Koi

Sealing your pond is the last stage of the build before the water and fish can go in. Two of the most common methods that Koi is asked about are liner and fibreglass. Keith Holmes examines the pros and cons of each…

Having decided to build a Koi pond, one of the first major decisions that you’ll need to make is how to seal it and make it watertight. This is perhaps one of the most important judgments that you’ll have to consider, and although in today’s world a lot of choices are made according to price, this is one that shouldn’t be made purely on this basis. Making the wrong choice could end up costing you more than going for the pricier option.

There’s a mass of different pond paints and sealants on the market, but the two most popular choices for pond sealing are liners and fibreglass. Although in an ideal world every pond would be fibreglassed, this is not always possible; and liners are surprisingly versatile, and, in some instances, prove the
best option.

At the end of the day, you should look at both and make your choice, based on what is not just right for your budget, but also the location of your pond and chosen construction method. To help you make your mind up, though, we’ll look at the pros and cons of each, starting off with liners.

Get in line
This is perhaps the easiest and cheapest way of creating a pond, and is ideal for smaller constructions, or ones where the added expense of blockwork needs to be avoided. For a small ornate goldfish pond, or even a small Koi pond, when using a liner, it’s quite feasible to create a pond in a weekend once the hole has been dug. Pond liner comes in varying grades, and, as with most things in life, you get what you pay for. Cheap PVC liners are available for very little cost, but these generally offer a limited guarantee, and don’t possess the strength of butyl or Ecoseal. The most common types of liner are:

Ecoseal (formerly Greenseal): 0.75mm and 1mm EPDM rubber. This is manufactured in the UK by Gordon Low Products using their own unique welding process, and is available in up to 1,800m2 in a single sheet. For very large projects, this liner can be welded on site. Typically this liner retails for around £6.50 per m2, and is very popular in the 0.75mm thickness as it’s easy to fold and manipulate around corners.

Firestone: 1mm EPDM is produced by Firestone in the USA, and can be joined on site for very large projects. Being 1mm thick, this is a tough liner, but its thickness can make it tricky when going around tight corners and trying to limit the number of folds. Typically this liner is priced at around 60p per ft2, and it’s quite commonly stocked on a roll by many Koi dealers in sizes ranging from 12ft to 24ft wide.

Butyl: 0.75mm thickness is the most commonly seen butyl liner on the market. Butyl is now one of the most expensive types of pond liner, so many people who would have once only used butyl, now use Greenseal in the 0.75mm thickness. In most cases, butyl will not be a stock item, but ordered in specially.

PVC: 0.5mm typical thickness. PVC is the cheapest liner available and therefore only really suited to small-to-medium or temporary projects. The main advantage of PVC is simply cost, with it being as much as 50% or more cheaper than alternative materials. PVC is also lighter than the other materials, and hence easier to fit, but it shouldn’t really be the choice for a serious pond build.

Box-welded liners: many of the pond linings mentioned are also available in box-welded options, which means that they’re welded to an exact shape, typically a rectangle, square, or oval/circle. This is generally done at the factory in line with a drawing supplied by the customer with the exact dimensions on it.

The advantage of this method is that there aren’t any folds and the liner can be installed very easily and quickly by a few people, who can simply open the box and drop it in place. Shelves and more specific shapes can be taken into account, but the more complicated the pond, the higher the cost will be. Typically, for a formal-shape pond with no shelves, you’ll be looking at an extra 50% of the normal cost
of a sheet liner for the same sized pond.

Some companies will even supply box-welded liners for L-shaped and more unusual shaped ponds, including those with planting shelves in them. If your pond falls into this category, you should be aware that the more complicated the pond, 
the more expensive it will ultimately become.

If you’re using liner, it’s vital that you install a suitable underlay. This can take the form of sand, carpet underlay, or you can consider purchasing one of the many pond underlays on the market. The benefit of these is that they tend to be easier and quicker to put down than sand, and many manufacturers offer the same guarantee period as for the liner itself.

Although liner is looked upon by many as the entry point for sealing ponds, you shouldn’t be put off considering this option if you’re constructing a block-built Koi pond, as it makes it much easier to fit skimmers, drains and any other pipework.

Size matters
When using liner, the most important thing you’ll need to do is ensure that you get the right size for your pond: if it’s too small, it won’t fit, and if it’s too big, you’ll have simply wasted money! There are two ways of working this out:

The first, and most accurate, is, when the hole has been dug, run a tape across the widest and longest sides of the pond. The tape should be held to the sides and then to the bottom. This will then give you the sheet
size required.

The second method is, if the hole hasn’t been dug, you can work out the size of the liner using the following formulas:

Max length + (2 x deepest section) = length
Max width + (2 x widest section) = width

So, if the pond is going to be 10ft x 6ft x 4ft deep, then this works out as follows:

10ft + (2 x 4ft = = 18ft
6ft + (2 x 4ft = = 14ft

Therefore, before any overlap, you’re going to need an 18ft x 14ft liner.

Whatever method you use to work out the size of your liner, you also have to add in some overlap – this can be anything from 6in to 1ft all the way round. Some people will allow more, especially if there are tight bends in the design, as these will result in the liner folding and being pulled into the pond. As already mentioned, these folds can be secured in place by using cold glue tape.

To sum up, liner is the cheapest and the easiest to apply and install of all of the linings available. And if you’re on a budget, but still want to block-build your pond, a box-welded liner is a serious option to give you that professional finish but at a fraction of the cost of having the pond fibreglassed.

Manmade fibre
Fibreglass has its selling points as well as some drawbacks; the main one being that in most instances, before it can be applied, the pond will need to be brick built and rendered or shuttered in order to give a smooth surface onto which the fibreglass can be laid. In very special cases, fibreglass can be applied directly to the soil, but chicken wire or something similar is normally laid onto the soil first, and a render applied to this to create a smooth surface and eradicate the need for brickwork. At this point, it’s worth employing the skills of a specialist plasterer or renderer, as it’s the quality and smoothness of this work which will give the chosen method of sealing some strength and its final finish.

When rendering the pond, it’s a good idea to use a product such as Fibre Mix in with the render (available in 1kg bags for around £15, which will cover 10m2 of render at a 5mm thickness). This product is basically small strands of fibre that are mixed with the render before it’s applied. When the render has dried, the fibres can increase its strength by up to 10 times. However, if you opt to use something like Fibre Mix, you must remember to burn off any exposed fibres – any protruding strands may have a detrimental effect on the finished project.

When you’re having the pond rendered, you should try to smooth and round off any tight corners. This will not only make it easier for the fibreglass to be applied, but it’ll also improve the final circulation in the pond by reducing dead spots. Once the pond has been rendered and all of the pipework is in place, the next task is to wait.

Before the render can be glassed, the render itself must be completely dry, and this generally takes a few weeks. If bad weather is expected, it’s worth investing the money to have some form of cover put over the pond. This will protect the pond from the elements, and also allow the project to continue. When the construction work has been completed, the fibreglassing can then start.

It should be stressed that that this isn’t a job for the inexperienced, and if it goes wrong, it could result in the loss of your fish. If you’re going to have your pond glassed, you should consider employing someone who specialises in ponds to do it for you. Otherwise, you should do your homework, and even consider going on a short course to give you a limited amount of experience before embarking on such a project.

A very basic guide to fibreglassing
Once you’re satisfied that the render is dry, you’ll need to coat the walls of the pond with resin.

Then, place a sheet of chopped strand fibreglass matting on the wall from top to bottom. An area which will be covered by two sheets should be worked on at one time.

Continue doing this around the pond, but ensure that an overlap of at least 3in is allowed.

Once in place, a roller loaded with resin should be firmly moved across the matting. Care should be taken to cover all corners to ensure that it’s secured in place.

Once two sheets have been positioned, a third sheet should be laid, overlapping the first two sheets by a half.

Apply a third coat of resin using a paddle roller, which will release any air and ensure a good even spread.

Once the whole of the pond is covered in this manner, it must be allowed to dry for at least 24 hours.

After this time, it can be sanded down to remove any sharp surfaces.

Finally, once you’re happy with the finish, the final topcoat can be painted onto the fibreglass in your chosen colour.

It’s important when doing this to ensure that the whole pond is completed in a single application.

Once this is done, 72 hours should be allowed for drying, before the pond is washed down prior to filling.

As already mentioned, when using fibreglass, there’s always the risk of there being some styrene leach, thus it’s a good idea to place a few test fish in the pond for a week or two before going ahead with full stocking.

This is a very simplistic guide to what’s involved in fibreglassing, and as a rule it’s not something that should be undertaken by a novice. Mistakes will be costly, so it’s worth employing a professional.

If this still hasn’t put you off, you should spend some time talking to someone who does this for a living and follow their advice. This point really cannot be emphasised enough – fibreglassing is difficult.

Equipment: Resin, chopped strand matting, lambswool rollers, paddle rollers, gloves, 
coloured pigment, topcoat, catalyst, and some acetone for cleaning.