The first thing that needs to be done before attempting to dismantle any tap assembly is to turn off the mains water supply. This is done by turning the main stopcock off, which, in most cases, is located beneath the sink unit in the kitchen or where the actual water mains is piped into the property. This could be in a utility room or basement even. There are properties around still where the only stopcock is located outside the property.

The External Stopcock

The water supply to a property is fed from a pipe that comes off the mains supply. The pipe to the property is normally called the rising main. The rising main usually has a stopcock fitted just outside the property. These are simply taps, most probably made of brass, which control the flow of water to a property. To turn off the supply simply turn the stopcock clockwise and turn it anti-clockwise to turn it on.

The external stopcock is usually in a purpose-made cavity in the ground with a metal hinged lid. Depending on when it was last opened you may need to use a flat blade screwdriver or small jemmy-type lever to open the lid. It's not unusual to find the cavity full of debris such as soil or water. There's also the possibility of other types of debris that could be harmful, such as broken glass. It's not unheard of on some housing estates for drug users to dump used needles in these cavities as they are easily accessible.

Safety first in all cases: never put your bare hands down inside the cavity unless you are sure it's safe to do so, and in any event you should wear safety gloves at all times.

If the stopcock is set deep in the cavity then you may require a key that fits over the stopcock handle. You could make your own or one could be purchased, they're not overly expensive and are available at most good DIY shops. A key is simply a long handle with a 'U'-shaped end that fits over the tap handle. Once the stopcock is turned off turn the tap on where you intend replacing the washer to ensure no water flow, then turn the tap back off again.

It's also worth noting that external stopcocks can sometimes isolate your immediate neighbour's house as well, so it would be worthwhile to check if this is the case and warn the neighbour that the water supply may be interrupted for a while.

The Internal Stopcock

Most modern properties have an internal stopcock which is usually found in the kitchen, normally under the sink unit. Access is usually only hindered by what's in the cupboard. It's only when something goes wrong we realise just how difficult we make it to gain rapid access to a stopcock located in cupboards. Also because this is basically another tap that gets very little use, it does tend to corrode and becomes difficult to turn. Don't use force, the result could be disastrous and you'll then need to use the external mains stopcock to isolate the supply. If the tap feels tight then use a loosening spray such as WD-40 or similar. You can, if safe to do so, apply a little gentle heat with a blowlamp.

Bear in mind that, in the UK at least, any leaks or burst pipes prior to the external stopcock on the rising main is the responsibility of the appropriate water company; any leaks or burst pipes after the rising main stopcock is the responsibility of the property owner.

Dismantling and Re-assembling

In order to replace the washer we now need to dismantle the tap assembly. There are numerous styles, shapes and types of taps but the basics remain the same for most taps in use today. You need to remove the tap handle and decorative surround in order to expose the inner working parts. A useful tip here is to leave the plug in the sink, bath or basin, that way if you drop any small parts they won't disappear down the plughole! Using a small flat-bladed screwdriver, gently prise the tap cover off the top of the tap, which then exposes a screw¹ underneath, and unscrew. On a mixer tap assembly you gain access the same way, by removing the small plastic cap on the cold side.

As this part of the tap you are removing is the decorative or finished part of the tap, care is needed not to damage the surface. It's usually fitted on either a splined or offset pin. The outer decorative case of the tap can also be screwed to the main assembly but is usually only finger tight. However, if it is tight do not use tools with a serrated face such as Vice Grips or pipe wrenches. Wrap an old cloth around the decorative cover and use either an adjustable spanner or non-serrated grips, being careful not to over set the grips as this may damage the finish — or worse, crush the decorative section.

When this cover is removed, the internal workings of the tap can be seen. This part of the tap is known as the head gear. Holding the tap firmly to prevent it turning on the basin, sink or bath, use an adjustable spanner to undo the head gear by turning anti-clockwise.

If the head gear is excessively tight and you feel the tap is starting to turn then you may have to use either an extra set of non-serrated pipe grips to hold the tap assembly. Or, if the tap is adjacent to a wall, you may be able to wedge the tap using a block of wood between the tap and the wall. Once the head gear is removed you will then see the washer on the underside.

Most standard sink and basin taps in the UK use a ½ inch washer and bath taps normally use ¾ inch washers. The washers are normally held in place with a small nut; usually about 8mm or 10mm spanner size. Undo the nut, again anti-clockwise to loosen, then remove the old washer. Fit new washer and re-tighten the nut being careful not to over tighten as this could damage the new washer. If there is no nut then usually the washer fits over a 'lug' and has to be just pushed on over the lip of the lug. A smear of Vaseline on the lug can help facilitate the fitting of the washer.

Some taps have an 'O' ring fitted where the head gear fits the body of the tap. Check this and if fitted ensure it is in good condition before refitting the tap. If in doubt replace it, again they only cost pennies so it's not worth risking further leaks for the sake of both a washer and an 'O' ring.

If there is no 'O' ring and the head gear fits straight onto the base of the tap, it's possible there could be corrosion and pitting on that face. If this is the case then a special tool called a 'reamer' will be required to re-cut the face. This is a specialist job and unless you have some knowledge in the use of a reamer then either call in a plumber or replace the whole tap assembly. If the face is cut back at the wrong angle, or cut back too much, then you will have major leak problems and the tap assembly will be scrap anyway.

Refitting is a straight reversal of removing, remember to hold the tap firmly when tightening the head gear to prevent the tap turning on the basin or bath.

If you're unlucky enough to have a badly-leaking tap during a holiday period when shops are closed, if you are careful while removing the washer and it doesn't break up, then you can turn the washer upside down and re-assemble. This will, in most cases, stop the leak until a replacement can be obtained.

When all is reassembled turn the tap on, but not full on, and then turn the mains stopcock back on. Leaving the tap you've just repaired turned on will allow for any loose debris to flush out straightaway. Turn the tap off, and if all is well there should be no more dripping or leaking tap.