How to build a Rock waterfall or backyard stream ...

Building rock waterfalls into the ground by using liners or concrete is difficult. It is not just a matter of placing a liner on a piece of sloping ground. It is far simpler and lower cost to buy an imitation but highly realistic rock waterfall preformed from fiberglass or polyurethane.

• Knowing how to build rock waterfalls without them leaking is tricky.

• Knowing how to build waterfalls from real rock is skilled work, heavy work and not for the feint-hearted.

This question asked of Peter May highlights the potential problem of the DIY approach to how to build a rock waterfall ...

How Do I Seal a rock and mortar waterfall into a Koi pond

Do you have the knowledge or know someone who knows how to seal a rock and mortar waterfall into a Koi pond? The water seeps through the rock and mortar even after we have tried three different sealers and even placed a plastic sheeting behind the rocks and then reset the rocks. The water will even travel sideways and lead into my wall and crawl space. NEED HELP!!!!! Thank You, Jim Patrick, Cedaredge, Colorado USA

ANSWER by Peter J May:

I don't know whether you have 'Murphy's Law' or 'Sod's Law' over there, but it is essentially about the law of nature that if you think you have every possibility covered to stop anything from going wrong, but you leave the slightest loophole in your plans or arrangements then it will go because of that. In fact when you are building ponds you find that even other laws of nature, like gravity, just go completely out of the window.

What is happening there is a sort of capillary reaction. This can be quite an effective way of siphoning a pool almost dry overnight. You get between the inside walls of stone faced pools between the liner and the mortar of the wall.

If you have soil or turf coming down into the pool, little capillary siphons set up in the soil. If water gets down behind the stone facing a waterfall, the pressure of the water coming down the stream can push the water yards side ways because it has nowhere else to go.

The water in the pool can act as an air lock, or just a barrier itself that aint going nowhere. I had a raised pool that I built for someone that had a stone face down to the marginal shelf about a foot down. It had a really long stream down to it that would cause a fair amount of water to evaporate. It would lose about an inch a day in the hot days of summer.

BUT some days it would lose nearly a foot. The customer thought there was a leak in the stream, which we dutifully dismantled suspecting mice having gnawed through the liner. No holes. That took two of my three days.

What seemed obvious now was that the liner had something wrong with it behind the wall because it never lost water below the level of the wall. But before we dismantled the wall I asked the customer if perhaps he had filled the pool right up to the top the night before there was a big leak. He said of course he did, because how would he know that it had leaked some much otherwise.

So then I asked how full he filled it. He said he just left the hose in it and let it fill to over flowing.

Instead of dismantling the wall, drilled little holes in the pointing of the capping slabs round the pond. And they solved the problem. There was a little siphon setting up in the mortar that sucked the pool dry, dragging the water up behind the wall and behind the outside wall, out onto the gravel around it.

When we build waterfalls we insert little plastic tubes (about every foot or 18inches) between the stones at the base and these relieve the pressure from the waterfall and provides somewhere for the water to drain. Also I use a lot of smooth pea shingle to backfill behind big stones.

That helps.

When I first read you email I though of the best sealant, before I thought of the real solution. With every sealant there is a time when it eventually packs in on you and you are back to square one. This is generally after the first icy weather. Anyway, here it is. We have a product in the UK called G4 which I think is a cellulose sealant. What is good about is that you can paint it cement or concrete when its wet and it will follow the water down into the material to seal it deep down in. It also seals one material with another. So if you had a wooden chute coming down to a stone sill, it would seal the two into a unit. it is very useful for sealing in the lime that washes off fresh concrete products into pools. Also, you can get in a black, brown, green and red and clear gloss or matt. Brilliant stuff and stuff that good doesn't come cheap.

IF you want I can find out where you get a G4 equivalent in the States, But I'd recommend some experiment with a small masonry drill first. BUT mind you don't drill through the liner!.

All the best Pete

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If you would like to test how much water looks best for your rock waterfall do the following .....

i) Place a hosepipe into the waterfall inlet area and prevent it "back-firing"

ii) Adjust the water flow by regulating the garden hose tap. Do this until you're perfectly happy with the flow effect

iii) Without closing the hose fill a known volume container from the hose pipe. Ideally the container should be about 25 litres or 5 gallons because this will give you good accuracy.

iv) Calculate the flow rate by dividing the volume of the container by the time taken

to fill it. These free calculators could come in useful here.

v) This calculator would also allow you to weigh the container as an alternative to knowing its volume. For water use the following:

• 1 US gallon weighs 8.83 lbs

• 1 Imperial gallon weighs 10 lbs

• 1 litre weighs 2.2 lbs

vi) If one hose does not deliver sufficient water then use 2 hose pipes.

If you prefer to work out how much water you need for your rock waterfall then take a look here but also bear in mind that fittings play a role

Consider pressure drop in pipe systems ...