grooves up or grooves down. First establish whether the board is genuinely dual sided. If both faces are properly finished, either face may be suitable. If one face is clearly unfinished, the properly finished face is the one intended to be visible.
The Timber Decking and Cladding Association (TDCA) gives the same basic test: if both sides are dressed and the edges are chamfered or eased, a dual-sided board can normally be installed either way according to preference.
How to tell which face should go up
Do not decide from the groove pattern alone. Put a board on its edge and compare both faces before you start fixing.
- Look at the machining on both faces. Are both sides cleanly finished, or does one side look like an obvious back face?
- Check the edges. Rounded, eased or chamfered edges are a useful sign that a face has been finished for exposed use. A plain face with sharp square edges can indicate that it is not intended to be the top surface.
- Compare the whole pack. Make sure the boards share the same profile before choosing the orientation for the deck.
- If both sides are properly finished, choose the face you prefer. Then keep that orientation consistent across the whole deck.
Do grooved decking boards have to go grooves up?
No. A purpose-made reversible decking board may have grooves on one face and a different groove pattern or a smooth face on the other. If both faces are properly finished, the visible face is normally a matter of preference.
However, if the reverse face is rough, unfinished or has sharp square edges, do not assume it can be used as the walking surface just because it is flat.
Are the grooves there for extra grip?
Not necessarily. The TDCA specifically warns that ordinary grooved decking should not be treated as an enhanced-grip or anti-slip product simply because it has grooves. Purpose-made enhanced-grip decking is a different product specification.
That is particularly important when choosing decking for steps, ramps or areas where verified slip resistance is required.
What direction should the grooves run?
Where a grooved face is used, the TDCA recommends laying the grooves in the direction of the deck's fall so water can be channelled away rather than trapped across the surface.
That means board orientation and board direction should be decided together. Do not choose the face first and only later discover that the groove direction works against the fall you have built into the frame.
What if the two sides have different groove patterns?
If both faces and edges are properly finished, different patterns do not automatically mean one side is the top and one side is the bottom. It may simply be a reversible board offering two appearances.
Pick the side you prefer, then lay the boards consistently. Changing between profiles from board to board will usually make the finished deck look accidental rather than designed.
What if I am replacing boards in an existing deck?
Match the surrounding deck where possible. Even if a replacement board is technically reversible, fitting the opposite profile to the existing boards can make the repair stand out.
Also check the actual replacement boards before ordering around an old profile. Decking patterns can change between suppliers and batches, so the dimensions and suitability matter more than assuming every 125 × 32mm board has exactly the same groove pattern.
Our current treated timber decking boards are nominally 125 × 32mm and finish at approximately 120 × 28mm, but the exact profile can vary with supplier availability.
Should I put the smoother face up because it is easier to clean?
If the board is genuinely reversible, practical preference can be part of the decision. The important point is to confirm that the face is properly finished for exposed use rather than assuming that any smooth-looking reverse is a usable top face.
If the manufacturer supplies a specific installation direction, follow that instruction instead of treating the board as reversible.
Check the boards before you start fixing
Choose the face and direction before laying the first row. Once several boards are fixed, discovering that one face is not suitable or that the groove direction does not match the fall creates unnecessary rework.
At the same time, check your board spacing and full deck layout. See What Gap Between Decking Boards? and How Many Decking Boards Do I Need? before fixing the first row.
Related decking guidance
- What Gap Between Decking Boards?
- How Many Decking Boards Do I Need?
- Decking Calculator
- Shop Timber Decking
Technical guidance on reversible profiles, grooves and finished edges referenced from the Timber Decking and Cladding Association. Always follow any product-specific installation instructions supplied by the manufacturer.
