By far the most popular method of making a pool nowadays – and this must say something for the simplicity and effectiveness, as well as the economics of the technique – is to line an excavation with a sheet of flexible plastic, referred to as a pool liner. Of the different kinds of sheet plastic that can be used for this purpose one is so different from the rest in its qualities that it requires a different installation technique, which had better be described separately. Polythene. I remember very clearly my first use of polythene to make a pool, at a time when the only alternative material was concrete. The ease and speed with which the pool was completed seemed almost too good to be true. In a way it was: before long the pool was suddenly empty, the material torn by an amphibious dog. The incident illustrated vividly both the possibilities of plastics and the vulnerability of polythene. It is still offered for pool making and it still has two outstanding characteristics: it is by far the cheapest and by far the least reliable of pool liners. If you want a temporary pool it may serve but, with much stronger plastics now ‘ available, polythene is completely out of the race as far as serious pool making is concerned.
The method of calculating the size of polythene liner required differs from that used to calculate a stretchable pool liner. Because polythene will not stretch to mould to irregular curves its use is generally limited to rectangular pool shapes. The size required is found by laying a tape in the hole, down the sides, over shelves, along the bottom and up again, first lengthwise, then widthwise, and adding to each of these two measurements an extra foot for overlap. Two sheets – or one folded double – to this size will be needed in 500 gauge polythene. The second offers some safety margin if the top one pinholes. Clear polythene is useless; blue is usual; black may last a little longer.
Again because it does not stretch, polythene must be tucked right down into the excavation before the water is run in. When the water is at shelf level a pause is desirable to fold and pleat the material in the corners before filling is resumed. The overlap is secured by paving stones projecting 2 in. over the water.
Both cold and sunlight work gradually to make polythene brittle above the waterline but whether it lasts two months or two years its useful life is usually terminated by some minor accident or by the spontaneous production of pinholes. Once punctured it is usually a write-off. I have, in emergencies, been able to repair a single puncture with a device used for mending holes in saucepans – a pot-mender to your ironmonger – but multiple holes or a tear are beyond treatment. There is no adhesive that will fix a patch irremovably to polythene and the only sensible thing to do when a polythene liner gives up the ghost is to replace it with something better – one of the stretch-able pool liners.
The great misfortune of PVC is that it looks very much like a sheet of polythene. Superficially it is so similar that it is often assumed to be a thicker version of that unreliable material. Nothing could be further from the truth, PVC is chemically a different kettle of polymers altogether and is immeasurably superior to polythene. It is far less susceptible to accidental damage, but if ever pierced by a sharp instrument repair is no problem. A patch of PVC applied with Bostik No. 1 clear adhesive is completely effective. The other great virtue of PVC is stretchability, which means that it can be stretch-fitted, a technique described below. It is sold under such brand names as ‘Juralene’, ‘Aqualene’ and ‘Lakeliner’. One excellent form is a laminate, blue on one side and stone on the other. Sizes widely available range up to 15 by 12 ft.
Reinforced PVC. A PVC sheet laminate with a close weave of nylon reinforcement is perhaps the most sophisticated pool liner to date. Designed specifically for pool making, its composition incorporates a bac-teriostat to prevent damage to the liner by soil bacteria and (optimistically perhaps) to inhibit the growth of algae on the liner surface. Sold under such names as ‘Flexilene’ and ‘Wavelock’, nylon-reinforced PVC has excellent qualities of strength, flexibility and stretch. Blue on one side and stone on the other, it is widely available ‘off the shelf in sizes up to 20 by 20 ft., and easily obtainable on order in any larger size that may be required, without limit. Butyl Rubber. Also available in almost any size you care to name, butyl rubber sheet is a strong and very elastic material whose qualities are generally comparable to reinforced PVC with, perhaps, even more stretch, and a little less strength in terms of resistance to puncturing. Its charcoal black colour offers an alternative to the sandstone and blue shades of the two PVC types.
The strength and elasticity which distinguish PVC, reinforced PVC and butyl rubber liners permit an installation technique which minimises wrinkling. The pool excavation is prepared by removing stones and putting down about half an inch of sand or sifted soil-just enough to provide an even bed for the liner without projections. With this precaution it is possible to stand heavy ornaments in the pool – and to stand in it yourself if need be – without any risk of damage. Damp sand will cling to the sloping walls, or damp newspapers will serve, if the walls appear undesirably rough. The liner is then stretched taut over the hole and held firmly – but not immovably with either a line of house bricks all round its edge, or by pieces of paving stone more widely spaced. On no account use massive lumps of knobbly rockery stone. Then it is only a matter of directing the hose towards the middle of the sheet, turning it on and standing back to admire the way the weight of water takes the liner down, spreads it along the bottom, smooths it up the walls, pushes it across the shelves and moulds it firmly to every contour of the excavation. Some wrinkling may occur, more particularly where the curves are sinuous and the pool shape complicated; another good reason for keeping it simple. With the pool full, the anchoring bricks can be removed. There is no danger of the liner snapping back under water when the weight is off. It is kept firmly in place by water pressure.
Surplus liner material is trimmed off with scissors. Leaving a flap 6 or 8 in. wide which may be nicked if necessary to allow it to lie flat and provide an even surface on which to lay edging stones. They can, if you like, be bedded on sand and cement, and they should certainly overhang the water by a couple of inches.
The finishing stone round a pool (of whatever material) should be as solid as possible. It is an area where there is going to be a lot of foot traffic, and at least part of the perimeter should be well and firmly paved. Projecting the slabs 2 in. over the water means that if the pieces are too small they will tend to rock on the edge, and might even tip an unwary visitor into the water. This is to be avoided, if only on the grounds that it is bad for the plants and the fish.
Calculating the size of stretchable liner needed for an excavation is very simple. You first need to measure the size of the rectangle which will enclose the pool shape, that is, the overall length and the overall width. To each of these measurements you add twice the maximum depth, and you have the size of the liner. For example, a pool 10 ft. long overall and 7 ft. wide overall, and H ft. maximum depth will need a liner 13 by 10 ft. This will serve for any pool shape within a 10- by 7-ft. Rectangle, and whether or not there are marginal shelves makes no difference whatever. There is no need to make any extra allowance for the flap. The slight slope of the sides and the elasticity of the liner fabric produces ample surplus for this, so the calculation is delightfully simple.
While this stretch-fitting technique is in accordance with the recommendations of the material suppliers, and is invariably found to work like a charm, I have a personal reservation in the case of butyl rubber. The method works as well for butyl as for the others, but I suspect that the odd cases of plant orgrowth penetrating this material have been due to stretching and it might be better to install it, like polythene, by tucking it down into the excavation before filling.
A stretchable pool liner is now recognised as the most reliable means of repairing a cracked concrete pool, because the liner’s elasticity can cope with any subsequent movement of the concrete resulting from further soil settlement. Stretch-fitting in such cases is not advised, because of the damage that could result from the liner being dragged, under stress, over the concrete pool edges, which can be very rough. It is better to put the liner down into the old pool (which must be scrupulously cleared of concrete fragments, and given a bedding layer of sand) and run in about 3 in. of water. This will be enough to anchor the liner. Then get into the pool (barefoot is easiest) and, as the water rises, smooth the liner up the walls and pleat it at the corners to obtain the neatest possible finish.
Flexible liners are more flexible when they are warm; on a really cold day it is surprising how they stiffen up. It is always a good idea to spread a liner on the lawn in the sun before installation: it makes it even more supple and easy to install. If it is a cold day, keep the liner indoors until the last minute, in an airing cupboard if possible. But please don’t put it close to a fire or sit it right on a hot radiator.
Length of Life
The question I am asked most often about pool liners (apart from which is .the best, to which I answer butyl rubber) is how long will they last? The answer, except where polythene is concerned, is that, in practical terms, nobody yet knows. Many of the early PVC liners that were put in ten years ago are still going strong. A few have been found to suffer from degradation due to soil bacteria and some from uneven distribution of plasticiser in the sheet. Since those days, however, there have been significant improvements in formulation and in production techniques so that the liners now on sale are bound to have a greater expectation of life than those made even a few years ago. There is no reason, based on laboratory tests, why the more sophisticated reinforced PVC and butyl liners should not last 50 years. Even more impressive ‘estimates based on accelerated ageing tests’ have been made, but how relevant they are to practical conditions remains to be seen. I would be more impressed by a firm guarantee, even for a period of ten years. I believe that 20 years would be an entirely possible, even conservative length of life to expect from the best of modern pool liners and that, without any doubt at all, is much longer than the useful life of the average concrete pool.
Choice of Colour
A question that bothers people a good deal more than it need, concerning glassfibre pools and liners, is which is the best colour. This must be a matter of personal taste. My own inclines strongly towards the more natural stone colour. The truth is that it doesn’t much matter in the long run because a thin film of algae and sediment soon obscures the original colour. It shows up badly on blue, much less conspicuously on stone or grey. The pool, after all, is not there to be
seen: it is just a neutral backcloth against which the stars of the show, the plants and fish, will be admired.
As far as costs are concerned there is little point in quoting prices which, in this inflationary age, are likely to be soon out of date. It may be useful, however, to indicate approximate relative costs which should remain broadly true unless some breakthrough in production techniques drastically alters the price of any of the listed materials.
Rather surprisingly the cost of concrete (which includes cement, aggregate, sand, waterproofing powder and Silglaze) proves to be on a par with the best stretchable liners. If the cost of labour was included concrete would move even higher in the cost table.