Mice Under Decking
Mice are the hardest rodent species to exclude from beneath raised decking. They are smaller, faster, and capable of passing through gaps as small as 6–7mm. Once beneath the deck frame, they nest rapidly and establish territory that is extremely difficult to break without closing the structural access point.
Why mice are particularly difficult to exclude from decking
The common house mouse (Mus musculus) and the wood mouse (Apodemus sylvaticus) are the two species most commonly found beneath garden decking. Both are capable of passing through gaps of 6–7mm — smaller than a standard pencil diameter — which means that even a very tight perimeter detail is insufficient to prevent access. The only gap size that reliably excludes both species is one no larger than 6mm, and even this requires consistent geometry across the full barrier.
Mice are also highly adaptable. They will test the perimeter of a structure repeatedly, finding and exploiting any weakness. A small crack in a concrete edging, a raised edge on a mesh panel, an unsecured join — any of these will be found and used. This is why perimeter-only approaches, however carefully installed, rarely prevent mouse access over time.
Beneath decking, mice find excellent nesting conditions. The boards above provide protection from rain and aerial predators. The sheltered, dark void is away from foot traffic. Nesting material — soil, plant debris, grass — is readily available at the perimeter. A mouse can establish a nest and begin breeding within days of finding suitable harbourage.
The damage mice cause beneath decking
While mice are smaller than rats, the damage they cause beneath decking structures should not be underestimated. They gnaw timber floor joists and bearers — not for access reasons, but to maintain their continuously growing incisor teeth. Over time, this gnawing weakens structural components in a way that is invisible from above.
Mice also target any electrical cable running beneath the deck — for outdoor lighting, power sockets, or garden speakers. Cable gnawing is a fire risk and requires professional repair. The presence of mice in a sub-base void is therefore not just a nuisance — it is a genuine safety issue in structures with underfloor electrical infrastructure.
Mouse droppings and urine accumulate rapidly. A single mouse produces up to 80 droppings per day. In a confined sub-base void, contamination builds up quickly and creates a persistent odour and hygiene problem, particularly in warmer weather when decomposition accelerates.
Why deterrents and bait fail against mice under decking
Ultrasonic deterrents, scent-based repellents, and bait stations are the most commonly tried solutions for mice under decking. None of them close the structural access point — and that is the fundamental problem.
Ultrasonic devices have no consistent evidence base for effectiveness in open sub-base environments. Mice habituate to novel stimuli quickly. Scent repellents dissipate rapidly in outdoor conditions and require constant reapplication. Bait stations kill individual mice but do not prevent re-colonisation from the surrounding garden — which, in any established garden environment, maintains a steady background population of mice.
The correct solution: fine mesh sub-base barrier
The only reliable prevention for mice beneath decking is a welded wire mesh barrier installed flat at ground level across the full deck footprint before the frame is fixed. The specification is critical: the mesh opening must be no greater than 6–7mm to reliably exclude mice. A 7.66mm clear opening, as used in BaseShield™ SubBase Defence, sits at the upper limit of mouse exclusion — effective against common UK species under normal conditions.
The barrier must cover the full footprint, not just the perimeter. Mice burrow. A perimeter-only barrier will be circumvented as rodents excavate beneath it and re-enter the sub-base from inside the perimeter. Ground-level coverage is the only approach that works.
The right product for this application
BaseShield™ SubBase Defence has a 7.66mm clear mesh opening — sized to exclude common UK mouse species as well as rats. The welded construction provides consistent geometry across the full mesh surface, with no distorted or enlarged apertures that mice could exploit.
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 flat across the deck footprint before the frame is fixed. For mouse exclusion, ensure every join is overlapped by a minimum of 100mm and perimeter edges are folded upward to eliminate the edge gap.
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.