Continuing our series of articles about how to build an inground pool, we now turn to shaping the bottom of your swimming pool. Vinyl inground pools usually have what's called a hopper bottom, so named because it resembles the funnel shape of a grain hopper. This can be the part of the inground pool installation that intimidates some homeowners. Don’t let it scare you, it’s not difficult at all. To shape the floor and hopper bottom of your inground pool kit, we simply set up stakes and strings. We place stakes at each corner and run string lines in between to guide the digging operation.
At this phase of your inground pool construction, your pool walls are installed and connected and the pool has been dug out - a few inches deeper than it's final depth. Now it’s time to start shaping the pool hopper bottom and get the floor ready for the pool liner. We start by locating the finished level of the pool floor by marking a line on your pool walls, which are generally 42" in height. Measure down the pool wall from the top to a point at 40”. With a Sharpie or Magic Marker, draw a line around the pool walls, this is the finished grade level, or the level of the floor.
Place stakes in the four corners of the pool, right up against the wall. Run a tight sting between these four corner stakes, at the level of the mark that you have drawn on the base of the pool walls. Your stakes should be 18-24" long, so that they don't move with the pull of the string, or if they happen to get kicked or knocked about by the digging operation. A piece of re-bar is usually the best material to use for these stakes. With a sledge hammer or maul, pound the stakes firmly into the ground, with 3" sticking up above ground. The string lines should line up, or be at the same level as the 40” mark that you drew on the walls, which will represent the finished floor level.
Assuming that the pool has a shallow area slope and a deep end, let’s continue to mark out the floor shape. With your spec sheet or pool plan diagram in hand, check the length of the shallow end floor. For purposes of illustration, let’s assume a 16’x 32’ rectangle pool. For this size, our shallow end will be 8 ft long (and 16' wide). Place two stakes on each side of the pool, exactly 8 feet from the shallow end corner stake. Run a tight string across the pool between the two stakes to indicate the end of the shallow floor and the beginning of the slope. This transition is usually referred to as the "break".
Next, measure from the first set of stakes 14 feet, towards the deep end. Keep the measuring tape horizontal, running parallel with the 40" mark that you drew on the walls. Don't measure "down the slope", but keep the measuring tape at the level of the 40" mark. At the 14' mark, position two more stakes - on both sides of the pool. This marks the end of the slope and the beginning of the deep end hopper bottom. Set these stakes right up against the pool wall, on both sides. Run another tight string across the width of the pool, between these two stakes.
From the point of this stake that marks the beginning of the deep end floor, measure another 6 feet towards the deep end wall, along the 40" mark that was drawn along the base of the wall. This will indicate the end of, or the back side of, the deep end floor. Again, place two stakes - up against the wall, at this measured point, and run a tight string across the width of the pool. You should now have 4ft remaining, from this stake to the deep end corner stakes.
Now measure across the string line that runs across the back wall, or deep end, of the pool. Measure in 4 ft from each corner stake in both the deep end and the shallow end of the pool. Place 2 more stakes against the deep end walls, and run a tight string from these stakes to two shallow end stakes - also positioned 4 ft from the corner. You will now have a square across the deep end of the pool, where the strings intersect. This square indicates your 8 ft deep bottom pad. To be sure that the measurements are correct, measure diagonally across the square, to make sure the two measurements match exactly.
Next, we will precisely locate the 4 corners of the bottom pad, or floor. At the point where the string cross for your bottom pad, take another stake and hold it right underneath the intersections of the strings, and pound it in place, into the four deep end corners of the pool floor. Tie a tight string between all four bottom pad stakes, 2" above the ground, and take another set of cross measurements, to insure that the pad is square.
From the two stakes placed on the back side of the deep end floor pad, you will now run tight strings to the stakes located in the deep end corners. These strings will be running at an upward angle, from the deep end floor, to the deep end corner stakes, at the base of the walls. From your 2 shallow end side corner stakes on the bottom pad, run string lines up to the stakes that were placed at the end of the shallow floor, or the break. This will designate the point where the angled side walls will intersect the slope from shallow to deep.
At this point, you can remove the strings and stakes that are intersecting across the hopper bottom, at the level of the shallow end floor. Your stakes & strings should now look like this.
Now you can finish digging out the floor to these contours. The angle from the bottom of the wall to the floor should be even, and close to 45 degrees in the deep end and 30 degrees going up the slope to the shallow end floor. After the heavy equipment has removed most of the material, you can get in there with a flat shovel and smooth out the rough stuff, to help to better define the angles beneath the strings. Leave a 2” gap between your string lines and the floor, to allow for the floor material that you will add on top of the bare earth.
Adding the floor finish material will fully define the wall and floor angles. You can use vermiculite, pre-mix, (vermiculite and concrete mix), or a 4 to 1 sand and Portland cement mix, rising up to the string lines on the floor and slopes. Finishing the floor should be done in stages, starting from the deep end back wall, and working your way up and out of the shallow end. Add the material to each deep end side wall, the bottom pad, your shallow to deep slope and finish up with your shallow end floor. In this manner, if you are finishing the floor on a part-time basis, you will have complete sections that will blend together. Let the finish material dry for a day and then you can walk in and remove your stakes and string lines, filling the holes with some leftover material. Sweep the pool well to remove any loose material.
Now that wasn’t so hard, was it? With the floor prepped and cleaned, you are now ready to install the pool liner in the inground pool that you built yourself!
SPP Pool Expert