Before expressing my opinions about the decision process let me first define the two major options for the top of the wall: cut-in and cut-away. When we say “cut-in” we are describing that the top of the wall is cut down into the pool so that the water surface extends to the outside edge of the wall – effectively submerging it even if the water is not flowing over the edge. When we say “cut-away” we are describing that the top of the wall is cut away from the pool so that the water surface terminates at the inside edge of the wall – resulting in the top of the wall being dry when the water is not flowing over the edge.
It is possible to have a perfectly level top of wall but I don’t recommend it. First, when the edge water is not flowing, the top of the wall will be very obvious visually. Reflection off the surface is a primary reason for having the vanishing edge in the first place so it doesn’t make sense to interrupt that reflection with a band of dry tile or stone. It is better to cut-in the edge and let the reflecting water surface extend over the wall or cut-away the edge so that you don’t see the top of the wall at all. One problem revealed while the water is flowing over a level wall is the hydraulic jump created when the slow moving volume in the pool suddenly races away over the wall. The jump is actually seen as an approximately ¼” (6 mm) drop in the water level, disrupting the reflections. The easiest fix is to simply lower the inside edge of the wall (“cut-in”) by at least ¼” (6 mm).
Why are so many vanishing edges cut-away? The most important reason to cut-away an edge is that it eliminates the otherwise submerged edge of the cut-in version. This creates the illusion that the edge of the vessel is paper thin or even non-existent. For the right pool configuration and location the cut-away edge can be dramatic. Unfortunately, I think the detail is often applied to the wrong projects and this mistake is repeated over and over by unskilled designers. I also think that designers place too much emphasis on hiding the inside edge to create the illusion – after all, if a cut-in edge is kept fairly flat the refraction doesn’t reveal the inside edge anyway.
If the cut-away edge can be so dramatic, why not use it all the time? The simple answer is that the top of an exposed vanishing edge wall cut away at a 30-45 degree angle is just plain hideous. If I can see the ugly top of that wall from anywhere then I prefer to use the cut-in detail. I estimate that 95% of our edges are visible from somewhere so consequently we use the cut-in version extensively. Good examples of the cut-away detail where it can’t be seen from the decking or the house would be pools with distant edges or pools so wide that there is no decking on either side that would provide a direct view.
A vanishing edge makes a bold architectural statement. A giant chamfer on the edge weakens that presentation – it’s soft and contrary to the sharpness for which the vanishing edge of water was intended in the first place. Then, consider what happens to the water as it rolls down the cut-away face: the trajectory launches the water off the lower edge into the air creating unnecessary water loss, cooling, and noise. Now that may be desired for some projects where the wall view is a water feature but typically you need to increase the flow rate to achieve the right downstream effect.
Understanding the cut-in detail will reveal what many professional designers already know: there is no need to hide the wall with an illusion – the wall is expected and indicates strength and confidence. We have actually had a few clients tell us that they don’t like vanishing edges because they don’t feel secure. The feeling that a paper-thin wall may simply fall away creates tension when they should be feeling relaxed in their buoyant bliss. I suspect their feelings may have originated in a pool with the cut-away detail because I would certainly feel confident about the structure when I can visibly see a thick wall that invites me to rest my arms on it.
Regardless of the top edge detail our vanishing edge walls are at least 12” (300 – 305mm) thick plus the veneers. For the cut-in detail we typically drop the inside edge ½” to 1” below the outside edge. This eliminates the hydraulic jump issue and there is no reason to make the outside edge any “sharper” by dropping the inside edge. In fact, lowering the inside edge might make it visible when the ambient lighting and view angle provide less reflection off the surface and more refraction through it. We try to keep the top edge as flat as possible but a rough material such as natural stone will require a little more slope than, say, polished black granite.
Regardless of the cut-in slope, the net effect is the same: the water surface extends to the outside of the wall, submerging it and making the pool look larger. With tight edge tolerances we can operate the vanishing edge at low flow rates and the water rolls over the edge and flows down the outside of the wall into the catch basin without jumping off the wall. It is quiet and calming.
Over the years we have received numerous design concepts from architects, landscape architects, and pool builders that needed our help with structural and/or hydraulic design. I’ve noticed that the pool builders tend to gravitate towards the cut-away option while the professionals educated and experienced in design utilize the cut-in detail much more frequently. The pool builders may be arriving at their decision by accident, but I’m convinced that the design professionals are making a conscious decision to maintain the crisp lines presented when the water rolls over the sharp ninety-degree edge and flows straight down the wall.
At the end of the day every project needs to be assessed on a case-by-case basis. The cut-in / cut-away decision is just the first step in the edge detail development. After that, the materials selection and tectonics provide a myriad of options that affect both aesthetics and hydraulic performance. I don’t think you can ever go wrong with a cut-in detail but you can certainly disfigure a vessel by using the cut-away detail when it’s inappropriate.