Figure 5. This foil-faced foam is blocking out the opening for an undermount sink. The foam and most of the wood forms pictured here will be removed as soon as the concrete is hard enough to finish. The undermount sink is already in place beneath the Kraft paper.
Undermount sink form. Foam insulation is good for blocking out undermount sinks because it's easy to cut and can be removed by breaking it into pieces. We put the paper template that comes with the sink on the foam and transfer the shape by poking a series of holes through the line. We cut the foam with a jigsaw, then smooth the edges with sandpaper. We wrap the edge with red plastic tape to fill voids and prevent concrete from sticking, then install the foam over the sink with cleats (Figure 5).
Faucets. We form faucet holes with short pieces of PVC pipe. The pipe can be removed when the slab begins to set. Most faucet stems are long enough to go through a couple of inches of material.
But if the combined thickness of counter and plywood is too great, the plumber will not be able to install the nut that holds the faucet in. To avoid this, we cut an oversized hole through the plywood under the faucet and cover the hole with a piece of foam.
The foam keeps the concrete out of the cabinet, but it's soft enough that we can poke holes in it and use it to hold the PVC. We continually check the pipes for plumb because it's easy to knock them out of position. When the slab is cured, the plumber can easily dig out the foam from below and seat the fastening nuts against the bottom of the slab (Figure 6).
Figure 6. These faucet holes are formed by PVC pipe set into a piece of rigid foam. The foam covers an opening in the plywood and makes that part of the counter thinner so the plumber has plenty of thread to install fixtures.
