Why a Few Millimetres Matter More Than You Think

Most of us picture a mouse squeezing under a door and assume the gap must be enormous. In reality, a mouse only needs about 6mm — roughly the width of a pencil. A rat needs closer to 20mm, though a young one will manage with less. Insects and spiders are far less demanding still: many will walk happily through a 1–2mm crack, and a trail of ants can pass through a gap you would struggle to see in daylight.

Multiply that by every door frame, window reveal, air brick and pipe entry point in a typical UK home and you have hundreds of potential doorways. The encouraging part is that sealing them is one of the cheapest and least disruptive jobs in pest control. No scaffolding, no builders, no mess — just a few pounds of materials and a free afternoon.

A Ten-Minute Survey of Your Home

Before you buy anything, walk around the outside of the property with a torch, ideally on a dry day. Go slowly and look at the details rather than the overall picture. The following spots account for the majority of unwelcome entries:

• The bottom, sides and top of every external door, including the gap between the door and its frame.
• Window frames where sealant has cracked, shrunk or peeled away — especially on the sunniest side of the house.
• Where pipes, cables, soil stacks and meter boxes pass through the wall.
• Air bricks, vents and the weep holes at the base of aluminium or uPVC window frames.
• Garage doors, shed openings and any integral door leading to a loft, cellar or utility area.

A sheet of paper makes a handy gauge: if it slides through a gap easily, so can an insect. For anything larger, a pencil or a torch beam will tell you whether a rodent could follow.

Doors: Brush Strips, Wipers and Letterbox Seals

Doors are the front line. A screw-on brush strip along the bottom of a door is inexpensive, easy to fit and hard-wearing, and it will close a gap of up to about 20mm. For a tighter, more weatherproof finish, a rubber or silicone wiper seal combined with a threshold plate works well, particularly on back doors that take the brunt of the rain.

Do not stop at the floor. Check the hinge side and the top of the frame too, where older doors often warp away from the frame. Sliding patio doors are a common weak point, as the track and meeting stiles can leave wide vertical gaps; a brush or fin seal fitted to the leading edge usually solves it. Finally, fit a brush to the inside of the letterbox, and consider a cover for the keyhole — both are classic entry points that are easy to overlook.

Silicone Sealant, Frame Sealant and Expanding Foam

Cracked sealant around window and door frames is a gift to insects. Scrape out the old material, clean the surface, and let it dry completely before applying new sealant — silicone will not bond to a damp or dusty surface, and any residue of old sealant will shorten its life. An exterior-grade low-modulus silicone or acrylic frame sealant will cope with movement far better than a rigid filler, and it can be smoothed with a damp finger or a sealant tool for a neat finish.

For gaps wider than roughly 10mm, sealant alone is not the answer. Expanding foam fills the space well, but mice and rats will chew straight through it. Push stainless steel wire wool or a strip of metal mesh into the gap first, then foam over the top so the foam holds the mesh in place. Used that way, it is a genuinely permanent repair rather than a temporary one.

Mesh and Vents: Sealing Without Blocking Airflow

Air bricks and vents exist for a reason, and blocking them completely can cause damp, condensation and, in the case of gas appliances, a serious safety risk. Never seal a vent that serves a boiler, fire or any other fuel-burning appliance — if in doubt, leave it alone and take advice.

Instead, fit a fine stainless steel or aluminium insect mesh with an aperture of 1–2mm behind the existing grille. It keeps insects, spiders and wasps out while letting air move freely. The same approach works for weep holes at the base of window frames, which must still be able to drain rainwater. Trim the mesh slightly larger than the opening so it sits snugly, and fix it with a few dabs of sealant or small screws.

Keeping It Sealed All Year Round

Seals do not last forever. Ultraviolet light, frost and the natural movement of a building all take their toll, so make a habit of checking your doors and windows each early autumn, just as insects and rodents begin looking for somewhere warm and dry. It is a fifteen-minute job that saves a great deal of trouble later.

Sealing works best alongside a few simple habits. Keep bin bags off the ground, store dry goods in sealed containers, fix dripping taps and clear up fallen fruit promptly. Trim shrubs and ivy back from walls so they do not form a bridge to your window sills, and keep external waste bins lidded. Do those things and your sealed gaps become a genuinely effective barrier — quiet, inexpensive, and far less hassle than dealing with an infestation once it has settled in.