How to Build the Perfect Pond

The initial building of a pond structure is the most important part of ensuring it lasts for decades, and requires a minimum of maintenance. When you build a house, you expect it to last forever. So why cut corners on a pond? It carries a large, heavy amount of water and livestock, and is constantly open to the elements, so It’s essential to get the basics right.

We’re going to assume that you’re just building a basic pond, and dropping a pump inside it. If you’re planning something more extensive (with bottom drains and skimmers) then I assume you know how to do this already - or wont be doing it yourself, as it isn’t a simple project for anyone with limited building skills.

1) Setting up the base

You’ll need a hole digging. This may sound easy, but the deeper you go into the ground, the more chance you’ll find it to be damp. When building a house, a damp proof barrier is used to stop moisture from rising up into the structure - so we will need to do exactly the same for your pond.

Once your hole is dug, you’ll need to fill the base with some small “type 1 MOT” stone. It’s basically a mixture of small stones and dust, which we will lay on the uneven surface in your hole and will help level it up and provide a drain away for any moisture that finds itself under the base. You’ll need a three inch thickness of stone for a small pond, but this should be increased to 4+ inches for a medium to large sized area.

This stone will need to be wacked down using a “wackerplate”, or a vibration plate. It resembles a lawnmower, but its job is to vibrate the surface below it, and compact your stone into a nice flat surface. From here, you’ll cover the stones with a layer of sand. Again, the thickness will depend on the size of your pond, but I’d have at least a inch of sand for a small pond - rising to 1.5 - 2 inches for a medium / large pond. Flatten this down onto your stone using the vibration plate again. You now have a hole, lined with stones, and covered with a layer of sand.

We now lay down the damp proofing or “visqueen” sheeting, which will stop moisture from coming up through your concrete base and into your pond build. Put this barrier membrane up and out the sides of your pond too.

You should now have what looks like a plastic bag inside a big hole, with the bag spilling out over the top of the pond onto the ground area around the hole.

2) Lay the base.

Most people use a concrete mixer, or order ready-mixed concrete to pour on top of the damp proof membrane. If you have a small pond, you’ll need 4 inches of concrete. Medium or larger ponds can use the same amount, but we’d recommend using metal mesh re-inforcing bars (or metal mesh in sheets), which you’ll prop up an inch or two ontop of the damp proof barrier before pouring the concrete in. This simply provides strength to the concrete, and will be hidden inside the 4 inches of concrete you’re just about to pour.

The concrete mix will require a waterproofer added (render mix waterproofer on the products page), and plenty of re-inforcing fibres (products page) to minimise shrinkage and cracking. This is very important!

You should now have a big hole with a nice flat concrete base, with a waterproof barrier coming up and out of the sides.

3) Bricklay the walls.

Depending on the size of the pond, you’ll need to ensure that your walls are strong enough to withstand the force of the water inside them. We’ve seen many people use a double skinned brick wall with nothing inbetween but a few wall ties. This is NOT a good idea, as the inner skin can collapse into the outer one unless you fill the cavity with reinforcing bars and concrete.

Personally, we use a hollow concrete block, as it is twice the thickness of a standard breezeblock - and effectively gives you double the thickness without laying two lots of blocks. You could also use high density “7nm” blocks - but I would lay these flat, which will give you a nice wide wall. Both are good, but using a single thickness breezeblock will take longer, as they are half the size of the hollow blocks.

You’ll need to mix your mortar, which will be combined with rendermix waterproofer and reinforcing fibres. This will ensure your block joints are strong, less likely to crack, and waterproof.

Once you’ve built this up inside the waterproof barrier, pull the damp proof material up and over into the pond and secure it with some spare blocks. You should be able to see down the back of the material, which should be just soil from when you dug the hole. Some people backfill this area with soil (but this will expand and contract as it gets wet and dries out, causing movement within the area of your pond - which can cause cracking! You’ll need to mix some more concrete, with plenty of small stones inside it for strength, and pour this into your void between the soil and waterproof barrier. Mix another load of concrete, but make it runnier. Pour this into the breeze block holes, and the solution should run right down to the bottom of the blocks, until you’ve filled up all the wall. It’s a lot of concrete you’ve used so far, but this structure will still be there in 100 years time and holding water (unlike most ponds).

4) Render your walls.

Why should you have to do this? The pond is built like a tank already - so why render too? There’s a number of reasons, but the main important two for us are:

(a) it will make the structure stronger, and keep the frost away from the blocks and mortar. Some blocks or badly mixed mortars can crack when temperatures drop below freezing - and the block and mortar can freeze in different ways if water has got inside one…. So why take the risk? Plus, once you get a crack forming in a block wall, it will get worse and worse - and how do you fix it now you’ve encased it in so much concrete? You will spend years trying to patch it up, and trying to seal it - and it’ll never be right. Why not let the frost and ice attack the render layer, which can easily be replaced ?

(b) most blocks are cast with a large number of gaps and holes in the surface (many hundreds per block). When you just paint your block without rendering it, can you be sure you’ve covered every tiny hole in every block to stop the water leaking into the block? Once the water is inside the blocks, this can then freeze, causing expansion and cracking of your block wall. Many people skip the rendering step, and some get away with it, as It may not drop below zero for long periods of time…. but it only takes one harsh winter and you’ll come out one day to an empty cracked pond with no water…...

Hopefully you’ve seen now that rendering is important to guarantee a perfect seal. There are so many different opinions on what mix to use, from 3 sand to 1 cement, to a mix of either 4 or 5 plastering sand, to 1 concrete and 1 lime - but it all depends on conditions. You may need to Google these conditions, as there are many which we can’t go into here, as some people wont be affected by them - plus we tend to find one builder will swear by one mix, and another will say that mix is incorrect. This is where you must do your own research, but we use 5 plastering sand, to 1 cement and 1 lime. Once lime goes off, it hardens like bell metal.

We add waterproofer and fibres into the mix again which should stop the render cracking, and keeps any water away from your blocks. Many people argue about if you should do one thick layer of render, or two thinner ones. This again seems to be down to how individual builders work. Personally, we do two thinner layers of render. Make sure you round off any corners, so there are no right angled edges, which become the render weak points.

Do not use PVA adhesive on your blocks, which is a massive failure point of ponds. This is a great adhesive, but should be used indoors for plasterers, not outdoors where there is a lot of water - it will simply rot away, and allow your render to fall apart. Ever wondered why your ridge tiles on your roof always need pointing - even if they mixed PVA in with it for strength? Most builders don’t realise that the PVA will rot away, leaving your mortar weakened…. We use a product called SBR Bond, which is around the same price, but suited to the task and can be applied to your surfaces before rendering. It is suitable for outdoor use.

5) Don’t rush it!

You’ve finally got your pond rendered, and the base is down. Can you apply the paint now? I’m afraid not. The moisture in concrete needs to work its way out before you can paint, or it will be trapped inside. If you paint on unset render or concrete, the water will try and force its way out through the paint (causing little bubbles in the paint), or remain inside the render, keeping it damp and providing a potential cracking risk in winter months.

Usually after 7 days, the render is set. However, it takes upto 28 days for thicker concrete base to fully set, and expel all moisture. This is the number one reason for bubbles forming in painted ponds - as it is often painted within 2-3 days after the concrete being laid, or walls being rendered.

Keep your pond covered away from rain while it dries.

6) Almost there!

Use fish friendly silicone around any fittings, windows, gaps or cracks. Make sure you sprinkle some sand onto the silicone while it dries to give it a good key for the paint to stick to.

If you’re repairing an old concrete pond, which is badly cracked - or your new render hasn’t gone very well and it’s all dusty - then you’ll need to strengthen the render using concrete hardener and dustproofer. This isn’t needed if you’ve rendered it properly, or you’re not fixing up an old badly built pond.

It’s time to apply the paint. Your pond will be waterproof already (hopefully!) but the chemicals in cement will kill your livestock, so the paint is not only a rubberised flexible waterproof coating, but one that also keeps your fish away from the render and cement. Ensure you follow the guidelines to the letter, and keep your pond covered in case it rains.

7) You’re all done!

Fill, Sit back, and enjoy all your hard work!