Black house spiders: the window-web builder explained

Those dense, messy grey webs in the corners of your window frames — with a spider that vanishes into a crack the moment you get close — belong to the black house spider. It's one of the most common spiders on Australian buildings, one of the most frequently mistaken for something dangerous, and the main food source drawing white-tail spiders into your home.

What a black house spider looks like

Black house spiders (Badumna insignis) are robust, dark charcoal-to-black spiders with a velvety look; the abdomen often shows a faint grey patterning. Females — the ones that build the big permanent webs — reach about 18mm body length with a legspan up to 30mm; males are smaller at around 10mm and wander looking for females. Legs are stout rather than spindly. Their smaller cousin the grey house spider (Badumna longinqua) shares the same habits.

The webs on your windows

The web is the signature: a dense, tangled, slightly funnel-shaped mess of grey silk with one or two tube-like retreats leading into a crack, gap or corner. Fresh silk looks lacy; older webs collect dust and turn into the grubby grey mats you see around window frames, eaves, vents, weep holes and outdoor light fittings. The spider sits hidden in the retreat and darts out when an insect hits the web — which is why the webs cluster around windows and lights: that's where the flying insects are at night.

Not a funnel-web: telling them apart

Because the web has a funnel-ish entrance and the spider is black and solid, black house spiders are constantly mistaken for funnel-webs. Three practical differences: location — funnel-webs are ground burrowers (and essentially absent from metropolitan Melbourne), while black house spiders live up on structures; web — a funnel-web's burrow has trip-lines radiating from a hole in the ground, not a woolly grey mat on a window frame; behaviour — black house spiders are shy and retreat instantly, while disturbed funnel-webs stand their ground. A black spider in a messy web on your window frame is, overwhelmingly, a black house spider.

Are they dangerous?

Not seriously. Black house spiders are timid and bites are rare — they'd rather retreat into the crack than defend the web. A confirmed bite can be painful, with local swelling and occasionally nausea, so it's not trivial — cold pack, and see a doctor if symptoms go beyond local pain (the Better Health Channel covers spider bite first aid). But no deaths are attributed to the species, and the main cost of a black house spider population is cosmetic — plus the predators it attracts.

Why your house has them

Two ingredients: insect traffic and cracks. Outdoor lights left on all night, gappy weatherboards, unscreened vents and weep holes make a building prime habitat. And here's the part most people don't know: a healthy black house spider population is a buffet for white-tail spiders, which hunt them specifically. Houses with lots of window webs get white-tail visits — controlling the webbing spiders is also how you stop the wanderers that end up in your laundry pile.

Getting rid of them

Brushing webs down helps but doesn't last — the spider is safe in its retreat and rebuilds overnight. Effective control combines: web removal, a residual treatment applied into the retreats and across the surfaces they build on (eaves, window frames, vents, fence lines), switching outdoor lights to warm/yellow LEDs that attract fewer insects, and sealing the cracks that make retreats possible. That's the method behind our spider control service — one treatment typically keeps buildings clear for many months, and for commercial frontages it's the difference between clean shopfront glass and webs over the signage. We treat homes and businesses across Melbourne year-round; late winter and spring treatments get ahead of the summer web-building season.

Frequently asked questions

How big do black house spiders get?

Females reach about 18mm in body length with a legspan up to 30mm; males are smaller, around 10mm. If you're seeing a much bigger dark spider away from a web, you're probably looking at something else entirely.

Are black house spiders dangerous to humans?

Not seriously. They're shy, bites are rare, and while a bite can be painful with local swelling, no deaths are attributed to them. Their main significance is the messy webs — and the white-tail spiders their presence attracts.

Is the spider in my window web a funnel-web?

Almost certainly not. Funnel-webs burrow in the ground and are essentially absent from metropolitan Melbourne; black house spiders live in webs on structures — window frames, eaves, vents. The woolly grey mat with a tube retreat is the black house spider's signature.

Why do the webs keep coming back after I clean them?

The spider survives web removal by hiding in its crack retreat and rebuilds overnight. Lasting control needs a residual treatment into the retreat plus reducing the insect traffic (outdoor lighting) that feeds the web.

Do black house spiders attract other spiders?

Yes — white-tail spiders hunt them as preferred prey. A building with lots of black house spider webs effectively baits white-tails inside, which is a strong reason to keep the webbing population down.

A1 Pest & Weed Control handles this for homes and businesses across Melbourne — licensed technicians, guaranteed results, and fast response: 0411 859 516 or get a free quote.