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Fitting an under-mounted can be reasonably straightforward but it can also turn out to be technically and skilfully demanding. Under mounted sinks are fitted by means of clips, adhesives or even a combination of both. The difficulty arises in attaching fastenings to the underside of the worktop. Undermounts are usually fitted to solid surfaces, if you are fixing to wood that may not be a problem but if you need to drill granite for example, that will require specialist drill bits and a considerable amount of expertise.
One method of attaching undermount sinks to the work surface is to place the worktop upside down and simply silicone it in place, an extra bead around the perimeter decreases the likelihood of failure. This method can work but good surface preparation is essential. The weak point in this system is if the silicone seal gets a nick off a stray sharp object while using the sink. A small cut in the silicone will cause a tear over time under the weight of the sink and water causing the fixing to fail.
Some fitters will use two pack epoxy instead of silicone which should be much more effective if done properly.
The most popular method of fitting under-mount sinks is by using the silicone as a sealant and attaching the sink by means of clips or brackets.
Most sink manufacturers include clips with the sink, you may wish to change this for something more suitable for your needs or you may just wish to undermount a sink primarily designed to be inset or flush fitted. There are a number of different brackets on the market all of which seem to be at least adequate of holding the sink.
The clips are attached to the work surface by either epoxy, drilling or even by just cutting a slot.
These clips use a base that is epoxied to the work surface and the sink screwed on with brackets and wing nuts after the epoxy has set.
Most sink manufacturers supply bolts and usually inserts that are inserted into a hole drilled in undeside of the work surface. Some designs use epoxy to glue the inserts in place.
G clips are provided by Franke and some other manufacturers. A slot is cut into the underside of the work surface and the sprung G clip hammered in. A spacer is inserted to cause the spring of the G clip to put pressure on the rim of the sink.
There is a selection of aftermarket clips on the market, but remember, some are only suitable for stainless steel or other thin wall sinks, some are universal, some you have to request the ceramic version if your sink has a thick wall. If you are undermounting your sink to wood, you can simply disregard the bolts and use screw to hold the mounting brackets.
If you are fitting a very heavy sink such as a cast iron or natural granite, you will most likely need to build a sub frame in timber to support the added weight of the sink.
If you are replacing or refitting your old undermounted sink, you will most likely not be wanting to remove the worktop to attach from the underside. Use the existing clips if you can, if not you can glue the sink in place and hold it with timber clamps or even substitute clamps for rope if the need arises. Be exceptionally careful not to break the worktop. Watch for any distortion of the work surface as you tighten and stop if you notice any. For added security you could epoxy on brackets or small plates, spanning the join of the work surface and sink.