What a valve gasket does and why it fails
A valve gasket is a thin seal that sits between two metal surfaces — typically between a valve body and its bonnet (the removable cap on top) — to prevent fluid or gas from leaking where those surfaces meet. Over time, gaskets harden, crack, or compress from repeated pressure cycles, and leaks develop.
Most valve gaskets fail because of age, temperature swings, or corrosion of the metal surfaces underneath. A leaking gasket usually shows up as a slow drip from the top of the valve, even when the valve handle is fully closed. Replacing the gasket is straightforward if you have basic hand tools and can turn off the water or gas supply to that section of pipe.
The job takes 30 minutes to an hour for most people and costs between $5 and $30 for the gasket itself, depending on the valve size and material. You do not need a plumber's license or special certifications to do this work on your own property.
Key Takeaways
- Turn off the water or gas supply to the valve before you start, and open the valve handle to release any remaining pressure in the line.
- Use an adjustable wrench or the correct socket size to loosen the bonnet nut — the large nut at the top of the valve — and lift it straight up.
- Remove the old gasket by hand or with a flat screwdriver, then clean the metal surfaces with a rag and inspect them for pitting or corrosion.
- Install the new gasket, reassemble the bonnet, and tighten the nut firmly but not so hard that you strip the threads or crack the bonnet.
- Turn the supply back on slowly and watch for leaks at the bonnet for at least five minutes before you consider the job done.
Gather your tools and materials before you start
You will need an adjustable wrench or a socket set that fits the bonnet nut on your specific valve. Most residential valves use a nut between 1/2 inch and 1 1/4 inches across, but measure yours first. You will also need a flat-head screwdriver, a clean rag or paper towels, and a replacement gasket that matches your valve type and size.
Buy the gasket before you take the valve apart. Bring the valve model number or a photo of the valve to a hardware store or plumbing supplier, or look up the model online to find the correct gasket part number. Common gasket materials are rubber, PTFE (Teflon), or graphite-impregnated rubber; the right one depends on what fluid or gas the valve controls and the temperature it sees.
Have a bucket or towel ready in case water drains from the valve when you open it. If the valve controls gas, make sure the area is well-ventilated and that you do not have any ignition sources nearby.
Turn off the supply and release pressure from the line
Locate the shutoff valve upstream of the leaking valve — usually the main water shutoff for the house, or a dedicated shutoff for that section of pipe. Turn it clockwise until it stops. If you are working on a gas line, turn the knob on the gas shutoff valve to the OFF position (perpendicular to the pipe).
Open the leaking valve by turning its handle counterclockwise (for a gate valve or ball valve) or lifting the lever (for a lever-handle valve). This releases any pressure trapped in the line between the shutoff and the leaking valve. If water comes out, let it drain into a bucket or onto a towel.
Wait 30 seconds to make sure no more water or gas is flowing. If you are working on a gas line and smell gas, stop when ready, leave the area, and call your gas company's emergency line.
Remove the bonnet and extract the old gasket
The bonnet is the cap-shaped part at the top of the valve. It is held on by a large nut called the bonnet nut or packing nut. Use your wrench or socket to loosen this nut by turning it counterclockwise. Do not remove it completely yet — loosen it about one full turn, then use your hand to unscrew it the rest of the way.
Lift the bonnet straight up and off the valve body. The gasket sits in a groove or on a flat surface just below the bonnet. If it is stuck, use a flat-head screwdriver to gently pry it out. Do not scratch or gouge the metal surface underneath, because any damage there will cause the new gasket to leak.
Wipe the metal surface clean with a dry rag. Look for pitting (small holes), corrosion (white or green discoloration), or deep scratches. Minor surface roughness is normal, but if the surface is heavily pitted or corroded, you may need to have it resurfaced by a machine shop or replace the entire valve.
Install the new gasket and reassemble the bonnet
Place the new gasket in the same groove or position where the old one sat. Make sure it sits flat and is not twisted or bunched up. If the gasket is a ring shape, check that the opening faces the right direction — usually toward the stem (the rod that moves up and down when you turn the handle).
Lower the bonnet back onto the valve body, making sure the gasket stays in place as you do. Start the bonnet nut by hand, turning it clockwise until you feel it catch the threads. Then use your wrench to tighten it snugly — firm enough that the gasket compresses and seals, but not so tight that you crack the bonnet or strip the threads.
A good rule is to tighten until you feel solid resistance, then give it one more quarter turn. If you over-tighten, you risk cracking the bonnet or damaging the gasket. If you under-tighten, the gasket will not seal and you will still have a leak.
Turn the supply back on and check for leaks
Slowly turn the upstream shutoff valve counterclockwise to restore water or gas to the line. Open it gradually over 10 to 15 seconds so pressure builds slowly and does not shock the system. Watch the bonnet area closely for any drips or mist.
Let the valve sit under pressure for at least five minutes. If you see a drip, turn the supply back off, wait for pressure to drop, and tighten the bonnet nut another quarter turn. Then turn the supply back on and watch again. Repeat this process until the leak stops.
If the leak continues after you have tightened the bonnet as far as it will go, the metal surface under the gasket may be too damaged to seal. In that case, you will need to have the valve resurfaced or replaced.
Common mistakes and how to avoid them
The most common error is over-tightening the bonnet nut. Many people assume that tighter always means better, but a gasket only needs enough pressure to compress and seal — extra force just cracks the bonnet or damages the gasket. Tighten until you feel resistance, then stop.
Another mistake is installing the gasket upside down or twisted. Take a moment to look at how the old gasket sat before you remove it. If it is a ring, make sure the new one sits in the same groove. If it is a flat disc, make sure it lies flat with no wrinkles or folds.
A third error is not cleaning the metal surface well enough. Even a thin film of mineral deposits or corrosion will prevent the new gasket from sealing. Use a clean, dry rag and wipe the surface several times until it looks shiny and smooth.
Frequently Asked Questions
Do I need to turn off the main water shutoff, or can I just shut off the valve above the leak?
You can shut off the valve closest to the leak — usually a dedicated shutoff for that section of pipe. If there is no shutoff between the main and the leaking valve, then you will need to use the main shutoff. Either way, always turn off the supply before you open the bonnet.
What if the bonnet nut is stuck and won't turn?
explore penetrating oil (like WD-40) around the nut and let it soak for 15 minutes. Then try again with your wrench. If it still will not budge, tap the wrench gently with a hammer to help break the corrosion. Do not force it hard enough to strip the nut or crack the bonnet.
Can I reuse the old gasket if it looks okay?
No. Gaskets harden and lose their ability to seal once they have been compressed under pressure. Even if the old gasket looks fine, it will not seal as well as a new one. Always install a fresh gasket.
How do I know what size gasket to buy?
Bring the valve model number or a photo to a hardware store or plumbing supplier. You can also measure the diameter of the bonnet opening and the thickness of the old gasket, then search online for a gasket that matches those dimensions. Most suppliers can cross-reference by valve brand and model.
What if water leaks from the handle stem instead of the bonnet?
That is a different seal called the packing nut, located below the bonnet. The fix is similar — loosen the packing nut (usually smaller than the bonnet nut) and tighten it slightly. If that does not work, you may need to replace the packing material inside, which is a more involved repair.