How to Install a Toilet on Concrete
Updated February 25, 2026
Mount a toilet on a concrete slab floor using a flange, concrete screws, and a wax ring -- without cracking the slab or creating leaks.
Overview
Installing a toilet on concrete is different from a wood subfloor. You cannot use wood screws. You need concrete anchors or a flange designed for slab floors. The drain pipe comes up through the concrete, and the flange sits on top and bolts down. Get this wrong and you get rocking, leaking, and sewer gas. Get it right and it is rock-solid for 20+ years. This guide covers both scenarios: setting a new toilet on an existing flange and installing a new flange on a concrete slab.
What You'll Need
Safety First
- Wear safety glasses when drilling concrete. Chips and dust fly. A hammer drill throws debris at high speed.
- Concrete dust is a lung irritant. Wear a dust mask or N95 respirator, especially in enclosed bathrooms.
- Do not overtighten the closet bolts. Porcelain cracks without warning. Snug plus a quarter turn -- stop the moment you feel resistance.
Step-by-Step Instructions
- Remove the Old Toilet (If Replacing)
Shut off the supply valve. Flush the toilet, hold the handle down to drain the tank. Sponge out remaining water from the tank and bowl. Disconnect the supply line. Pop the bolt caps, remove the nuts from the closet bolts. Rock the toilet gently to break the wax seal, lift straight up. Stuff a rag in the drain opening to block sewer gas. Scrape the old wax off the flange with a putty knife.
Tip: Old toilet weighs 60-100 pounds. Have a helper or at least a clear path. Set it on cardboard to protect the floor. Old wax gets on everything -- keep paper towels handy. - Inspect and Repair the Flange
Check the existing flange. It should sit on top of the finished floor or no more than 1/4 inch above it. Flange below the floor? Use a flange extender kit ($5-10). Flange cracked or broken? Replace it. PVC flanges glue to PVC drain pipe. Cast iron flanges may need a repair ring that bolts on top. The flange must be solid -- it holds the toilet down and seals the connection.
Tip: Flange height matters more than most people realize. Too low and the wax ring cannot make a proper seal. Too high and the toilet rocks. Measure from the top of the flange to the finished floor surface. Level with the floor or up to 1/4 inch above is the target. - Secure the Flange to Concrete
If the flange is not already anchored, you need to drill into the concrete. Use a hammer drill with a 3/16-inch masonry bit. Drill through the flange mounting holes into the concrete -- 1-1/2 inches deep minimum. Blow out the dust with compressed air or a bulb. Drive 1/4-inch Tapcon concrete screws through the flange into the slab. Alternate sides to pull the flange down evenly. The flange should not move at all when you push on it.
Tip: Tapcon screws are the standard for toilet flanges on concrete. Pre-drill the exact bit size printed on the Tapcon box -- too big and they strip, too small and they snap. A $50 hammer drill rental pays for itself on this job. - Set the Wax Ring and Closet Bolts
Insert new closet bolts (5/16-inch x 2-1/4 inch is standard) into the flange slots. Slide them into position -- 12 o'clock and 6 o'clock, equal distance from the wall. Press a new wax ring onto the flange, centered over the drain opening. Standard wax ring for flanges at floor level. Extra-thick wax ring or wax ring with a horn (plastic funnel extension) if the flange is slightly below the floor.
Tip: Wax-free gaskets (Fluidmaster Better Than Wax, Sani Seal) are reusable and work on uneven surfaces. They cost $10-15 vs $3-5 for wax, but you can reposition the toilet without replacing the seal. Worth it on concrete where the surface may not be perfectly flat. - Set the Toilet and Tighten Down
Lift the toilet, align the base holes over the closet bolts, and lower it straight down. Press firmly with your body weight to compress the wax ring. Do not twist or rock -- wax rings seal once and any movement breaks the seal. Hand-thread the washers and nuts onto the closet bolts. Tighten alternating sides: a few turns on the left, a few on the right. Snug plus a quarter turn. Check with a level across the bowl. Shim if needed -- plastic toilet shims ($3 for a pack) work on concrete. Trim the shims flush and caulk around the base.
Tip: Caulk the base with silicone, but leave a 2-inch gap at the back. This lets you see if the wax ring ever fails (water escapes at the back) while still giving a clean look from the front and sides. - Connect Water and Test
Hand-tighten the supply line to the fill valve. Then a quarter turn with pliers -- no more. Turn on the supply valve. Let the tank fill. Flush 3-4 times. Check every connection: supply line, base (wax ring seal), and tank-to-bowl bolts. Any water on the floor? Tighten the relevant connection. No leaks after 4 flushes? Put the bolt caps on, trim the closet bolts with a hacksaw if they stick up too far, and you are done.
Tip: Sit on the toilet and shift your weight around. Any rocking? Shim the low side. Rocking on concrete eventually cracks the wax seal and you get sewer gas and water damage. Fix it now.
Pro Tips
- Renting a hammer drill costs $30-50 for a half day. Buying a corded hammer drill starts at $60. If you own a home on a slab, you will use it again -- anchoring shelves, running cable, mounting hardware. Worth buying.
- Concrete screws (Tapcon) come in flat-head and hex-head. Use flat-head for toilet flanges -- the flat head sits flush in the countersunk flange holes and does not interfere with the wax ring.
- Toilet rocks even after shimming? The concrete may have a hump. You can grind a small area with a masonry grinding wheel ($8 attachment for an angle grinder) to level the surface under the toilet base.
- If you are setting a toilet on a brand new slab (new construction), the plumber typically sets the flange before the concrete is poured. The drain pipe and flange are embedded in the slab. If this was not done, you need to core-drill the slab to run a new drain -- that is a professional job.
- Closet bolts corrode on concrete floors faster than wood floors because of moisture wicking through the slab. Use brass closet bolts ($4-6) instead of zinc-plated steel. They last the life of the toilet.
When to Call a Pro
Call a plumber if you need to move the drain location (requires cutting concrete and rerouting the drain pipe), if the existing flange is cast iron and crumbling (may need a full flange replacement down into the pipe), if there is no drain pipe in the slab and you need one installed (core drilling and connection to the main line), or if the slab is cracked around the drain area (structural issue that needs assessment before mounting a toilet).
Frequently Asked Questions
When should I call a plumber to install a toilet on concrete?
Call a plumber if you need to move the drain location (requires cutting concrete and rerouting the drain pipe), if the existing flange is cast iron and crumbling (may need a full flange replacement down into the pipe), if there is no drain pipe in the slab and you need one installed (core drilling and connection to the main line), or if the slab is cracked around the drain area (structural issue that needs assessment before mounting a toilet).
What are some expert tips to install a toilet on concrete?
Renting a hammer drill costs $30-50 for a half day. Buying a corded hammer drill starts at $60. If you own a home on a slab, you will use it again -- anchoring shelves, running cable, mounting hardware. Worth buying. Concrete screws (Tapcon) come in flat-head and hex-head. Use flat-head for toilet flanges -- the flat head sits flush in the countersunk flange holes and does not interfere with the wax ring. Toilet rocks even after shimming? The concrete may have a hump. You can grind a small area with a masonry grinding wheel ($8 attachment for an angle grinder) to level the surface under the toilet base. If you are setting a toilet on a brand new slab (new construction), the plumber typically sets the flange before the concrete is poured. The drain pipe and flange are embedded in the slab. If this was not done, you need to core-drill the slab to run a new drain -- that is a professional job. Closet bolts corrode on concrete floors faster than wood floors because of moisture wicking through the slab. Use brass closet bolts ($4-6) instead of zinc-plated steel. They last the life of the toilet.
What tools do I need to install a toilet on concrete?
You will need: Hammer drill with 3/16-inch masonry bit (essential for concrete), 1/4-inch Tapcon concrete screws (for flange mounting), Adjustable wrench (for closet bolt nuts and supply line), Wax ring or wax-free gasket, New closet bolts (brass recommended for slab floors), Level (to check toilet is sitting flat), Plastic shims (for leveling on uneven concrete), Putty knife (for scraping old wax), Hacksaw (for trimming closet bolts), Silicone caulk and caulk gun.
When should I call a professional instead of doing this myself?
Call a plumber if you need to move the drain location (requires cutting concrete and rerouting the drain pipe), if the existing flange is cast iron and crumbling (may need a full flange replacement down into the pipe), if there is no drain pipe in the slab and you need one installed (core drilling and connection to the main line), or if the slab is cracked around the drain area (structural issue that needs assessment before mounting a toilet).
How difficult is this project?
This project is rated intermediate. It requires moderate DIY skill and the right tools.
How long does this take?
Plan for approximately 1 - 3 hours. First-timers may need extra time.
How much will this cost?
The estimated cost is $15 - $60 (hardware only, toilet not included). Costs vary by location and materials chosen.