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As in the selection of basket strainer wastes, the most important factor to consider is the thickness of the material of your sink. Simply put, some are suitable for thin materials such as stainless steel, others are only suitable for thick materials like ceramic and occasionally some are suitable for both. Usually the shape of the overflow has been determined by the sink manufacturer so it's important to purchase one with the correct dimensions. Follow overflow advice on ceramic sinks for other thick materials such as marble, soapstone, slate and wood.
Some stainless steel and ceramic sinks have a round overflow hole punched or moulded into them. The overflow outlet is screwed in from the face of the sink into the thread of the overflow connector which is held to the rear of the sink. The rubber sealing washer is usually placed to the rear side of the sink so it will not be seen.
Most stainless steel sinks have the overflow grill outlet punched into the sink. The overflow connector is held to the rear of the sink and a stainless bolt or screw is inserted through a hole in the sink into the overflow body and tightened.
The sink manufacturer Franke has developed its own method which it calls "hygienic waste." It is a combination of both methods.
A very popular design of overflow is a metal plate design. The plate is simply bolted to the overflow body behind the sink. A rubber washer is placed behind the sink to make it watertight. They are commonly used on ceramic and thick wall sinks but occasionally on stainless steel.
Two similar style sinks. The first with a rectangular plate, bolt in overflow and another with a round screw in overflow. Some buyers have a preference for one style or the other, to others it makes little difference.
If you are fitting a ceramic or other type of thick wall sink, it is important to ensure your bolt, screw or round screw in overflow is long enough to reach through the wall of your sink. With plate overflows, some manufacturers supply you with two lengths of bolts, for others you will have to cut it for a metal sink. For round screw in types only one screw in overflow is usually supplied. If it is too long, with a bit of trouble you can cut it shorter, if it is too short you will be faced with buying another one. Purchasing a single round screw in overflow is usually not that straightforward so it pays to get it right first time.
Some sinks will not be fitted with an overflow. Many twin bowl sinks for example. The rational is that each bowl will act as an overflow to the other other bowl, notice the centre bar is usually lower than the perimeter walls to allow flow of water. Single assembly half bowls rarely have an overflow as they are designed to be just a utility bowl. It is rare to see an overflow to a copper or solid stone sink.
A traditional Belfast sink will be fitted with a "weir" style overflow. Any overflow water travels down the weir and into the waste. This is why it's important to fit the correct, slotted waste outlet.
If you are using a 50mm waste and a conventional plug, you can buy an overflow with a chain point or even a bolt with a chain attachment hole.
If you are considering a particular finish for your basket strainer you may struggle to find an overflow to match. Gold and brass are reasonably well catered for, but other finishes may be difficult to obtain.
As mentioned, some manufacturers like to add a little of their own style in the design of the overflow outlet.