Once you've selected your liner material and thickness, you're ready to measure your pond and place an order. You may have guessed by now that this isn't a simple matter of using a tape measure on the ground you've marked out with spray paint or a carefully shaped length of hose. Let's make sure you include all the necessary details in your measurements so that success is yours from the very first step!
Ponds are not only irregularly shaped; they're three-dimensional and can be somewhat challenging to measure. If your pond is roughly rectangular, it's a simple matter of measuring width and length as described below. If your pond's shape is highly irregular, then see if you can look at your pond from a birds-eye perspective. Determine the pond's longest dimension, and then mark the widest point perpendicular to the first.
Now, imagine looking at your pond as a cross-section. Your liner will travel from one edge, down the pond's wall, across the bottom, including any depth changes, and up the other side. If you've already dug the hole, you can use a string, ensuring it lays flush to the ground and tucks into any corners. Mark your string, and measure it in feet, rounding up to the nearest six inches. Take measurements for the side that's perpendicular to this one, using the same technique.
If you haven't begun excavating, use your most detailed plans and calculate your pond's deepest point. Just make sure that you don't get over-enthusiastic when you do start digging -- a slightly deeper and broader pond that's too big for its liner will only create an endless list of problems.
Once you have the pond's dimensions, it's time to add an allowance for the apron. The apron is critical because it's what you'll use to anchor the liner in place to prevent leaks, erosion, and other serious problems. We'll talk about that next.
Aprons are for Anchoring
The edges of your liner should overlap the edge of your pond, generally by one to two feet all around. This extra will anchor the edges so the liner won't shift or pull down. A loose liner edge will tend to sink into the pond, and once it is underwater, water escaping over the edge will create a long list of problems. It's essential to develop a reliable solution to keep the liner in place.
The type of anchoring you'll use will depend on the size and style of the pond. Backyard-sized ponds are usually best served by an anchor trench. In this design, a trench, usually 6 inches deep and 6 inches wide for a small pond, is dug around the pond's perimeter, at least 12 inches away from the water's edge. The liner is pulled over the edge and down into the trench. It should extend across the floor and a few inches up the opposite side. Use sand or gravel to cover the liner and fill the hole. Next, you can add an attractive covering like pebbles or river rocks to extend below the waterline. This treatment protects the liner from scuffs and scrapes and UV exposure.
Anchor shelves are another option. Generally, they're simpler to construct, but since they may be five feet wide or more, they're usually used only on large ponds or lakes.
If you have a moderately steep slope descending into the depths of your pond, you may want to include a curb or a speed bump in your design. A speedbump helps support any decorative fill that would otherwise work its way down to the bottom. The curb can be something as simple as a perforated pipe which acts as a stop near the bottom of the downward slope.