Quick answer: A leaking tank-to-bowl gasket drips at the joint under the tank where the tank sits on the bowl — not at the floor wax ring. Shut off the supply, drain and sponge the tank, lift the tank carefully, install a matching new gasket (often with new tank bolts), snug hardware evenly without overtightening porcelain, then refill and prove the joint stays dry.
This guide covers the seal between tank and bowl on two-piece toilets. If water pools at the floor around the base, use toilet leaking at the base instead. For a full internals refresh, see complete toilet tank rebuild and how to replace a toilet flush valve.
Educational illustration — diagram for teaching, not a photo of a real repair.
Tank-to-bowl gasket leak vs base / wax leak
| What you see | Likely seal | Wrong fix to avoid |
|---|---|---|
| Damp under the tank at the bowl shelf / rear joint | Tank-to-bowl gasket (± bolt washers) | Pulling the toilet for a wax ring while the tank joint is the wet zone |
| Puddle at the floor around the closet flange after flushes | Wax ring / flange / rocking bowl | Cranking tank bolts harder hoping the floor dries |
| Rust streaks only at bolt tips; joint otherwise dry | Tank-bolt washer path | Ignoring bolts while only swapping a gasket (or the reverse) |
Wipe the area dry, flush once, and watch where water reappears. That wet zone decides the repair path.
Safety first
- Shut off water at the toilet stop and prove it closes. If the stop is frozen or corroded, do not force it — isolate at the house main and see toilet shutoff valve leaking, stuck or won’t close.
- Flush, hold the handle, and sponge the tank dry before loosening bolts.
- Protect floors; expect leftover water when the tank lifts.
- Never overtighten locknuts or tank bolts on vitreous china — manufacturers warn that overtightening can crack porcelain.
- Stop DIY if bolts seize, shear, or you see a crack near a bolt hole.
Educational illustration — diagram for teaching, not a photo of a real repair.
Diagnosis before you buy parts
- Confirm the floor under the base is dry while the tank/bowl joint is wet (or vice versa).
- Look for a crushed, flattened, off-center, or hardened foam/rubber gasket once the tank is up.
- Check whether the flush-valve shank / locknut area is the wet path (gasket often travels with flush-valve work).
- Note bolt condition — rusted bolts often get replaced in the same visit.
Many two-piece toilets use a triangular foam gasket kit or a brand-specific DryLock-style gasket. Kohler, for example, documents tank gasket installation (including GP51487-style kits) and warns against overtightening tank nuts. Match the gasket style to your flush-valve shank — do not assume one foam ring fits every tower or canister design.
Tools and materials
- Matching tank-to-bowl gasket (and flush-valve kit if the valve is coming out)
- Tank bolt set (bolts, rubber washers, metal washers, nuts) if the old set is rusted
- Adjustable wrench / basin wrench as needed; towels; sponge; helper for tank lift
- The manufacturer’s install sheet for your exact kit
Replacement outline (two-piece, classic flush valve)
- Isolate: Shut off supply, flush, sponge the tank, disconnect the supply line.
- Unbolt evenly: Support the tank. Loosen tank nuts a little at a time left/right so the tank does not twist on one bolt.
- Lift: Lift the tank straight up with help and set it on towels. Avoid twisting the china against seized studs.
- Clean: Remove the old gasket. Wipe tank bottom and bowl inlet dry and free of grit.
- Install gasket: Seat the new gasket centered on the flush-valve shank / bowl inlet per the kit (orientation matters on brand-specific designs).
- Locknut caution: If you also replaced the flush valve, snug the locknut per kit language (a common Fluidmaster pattern is hand-tight plus about ½ turn — never force).
- Reseat tank: Lower the tank evenly. Install rubber washers inside the tank and metal washers/nuts underneath. Snug evenly until the tank is level and the joint is firm — stop short of “cranking.”
- Restore and prove: Reconnect supply, open the shutoff slowly, set the waterline below the overflow, flush several times, and wipe the joint dry. Watch for new dampness.
If the flush valve itself is the worn part, follow flush valve replacement. If fill valve, flapper, gasket, and bolts are all aged together, prefer the ordered sequence in complete tank rebuild.
Educational illustration — diagram for teaching, not a photo of a real repair.
What not to do
- Do not overtighten tank bolts to “stop a drip” — that is how tanks crack.
- Do not force rusted bolts; shearing a bolt mid-job creates flood risk.
- Do not treat a base/wax leak as a gasket problem (or the reverse).
- Do not reuse a permanently crushed foam gasket.
- Do not claim any tank tablet or cleaner as a fix for a failed mechanical seal.
When to call a plumber
- Cracked or chipped porcelain at bolt holes or tank bottom
- Seized or shearing tank bolts
- Tank will not sit level and dry after a careful reseat
- One-piece toilets or proprietary sealed tank designs you cannot service safely
- Shutoff will not close — isolate at the main, then get a pro
FAQ
Can I replace only the gasket without the flush valve? Sometimes, if the valve and seat are sound and the gasket is a serviceable style. Many jobs still open the locknut path — follow the kit that matches your toilet.
Should I replace the tank bolts at the same time? Yes if they are rusted, weeping, or the kit includes encapsulated bolts with the gasket.
Why is the floor wet but the tank joint dry? That points to the base/wax path — see toilet leaking at the base.
How do I know it worked? The tank/bowl joint stays dry after refill and several flushes, with no new rust streaks at the bolt tips.
Sources
- Kohler Assist — two-piece toilet tank and gasket installation
- Kohler install sheet — tank gasket / do not overtighten nuts (PDF)
- Fluidmaster 507A flush valve instructions (PDF)
- EPA WaterSense — Fix a Leak Week