Working under a sink
Empty the cupboard first, put something absorbent down, and find out where the water is actually coming from before undoing anything.
Find the leak before you undo anything
Water runs along pipes and pools at the lowest point, which is almost never where it came from. A wet cupboard base tells you very little on its own, and the temptation is to tighten everything, which sometimes makes it worse.
Dry the whole area, lay kitchen roll under each joint, and run the tap and the waste separately. A supply leak shows with the tap off; a waste leak shows only when water is running through. That one distinction halves the problem.
- Dry everything first
- You cannot find a new drip in an old puddle. Ten minutes with a cloth saves an hour of guessing.
- Paper under each joint
- Kitchen roll shows which joint is wet, and it shows a drip too slow to watch for.
- Supply or waste
- Supply leaks with the tap off. Waste leaks only while water runs. Test them separately.
- Check the sink seal too
- Water running down the outside of the waste is often the sink rim or the tap base, not the plumbing.
Traps are hand tight, and that is deliberate
A plastic trap seals with a rubber washer compressed by a hand tightened nut. It is designed to be undone without tools, and using pliers on a new one usually cracks the thread or crushes the washer.
The exception is an old one. A trap that has been in place for fifteen years will not move by hand, and that is when water pump pliers or a strap wrench come out. Replace the washers rather than reusing them, because a compressed washer will not reseal.
- Hand tight is correct
- The washer seals, not the force. Pliers on a new trap crack the thread or crush the seal.
- New washers
- A compressed washer has taken the shape of its old position and will not seal in a new one.
- Check the water seal depth
- A trap needs its water seal to block drain smells. A trap fitted dry or too shallow will smell.
- Note the layout before dismantling
- Photograph it. Reassembling a multi appliance waste from memory is how the dishwasher backs up.
Isolating valves are the best thing in the cupboard
A pair of screwdriver operated valves on the tap tails means the tap, the trap and the tails can all be worked on without touching the mains stopcock. If they are not there, fitting them is the best half hour you will spend under that sink.
Do check they actually close. Old service valves seize partly open, and discovering that after undoing a tail is how a small job becomes a wet one. Open the tap to confirm before committing.
- Fit them if they are missing
- It turns every future job in that cupboard into a screwdriver job.
- Test before trusting
- Old valves seize part open. Confirm at the tap rather than assuming the screw turned.
- Mark which is which
- Hot and cold are not obvious under a sink. A dot of paint saves working it out every time.
- Drain the head
- Even isolated, the pipe above holds water and it comes out of whatever you undo first.
Washing machines and dishwashers
Appliance connections cause a disproportionate share of under sink leaks. The waste spigot is often connected with a jubilee clip that has never been checked, and the fill hose usually connects to a self cutting valve or a tee that is decades old.
The waste hose also needs a high loop or a standpipe, or the sink drains into the machine. That is a plumbing fault rather than an appliance one, and it is common in kitchens that have been rearranged.
- Check the clip
- A jubilee clip on a waste spigot loosens over years of heating and cooling. Nip it up while you are there.
- High loop or standpipe
- Without it, sink water siphons back into the machine. A very common fault in rearranged kitchens.
- Replace old fill hoses
- They fail by bursting under full mains pressure, which is why they are worth changing on age alone.
- Blank unused spigots
- An open unused appliance spigot on a trap will leak the first time the sink is filled.
Making the space workable
Empty the cupboard completely first. Working around bottles of bleach on your side is how things get knocked over, and you cannot see what you are doing with the cupboard half full.
Then put a towel or a shallow tray down before anything is undone, and lay your tools out where you can reach them lying down. Once you are in position you cannot get up for the thing you forgot without losing it.
- Empty it completely
- Not partly. You need the floor space and you need to see the base to spot where water is tracking.
- Absorbent down first
- A towel or a shallow tray. Chipboard cupboard bases swell permanently once wet.
- Tools within reach
- Laid out beside you before you get down. Getting up costs you your position and your line of sight.
- Two lights
- A head torch plus a clip light from another angle. One light source leaves the shadow exactly where the leak is.
Frequently asked questions
- What tools do I need to work under a sink?
- Water pump pliers, a basin wrench, adjustable spanners, a screwdriver, a bowl and cloths, a head torch and new washers. Add a strap wrench for chrome, and a wet vacuum if the cupboard is already wet. Most under sink jobs need nothing more than that.
- How do I find where a leak under the sink is coming from?
- Dry everything, lay kitchen roll under each joint, then run the tap and the waste separately. A supply leak shows with the tap off, a waste leak only while water is running. Water tracks along pipes, so the puddle is almost never below its source.
- How tight should a sink trap be?
- Hand tight. The rubber washer does the sealing, not the force, and a plastic trap is designed to be undone without tools. Using pliers on a new one usually cracks the thread or crushes the washer, which creates the leak you were trying to prevent.
- Why does my sink smell?
- Usually the trap has lost its water seal, either because it has dried out through lack of use or because it was fitted at the wrong angle so the seal is too shallow. Occasionally an unused appliance spigot has been left open, which bypasses the seal entirely.
- Should I fit isolating valves under my sink?
- Yes, if they are not already there. A screwdriver operated valve on each tap tail means the tap, the trap and the tails can all be worked on without touching the mains stopcock, and every future job in that cupboard becomes much smaller.
- Why does water siphon back into my washing machine?
- Because the waste hose has no high loop or standpipe, so water from the sink runs down into the machine instead of away. It is a plumbing fault rather than an appliance one, and it is common in kitchens that have been rearranged since they were plumbed.
- Can I reuse the washers in a sink trap?
- It is not worth it. A rubber washer takes the shape of its position over years, so it will not seal in a slightly different one, and a set of new ones costs very little. A trap reassembled on old washers is the most common cause of a leak that appears after a repair.
- What is the first thing to do before working under a sink?
- Empty the cupboard completely and put a towel or shallow tray down. You need the space to lie in, you need to see the base to work out where water has been tracking, and a chipboard cupboard base swells permanently once it gets wet.
