Joist pads for a composite decking board terrace – when are they necessary and how do they reduce noise?

What are pads under joists and why does WPC “like” them more than you might think?

Pads under joists (sometimes called spacers, separators or EPDM rubber pads) are components placed between the joist and the base. Their role is even more important for a composite decking board terrace because WPC:

• is sometimes installed on composite joists or aluminium joists, which transmit vibrations differently from wood,
• expands and contracts with temperature, so the structure must be stable and level,
• is often installed over floor slabs, balconies and terraces above rooms, where noise is immediately more noticeable.

A pad is not a fitting “decoration”. It’s a quiet hero that does its job behind the scenes. 

When are pads under joists essential?

There are situations where they are not an “option”, but a sensible standard.

1. A terrace on concrete, paving slabs or block paving. 

A hard base + moisture + concentrated pressure from the joist = a risk of:

• water collecting beneath the joist,
• faster deterioration of the joist (even if treated),
• the transmission of vibrations and knocks.

Pads under joists break this contact and help maintain a gap.

2. A terrace on a balcony/floor slab with waterproofing. 

Two things matter here: protecting the waterproofing and acoustics. Pads (especially rubber/EPDM ones) act as a buffer:

• less risk of damaging the roofing felt/membrane,
• less footfall noise transmitted to the building structure.

3. An uneven base (which occurs more often than not). 

If the base has slopes, raised areas or dips, joists without pads will:

• rock,
• move,
• cause the fixing clips to click and create unpleasant noises.

4. If you want to reduce noise from a WPC terrace. 

If the terrace is close to a bedroom, in a terraced house or above a room, pads can make a noticeable difference.

How do pads quieten a composite decking board terrace?

Noise on a WPC terrace is usually not caused by just one thing, but by a combination of:

• thudding (footsteps resonate through the structure),
• knocking at hard support points,
• clicking (small movements at the joints),
• sometimes squeaking (components rubbing as the material moves).

Pads act like mini shock absorbers. A flexible material (e.g. EPDM) absorbs some of the energy and limits the transmission of vibrations to the base. In practice, this means:

• less of a “hollow” sound,
• a softer, more pleasant footfall,
• a lower risk of the terrace starting to “talk” after a season.

Note: if the terrace is poorly designed (joists spaced too far apart, inadequate support, looseness), pads alone won’t work miracles. But as part of a noise-reduction system, they are one of the simplest and most cost-effective solutions.

How do pads affect the durability of the substructure and the entire terrace?

The principle here is simple: dry + stable = long-lasting.

Pads help in three key areas:

1. Less moisture beneath the joist. 

When a joist sits directly on concrete, moisture has ideal conditions to remain there for weeks. A pad creates a gap and improves water drainage.

2. Better ventilation. 

A small gap beneath the joist improves air circulation. And the faster the structure dries, the lower the risk of problems.

3. Less movement in the structure = less looseness. 

Looseness is a deck’s number one enemy: fixing clips start to “pop”, joints wear, and boards may sit unevenly. Stable support on pads reduces small movements.

Which pads for joists should you choose for a deck made of composite decking boards?

You’ll most often come across three solutions. Each has its uses, but in different conditions.

Rubber / EPDM pads (the most common choice for WPC)

Who are they for? For people who want to reduce noise and isolate joists from the substrate.
Advantages:

• good vibration damping,
• moisture resistance,
• kind to waterproofing (lower risk of damage).

What should you watch out for? Choose a quality material — cheap alternatives can harden more quickly.

Plastic spacer pads (harder)

Who are they for? When you want stable, point support and a consistent height.
Advantages: water resistance, stability.
Disadvantages: less sound insulation than EPDM.

Adjustable deck supports (great for uneven ground)

Who are they for? When the deck needs levelling and the substrate is “moving”.
Advantages: quick levelling, easy installation.
Disadvantages: higher cost; a good layout plan is required.

The most common installation mistakes (and how to avoid them). 

• Pads placed too far apart.
  Result: flexing, clicking, reduced stability.
  Rule of thumb: more stable support points are better than “saving” on a few units.

• No slope or water drainage.
  Result: water collects under the deck and traps moisture.
  A slope away from the building and clear drains are essential.

• Mixing random materials. (cardboard, pieces of wood, tile fragments)
  Result: uneven levels, rotting pads, noise.
  Pads should be durable and resistant to water and pressure.

• Too small a gap and no ventilation.
  Result: moisture and accelerated deterioration of the components.
  Leave room for the structure to move and “breathe”.