Ants Coming Through Patio Doors - Where Is the Nest Likely to Be?

What Is Under a Patio

Paving is laid on sand or a dry mix, over hardcore, over soil. That is close to ideal for a black garden ant: excellent drainage, easy tunnelling, and a solid slab above it that holds the sun’s heat well into the evening.

Warmth is the part people underestimate. A south-facing patio is several degrees warmer than open ground, which speeds up brood development and lets a colony grow larger than it would elsewhere in the same garden.

The Signs Directly Outside the Door

Fine granular soil pushed up between the paving joints — small heaps of what looks like sand. Mortar or jointing compound washing or crumbling out of the same joints repeatedly. Slabs that have started to rock slightly.

Then watch the trail. In the warm part of the day the column will be obvious, and following it will normally take you to a joint, the edge of the slab run, or the gap where the paving meets the house.

The Other Usual Locations

Under a doorstep or threshold slab. Along the base of the wall where the patio meets the house, particularly if there is a gap at the damp course. Under decking. Beneath a planter or a pot that has not been moved. At the edge of the lawn where it meets the paving. Under a garden path.

Effectively: anything flat, warm and undisturbed with soil beneath it. If you lift one thing in the garden, lift the slab nearest the door.

Why They Come Through the Door Specifically

The track of a sliding patio door is an almost perfect entry route — a continuous gap, sheltered, running the width of the opening, with drainage slots straight through to the outside. French doors leak at the threshold and at the meeting stiles.

And it is the shortest route from a warm slab to a kitchen. The nest does not have to be far away, and it rarely is.

What to Do About It

Do not spray the trail. It kills what is on the surface, leaves the colony intact and stops gel bait working. Gel bait relies on foragers carrying it home, which is exactly the journey you have just traced.

Deal with the nest, then close the route: brush strips or a threshold seal on the door, and repointing the paving joints once the colony is gone. Ant treatment is £190 for a flat and £220 for a house.