The short answer
Water dripping from a toilet tank in a Northern Virginia home comes from one of five places: the bolts that hold the tank to the bowl, the big gasket between them, the nut where the supply line meets the tank, the shutoff valve on the wall, or a crack in the tank itself. On a humid day it may be sweat, not a leak. A few drops of food coloring tell you which in five minutes, and most of them are a part and an hour.
The five-minute dye test
- Dry the tank, the bolts underneath, the supply line and the floor.
- Put a few drops of food coloring in the tank. Don’t flush.
- Hold a dry paper towel under each spot: both tank bolts, the back of the bowl under the tank, the supply nut, and the valve on the wall.
- Colored water means tank water: the bolts, the gasket, or a crack. Clear water means the supply side: the line, the nut or the valve.
- Color in the bowl with no flush means it’s leaking inside, past the flapper. That’s not a drip to the floor, it’s a phantom flush.
That last one has its own guide: toilet refills by itself.
$29 Water Test and Plumbing Safety Check
A licensed plumber tests your water's hardness at your kitchen table, in grains per gallon, checks your water heater's age and health and walks the plumbing with photos. Book the $29 test
Where it’s leaking, and the fix
Swipe to see all columns →
Swipe to see all columns →
| The spot | What you see | The fix |
|---|---|---|
| Tank bolts | Drips from the bolt heads under the tank | New bolts and rubber washers. Cranking old ones tighter cracks the tank. |
| The tank-to-bowl gasket | Water at the back of the bowl, worse after a flush | The tank comes off for a new gasket. Bolts get replaced at the same time. |
| The supply nut | A drip where the line meets the tank | A snug quarter turn by hand. A cracked nut means a new line. |
| The supply line | A wet, bulging or rusty line | A new braided line |
| The shutoff valve | Water at the handle or the nut on the wall | Snug the nut behind the handle. If it still weeps, a new quarter-turn valve. |
| A crack | A steady seep from one spot, the tank level drops | A new tank or a new toilet |
| Sweat | Beads on the whole tank on a humid day | Not a leak. A fan or the AC dries it up. |
The shutoff valve is the one to watch
Most houses here have the original valves behind the toilets, and older ones with a round handle that takes several turns get stiff and start to weep at the stem. If you turn one and it starts dripping, or it won’t turn at all, don’t force it. Shut the main off and have the valve replaced with a quarter-turn one. Where your shutoff valves are shows you where the main is before you need it, and if the main itself is stuck, main water shutoff valve replacement covers that.
Supply lines are the same story. A braided steel line looks like it lasts forever, but a line that’s bulging, kinked, or rusty at the nut gets replaced now. Those are the parts that let go all at once and put water on the floor until someone finds the valve.
Sweat, not a leak
When cold water refills the tank on a warm, humid day, the outside of the tank sweats, and the drips collect on the floor under it. It looks like a leak and it isn’t. The giveaway is beads over the whole tank, not one wet spot. Running the bathroom fan or the AC helps. If it happens all summer, a tank liner or a valve that mixes a little warm water into the fill will stop it.
Why a small drip is worth fixing now
A tank bolt that drips a little with every refill is wetting the same spot of floor many times a day, and it soaks in long before the floor looks wet. Fixed now, it’s a bolt kit. Left for a year, it’s a soft floor, and sometimes a stain on the ceiling under the bathroom. If the water is pooling at the base with the tank dry, look at the wax ring instead: toilet leaking at the base.
Water coming out faster than a towel can handle? Shut the valve behind the toilet or the main, and call. We run 24/7 emergency plumbing.
A drip under a toilet tank is one of the small signs worth catching early. Every warning sign around the house, from a furnace’s blinking light to the sounds a house makes, is in one guide.
Fix it small, fix it now
A leak caught early is cheaper three ways. The water and sewer bill stops climbing. The water stops soaking the subfloor or the ceiling under it. And it stays a small repair, a part and an hour, instead of a bigger one later. Almost everything we find wrong with a toilet is repairable, and once it’s fixed right, you stop checking the floor every time you walk in.
What it costs when we fix it
Most leaks, wobbles and fill problems are a repair, toilet repair at $250 to $900, with the price in writing before we start. That covers a wax ring, a flange repair, tank bolts and gaskets, and fill valves. When the toilet itself is cracked or worn out, a new one installed runs $800 to $1,300, and this month’s coupons on our offers page take $100 off a new toilet, or $50 off any plumbing repair over $400. Any plumbing repair also comes with a whole-house plumbing inspection, a $149 value. If you can’t tell where the water is coming from, leak detection runs $200 to $500. A plumbing diagnosis visit is $49, or $250 after hours.
Call (703) 997-9757, day or night, or book a toilet repair online. Rated 5.0 stars across 1,138 reviews on Google, Angi, Yelp, the BBB and Facebook, Virginia Class A license 2705192276.
Questions people ask
Why is my toilet tank leaking onto the floor?
The tank bolts, the gasket between the tank and the bowl, the supply line nut, the shutoff valve, or a crack. Food coloring in the tank tells tank water from supply water.
How do I stop my toilet tank from leaking?
Find the spot first. Bolts and the gasket need the tank off. A supply nut often needs a quarter turn. A weeping valve or a bulging line gets replaced.
Is it normal for a toilet tank to sweat?
Yes, on humid days when cold water refills the tank. Beads over the whole tank are sweat. One wet spot is a leak.
Should I replace the shutoff valve behind my toilet?
If it’s stiff, won’t close, or drips at the handle, yes. A quarter-turn valve closes in one motion and doesn’t seize.
How much does it cost to fix a leaking toilet tank?
When we do it, tank bolts, a gasket or a supply line are a toilet repair, $250 to $900, priced in writing before we start.
