A cat net fails in 2 places: a gap wider than 5 cm, and an anchor that pulls out of 8 mm of plaster. Both are measurable before installation. Get the mesh to 5 x 5 cm (3 x 3 cm with kittens), keep anchor spacing at 30-40 cm, and size every fixing for 80 kg, and the net will hold a 5 kg cat arriving at 7.7 m/s.
Height decides whether this is worth doing at all. A fall from 3 m arrives at 7.7 m/s, from 6 m at 10.8 m/s, and from 10 m at about 14 m/s, or 50 km/h. Above 3 m, protection is not optional.
The fall thresholds, in metres per second
A cat that falls 1 m usually walks away. At 2-3 m, injuries are common but rarely fatal: split palate, fractured teeth, one foreleg. From 3-7 m the injury list lengthens to include the jaw, the hard palate, both forelimbs and the chest, and these cats need 24-72 hours of observation even when they look fine on landing.
Above 7 m the pattern changes again: cats right themselves and spread out, which limits speed but converts the landing into a chest and head impact. Any window or balcony with a drop over 3 m, which is every floor above the first, needs a barrier rated for the load below. The cat does not need to be a jumper; 60% of falls happen during a slip or a chase, not an escape attempt.
Mesh size: 5 cm for adults, 3 cm for kittens
5 x 5 cm is the working maximum for an adult cat. The skull is the limiting dimension, not the body, and a 5 cm square passes a 4 kg cat's head with 5-8 mm to spare — which means a 3.6 kg cat can push through. If any cat in the house is under 6 months or under 2.5 kg, go to 3 x 3 cm; a 6-week kitten fits through a 4 cm gap.
The same number applies at the edges: a 6-10 cm gap at the floor line, wall junction or top rail is where cats get stuck, and a stuck cat at 3 m is worse than a free one. Seal the bottom to within 2 cm of the floor with a batten, and run the top edge to the ceiling or fit a 30 cm overhang angled 45 degrees inward.
Material: 2 mm stainless against 4 mm nylon
304 stainless wire at 2 mm breaks around 150-180 kg per strand before knots; the same wire at 1.5 mm is about half that, which is still 4-6 times the working load. Nylon at 4 mm holds 40-60 kg with knots, 5 mm holds 60-90 kg, and both lose 30-50% of strength after 3-4 years of UV in direct sun. In a covered balcony nylon lasts 5-7 years; on a south-facing rail, plan on 3.
Wire costs more per square metre and is the right choice above 10 m, where wind load also enters the calculation: a 1 m² panel at 30 m/s wind sees roughly 550 N, or 56 kg, on top of the cat's impact. Below that height, 4 mm nylon with 5 cm mesh and correct anchors does the job.
Anchors: spacing, depth, and 80 kg per point
Anchor spacing 30-40 cm along every frame member, 50 cm absolute maximum. Each corner gets a double fixing within 10 cm of the corner, because that is where the membrane load concentrates: a 5 kg cat hitting the middle of a 3 m span loads the corner anchors about 2.5 times harder than the centre ones.
Into concrete or solid brick, use M6 x 60 mm expansion anchors in an 8 mm hole drilled 60 mm deep, and expect 80-120 kg pull-out each. Into hollow brick, aerated block or drywall, expansion anchors are useless — use M6 chemical anchors with a 80 mm screen, or through-bolt into the slab. Never fix into a window frame's plastic trim, aluminium flashing or a 10 mm render skin; those hold 5-15 kg and peel off quietly.
Span, sag and tension, in one table
| Span (m) | Anchors | Anchor spacing (cm) | Wire (mm) | Pull-out each (kg) |
|---|---|---|---|---|
| 1.0-1.5 | 6 | 30 | 1.5 | 80 |
| 1.5-2.5 | 8-10 | 30 | 2.0 | 80 |
| 2.5-3.5 | 12-14 | 30-40 | 2.0 | 100 |
| 3.5-5.0 | 16-20 | 30-40 | 2.0 | 120 |
| 5.0-7.0 | 24-30 | 40 | 2.0 + mid rail | 120 |
Tension the wire to 150-250 N by hand: firm enough that the panel does not touch the cat when pushed, loose enough that it gives 3-5 cm. Sag over 8 cm per 2 m section means the panel will bottom out against the railing, and the cat will climb it like a ladder. A 2 m span that sags 5 cm at 20 kg of push is correctly tensioned.
Installation, in the order that avoids rework
1) Measure the opening at 3 points (top, middle, bottom); balconies are rarely square and the difference is often 2-4 cm. 2) Mark anchor positions at 30 cm, adjusting to land on solid material. 3) Drill 8 mm, 60 mm deep, blow out the dust, and test one anchor with a 40 kg pull before drilling the rest. 4) Fix the frame or the first wire run, then tension from the top down. 5) Close the floor gap to 2 cm and the top to the ceiling. 6) Load-test with 10 kg for 24 hours, then 20 kg for 10 minutes at the centre of each span.
Keep the net 5 cm clear of AC units, drain pipes and cables so the cat cannot use them as a step. If the balcony has a 90 cm railing and no ceiling fixing, add 2 vertical posts at 1.5 m spacing rather than tying the top edge to the railing — the railing is rated for 50-100 kg of horizontal push, not for a cat's 148 J impact plus wind.
Inspection: 6 months, or after any storm
Check every 6 months and after any wind above 20 m/s: run a hand along each wire for slack, look for 1 mm nicks at anchor points (that is where wire fails), and re-tighten anything that moves more than 5 mm. Nylon goes chalky and stiff before it fails; if a 4 mm cord will not bend double in your hand, replace the panel.
Replace the whole panel, not single strands, when 3 or more wires are broken in one square metre. A repaired patch is 30-40% weaker at the knots and is where the next failure starts.
Do this today
- Measure the drop: anything over 3 m needs a net rated for 80 kg per fixing.
- Choose mesh by the smallest cat: 5 cm adults, 3 cm kittens under 6 months.
- Mark anchors at 30 cm, double at corners, and never into trim or render.
- Tension to 150-250 N and check sag stays under 5 cm over 2 m.
- Load-test 10 kg for 24 hours before the cat is allowed out.
Windows in the same room need the same treatment: a stopper limiting the opening to 10 cm, because a 10 cm gap passes an adult cat's shoulders. Fall mechanics are in window and balcony falls, the wider checklist in balcony and window safety. Budget anchors and wire before the cost breakdown.
Terms mentioned in this article
Frequently asked
What mesh size keeps a cat in?
5 x 5 cm is the maximum for an adult, because the skull is the limiting dimension and a 4 kg cat's head clears a 5 cm square by only 5-8 mm. Go to 3 x 3 cm for kittens under 6 months or any cat under 2.5 kg. Edge gaps of 6-10 cm at the floor, wall or top rail are where cats actually get stuck, so seal the bottom to 2 cm.
How much weight should each anchor hold?
Design every fixing for 80 kg. Use M6 x 60 mm expansion anchors in an 8 mm hole drilled 60 mm deep for concrete or solid brick, which gives 80-120 kg pull-out each. Hollow brick, aerated block and drywall need chemical anchors with an 80 mm screen, and window trim, flashing or a 10 mm render skin should never take load at all.
How high does the net need to go?
From the floor to at least 110-120 cm, and ideally to the ceiling, since a cat can jump 150 cm from standing. If there is no ceiling fixing, fit a 30 cm overhang angled 45 degrees inward or add vertical posts at 1.5 m spacing. Tension the wire to 150-250 N and keep sag under 5 cm over a 2 m span.