An all too common sight in some remodeled kitchens is that undermount sink that seems to be sagging and detaching from the granite countertop. This means that water is almost surely leaking below into the cabinet and will be causing mold and mildew buildup before long, unless the issue is corrected.
Why does this occur?
The installers from the big box stores want to get the job done quickly, and they likely don't know or care about ensuring that your sink is properly supported. They will simply apply a bead of silicone or epoxy around the rim of the sink, and clamp it to the countertop and allow it to cure. While this may last for some time after the job is complete, it is by no means a lifetime solution, and will fail - sometimes years down the road. However, many installers don't want to take the time to perform the installation properly and spend extra money on materials which will ensure lifelong customer satisfaction. If they can quickly spread some adhesive and convince you that it will hold, that means they can be onto the next task and out of your home sooner. Worse still, you as the homeowner are left with the impression that the job was done properly because the sink appeared to be fastened properly when they left. But, as the years pass, the stresses of everyday use will likely cause the sink to start separating from the countertop, leaving you with the cost of a repair bill long after the warranty period has passed.
Remember that an undermount sink that is used on a daily basis not only has to support its own weight - it also must cope with the weight of water and dishes. Because of this, you do need something in addition to the bead of adhesive or sealant around the rim of the sink to support that weight. Silicone on its own definitely has holding power, but it must not be the only thing doing the work. In situations where the underside of the countertop is not pre-drilled to accept the screws for sink mounting clips, I like to make use of pieces of wood around the underside of the sink to act as supports, which will ensure that the lip of the sink is secured against the countertop itself. I will use a high quality urethane adhesive, such as PL Premium to secure the blocks of 2 x 4 lumber and ensure all of them catch the lip of the sink while being securely fastened to the granite or quartz. Once this dries, you will have a lifelong solution that will never break free. The silicone around the rim of the sink acts as a sealant which keeps the water in the sink, and the mechanical coupling of the pieces of wood acting as a clamps ensure that the sink will remain mounted for a lifetime of use.
One caveat to this method that I have described is to beware of bleed through with certain countertop materials. You may want to be careful of using construction adhesive on lighter coloured stone such as marble, because the colour of the adhesive may show through to the top side of the lighter coloured stone after some time has passed.
Of course, there are other solutions as well. You can use a sink harness which is mounted to either side of the cabinet walls underneath, which will cradle the sink and support the weight once tightened. The caveat here is that your cabinets must be of decent quality construction in order to hold the mounting hardware. This means that particle board cabinets are not a good candidate for this method. If you are planning a kitchen renovation and are unsure of how your new undermount sink will be installed, consult your general contractor or project manager with your concerns before any materials are ordered. This way, you can have it written into the contract that your undermount sink will be properly installed, leaving you with one less thing to worry about after the renovation is complete.
If you are planning a kitchen remodel, keep this in mind:
The project planner from your local big box store who coordinated with you on that new countertop design probably did not coordinate with the fabricator of the stone to have mounting holes drilled into the underside of the slab to accommodate threaded brass inserts which would allow the installer to securely fasten the sink. Most undermount sinks include metal fastening clips that are designed to fasten into the underside of the countertop with screws - provided the countertop is pre-drilled at the factory to accept them. It is a shame that these metal clips are often not used and simply discarded or tossed aside in a ziploc bag. Most installers won't want to drill the holes themselves, and for good reason: drilling into granite is not an easy task, especially from underneath. You need a water cooled diamond bit to make the hole, and this can be very difficult to do after the countertop is installed. Plus, the jack-of-all-trades on the job site likely does not want to risk the damage to your granite countertop if they make a mistake.
If you don't ask the fabricator of the countertop to drill these holes and install the threaded insert to accept mounting screws, they simply won't do it, because they have no idea of knowing what size or even style of sink that you are planning on using. If you provide them with dimensions and a template, they should certainly be able to accommodate this simple wish. Whatever you do, please don't rely solely on a bead of adhesive/epoxy or silicone to hold up your new undermount sink - it likely won't last.