How to Protect the Underside of Decking
The underside of a raised deck is its most vulnerable surface — not to weather or decay, but to the animals that use the sub-base void as shelter. Rats, mice, foxes, and squirrels all exploit the space beneath raised decking. Protecting it requires a structural approach, not a deterrent-based one.
Why the underside of decking is exposed to animal access
Raised decking is constructed on a joist frame that elevates the deck surface above the ground. The joists are supported on bearers, pedestals, or posts — creating a horizontal void between the deck boards above and the ground below. That void is the decking sub-base, and it is accessible from ground level at the perimeter and wherever the frame support system creates gaps in the base detail.
The underside of the deck boards is the roof of this void. It is often overlooked as a surface requiring protection because the damage is not visible from above. But beneath the boards, the structural joists, the bearer connections, and any electrical or water services running through the frame are all exposed to whatever animals are using the sub-base.
The solution to protecting the underside of decking is not to treat the underside of the deck boards — it is to prevent animals from accessing the sub-base void in the first place. A ground-level barrier achieves this and protects all components of the deck structure simultaneously.
What animals access the underside of decking
Rats are the most frequently reported and the most structurally damaging species. They gnaw joists and bearers, contaminate the void, and in established colonies access the deck surface through gaps between boards. Mice are less immediately visible but cause rapid insulation and cable damage in decks with underfloor services. Foxes use the void for breeding dens, creating severe odour and contamination issues. Squirrels cache food and gnaw structural timber.
The full range of animals that use decking sub-bases is broader than most homeowners realise — domestic cats, hedgehogs, and various bird species also use the space for shelter. While not all species cause structural damage, any animal activity in the sub-base can displace soil, compromise bearer positions, and attract rodent predators that cause additional damage.
Approaches that don't work
Perimeter cladding and skirting boards are the most commonly installed 'protection' for the underside of decking. They conceal the gap but do not close it. Rodents burrow beneath skirting within days of installation. Foxes remove lightweight skirting materials physically. The sub-base remains accessible.
Bait stations and traps address existing populations but not the access route. Ultrasonic devices have no consistent evidence base. Scent deterrents require constant reapplication and lose effectiveness quickly outdoors. None of these approaches — individually or in combination — reliably protects the underside of decking long term.
The correct approach: ground-level barrier before construction
A ground-level welded wire mesh barrier installed across the full footprint of the deck — before the joist frame is fixed — eliminates the accessible sub-base void. Animals cannot burrow beneath the barrier from outside. The sub-base becomes a sealed space rather than an accessible cavity.
The specification for this barrier: welded wire mesh, clear mesh opening maximum 8mm (for mouse exclusion), wire diameter minimum 0.80mm (for gnaw resistance), hot-dip galvanised after welding (for ground-contact corrosion resistance). The barrier covers the full footprint — not just the perimeter — and is secured at the edges before any structural work proceeds.
The right product for this application
BaseShield™ SubBase Defence is the purpose-built solution for protecting the underside of decking from ground level up. Installed before the deck frame is fixed, it covers the full footprint and prevents multi-species sub-base access for the lifetime of the deck.
BaseShield™ SubBase Defence
Forti NovaA purpose-built rolled sub-base barrier system designed specifically to prevent rodent access beneath raised structures. Supplied in 1m × 25m rolls — covering 25m² per roll — with a 7.66mm clear mesh opening and hot-dip galvanised finish applied after welding.
Roll size
1m × 25m
Coverage
25m² per roll
Mesh opening
7.66mm
Construction
Welded wire
Finish
Hot-dip galvanised
Wire diameter
0.80mm
Installation
Deploy BaseShield across the full deck footprint before the joist frame is fixed. For a standard rectangular deck, the 25m roll length allows full coverage in parallel runs with minimal joins. Fold the perimeter edge upward against the outer joist before fixing.
Read the full installation guideFrequently Asked Questions
Stop access beneath your structure
BaseShield™ SubBase Defence is installed during construction to provide permanent sub-base protection. Designed for garden rooms, modular buildings, decking and screw pile foundations.