The skimmer box — the weir, in South African pool language — is the hardest-working hole in your pool wall. Every leaf, every sunscreen slick and every litre your pump moves passes through it, and because it is a plastic box cast into concrete at the waterline, it is also one of the most common leak points on older Cape Town pools. Here is how it works, how it fails, and what honest repairs look like.
What the skimmer box actually does
The weir skims the surface — where debris, oils and algae spores concentrate — by drawing the top layer of water over its floating flap and through its basket to the pump. That little flap (the weir door) matters more than it looks: it speeds the surface draw and stops swallowed debris floating back out when the pump stops.
Skimmer quick health-check (2 minutes):
- Flap present and swinging freely? (Missing flaps = lazy skimming)
- Basket intact — no cracks or missing lid? (Broken baskets feed debris to the pump)
- Water level mid-mouth? (Too high skims nothing; too low gulps air)
- Hairline cracks in the box corners or throat? (The classic leak)
The big one: the cracked skimmer box leak
Weir boxes crack — ground movement, sun-brittled plastic, and the mechanical stress of being the joint between plumbing and shell all conspire, typically at the corners or where the pipe joins the box. Because the crack sits at or below waterline, it leaks constantly, and because the loss looks like “a bit of evaporation,” it runs for months — undermining the coping and paving directly above it. The tell-tales: persistent loss that stops around mouth level, damp paving behind the weir, and dye snaking into a corner when tested (our DIY leak detection guide shows the method). Repairs range from epoxy rebuilds on sound boxes to full box replacement — a proper job, since the new box must bond to shell and coping, and exactly the sort of work our leak detection and repair service exists for.
The small faults that cost you anyway
- Missing weir flap: skimming efficiency drops hard; debris sinks and rots instead of being caught — a cheap part with an outsized effect;
- Cracked basket: lets leaves through to the pump basket and impeller — replace it the week it cracks;
- Perished lid or vacuum plate: air leaks that cost the pump its prime and mystify owners for weekends;
- Blocked equaliser line: on pools that have one, a blocked equaliser makes the pump gasp at marginal water levels.
Skimmer care that prevents all of it
Empty the basket weekly (it is the single highest-value 30 seconds in pool care), keep the water level mid-mouth, replace flaps and baskets the moment they fail, and treat any persistent water loss as a weir suspect until proven otherwise. When bigger work happens anyway — relining especially — have the weir inspected and re-bonded as part of the job; it is the cheapest moment to renew it, and we fold it into relining scopes as standard. Suspect yours already? Book a free assessment — weir faults are among the quickest diagnoses we make.
Frequently asked questions
Why does my pump lose prime when the pool is a little low?
The weir mouth starts gulping air below half-mouth level — top up to mid-mouth and the problem usually vanishes. If it persists at correct level, suspect the lid seal or suction line.
Can a cracked weir be sealed without replacing it?
Sound boxes with limited cracking — often yes, with proper epoxy systems. Crumbling or repeatedly cracking boxes want replacement; we quote both options where both are honest.
Do I need two weirs on a big pool?
Pools over roughly 9–10m skim noticeably better with two — usually only worth adding during major renovation work.
What size skimmer basket do I need?
The one made for your weir model — forcing a near-fit basket leaves gaps that feed debris straight to the pump. Bring the old basket or a photo of the box when replacing.
