Pool covers: heat, safety and fewer leaves

What a cover is actually doing

Water leaving the surface as vapour is the pool's biggest heat loss — larger than the walls, the floor and the cold night air put together. The physics is not negotiable: every litre that evaporates carries away roughly 0.6 kWh of heat. An uncovered 8 × 4 m pool on a warm, breezy day loses well over a hundred litres. That single line of arithmetic is the whole case for covers.

A lid stops it. Cover a heated pool overnight and the heating energy drops by 50–70%, and the water is a degree or two warmer at breakfast, so the heat pump starts the next day from a better place. Everything else is bonus: no leaves, no dust, no insects, and much less chlorine burned off by sunlight, so a covered pool is noticeably cheaper to keep in balance.

One caveat, said early: a cover is a lid, not a filter. The water underneath still needs its daily circulation, and a pool kept shut through a heatwave gets warm, flat and chemically lazy. Cover it at night and on cold days. Open it when you swim.

Solar blankets: the cheap workhorse

The bubble-foil blanket floats straight on the water, bubbles facing down. Over a sunny week it adds a few degrees, and every night it stops evaporation. Buy 400–500 micron material rather than the thin supermarket stuff; it survives handling. Have it cut to the water surface and about 5 cm short all round, so it lies flat instead of riding up the walls.

The whole question is handling. A blanket for an 8 × 4 m pool is 32 m² of wet, clingy foil; without a roller station nobody in the house will put it back on twice. With a roller at one end it is a thirty-second job for one person. Buy the roller with the blanket or buy neither — the roller is not an accessory, it is the thing that makes the blanket real.

Expect three to five seasons. Chlorine and UV are what kill it, so when it is off the water keep it rolled and shaded rather than spread on the lawn, and never leave it on while you shock the pool. It is cheap to replace, which is the point.

Slatted automatic covers: the one that gets used

Rigid PVC or polycarbonate slats roll out over the water at the push of a button, driven by a motor inside the roller tube. They insulate better than foil, they read as a surface rather than a tarpaulin, and closing them takes ten seconds — which is why they are actually closed every night. That last point is worth more than any line on the datasheet.

There are three ways to house the roller. A dry pit under a bench or the deck at one end looks best and needs a 60–80 cm recess cast with the shell. A submerged roller sitting in the pool at one end is cheaper, but it eats 50–60 cm of swim length — draw the pool longer to compensate. On-deck brackets are the retrofit answer, and they look like one.

Slats want straight, parallel walls, so rectangles and rounded rectangles are home ground; a Roman or oval pool needs an overhang or a custom shape, and a free-form pool cannot take one at all. Translucent solar slats with a sealed air chamber add gain as well as insulation; opaque white or grey slats keep the water cooler and age more slowly under strong sun.

Every cover works on paper. The one that pays for itself is the one that is on the water at ten at night in October.

Safety covers, and what safety means

A cover is a safety device only if it is certified as one and only while it is fully closed. The European reference most of the industry quotes asks a safety cover to carry an adult's weight and to resist a child getting under the edge. A bar-supported or tensioned mesh cover anchored into the deck does that. A floating blanket does the opposite: a child who slips beneath it is invisible from two metres away.

Walk-on rated slatted covers exist and are worth asking for, but read the conditions — the rating assumes the cover fully extended and the slats undamaged. A half-open automatic cover protects nobody, and a pool left with the key in the switch is a pool a five-year-old can open. Treat the cover as one layer: with small children in the garden, a fence, an alarm and the cover are not alternatives.

Winter covers and the off-season

A winter cover has a different job: hold five months of leaves, keep light off the water so algae stay asleep, and keep children and animals out while nobody is watching the pool. Tensioned mesh anchored into the deck is the practical choice — rain and snowmelt drain through, debris stays on top, and there is no pond of black water to pump off in March.

Solid tarpaulins cost less and need a cover pump, which needs somebody to think about it. A slatted cover can overwinter on the water in a mild climate, but ask the manufacturer first: ice crushes slats, and most want the water level dropped and floaters added. In mild regions active wintering — pool full, cover on, pump running on frost nights — increasingly replaces draining and blowing out the lines, and opening in spring is a morning rather than a weekend.

The pit is a structural decision

The most common cover regret is deciding on the automatic one after the shell is poured. The roller pit is cast with the structure. Retrofitting it means cutting finished concrete, re-waterproofing and re-tiling — that is not an upgrade, it is a different project with a different price.

So make the decision at design time even if you buy the cover two years later. Cast the pit, run the conduit for power and the key switch, and build the bench that hides it. An empty pit and a length of conduit cost very little; the cover then drops in over an afternoon. Draw the bench into your plan in the designer and you will see exactly what those 80 cm cost you in swim length — nearly always worth it.

Put the key switch where the whole pool is visible. You should never be closing a cover you cannot see across.

The only specification that matters

Covers are one of the few pool components where the engineering is easy and the behaviour is hard. They all work. The one that pays for itself is the one that goes on the water at ten at night in October, when it is drizzling and nobody feels like it.

So buy for effort rather than for numbers. A cover that needs two people and a spare shoulder gets used in July and forgotten in September — precisely the month it was bought to earn. A push-button cover has a use rate near 100%. A blanket with a good roller comes close. A blanket without one rounds to zero, whatever it cost.

What the supplier will ask

Have these ready and the quote will be right first time: the exact water-surface dimensions and the shape; whether the walls are straight and parallel; the water level relative to the coping; which end the roller goes and whether there is a pit; the power supply at that end; and whether you want a walk-on rating.

Then ask three things back. What is it rated to carry, and in what state is that rating valid? What does a replacement slat, or a new blanket, cost in five years? And how does it behave in wind — an automatic cover that lifts at the corners is a cover that will be damaged, and the fix is a detail at the ordering stage, not a repair afterwards.

What goes wrong in practice

1. The cover decided after the concrete

   Once the shell is poured, a roller pit means cutting, re-waterproofing and re-tiling. Cast the pit even if you buy the cover later; an empty recess costs almost nothing.

2. A blanket with no roller

   Thirty-two square metres of wet foil, handled by hand, gets used twice. The roller is what turns a good idea into a nightly habit — order them together.

3. Trusting a solar blanket around children

   It hides the water rather than holding weight, which is worse than no cover at all. Only a certified safety cover counts, and only when fully closed.

4. Cooking the pool under a closed cover

   Shut through a July week the water climbs past 30 °C, the chlorine collapses and dark slats can distort. Open it in a heatwave, and never shock the pool with the cover closed.

5. Ordering to the coping instead of the water

   A blanket cut to the deck rides up the walls and wears along the folds. It should finish about 5 cm short of the water line all the way round.