How to Core a Foundation Wall and Seal It

Why this matters

A hole through a foundation wall has to do two things for the life of the house: let a pipe or conduit through, and keep groundwater out around it. A ragged hole knocked through with a hammer drill and a chisel does neither well. It cracks the wall, leaves a cone of broken concrete on the far face that no seal can grip, and turns into the wet stain on the basement wall the customer calls about every spring. A diamond core drill makes a clean, round, sized hole that a seal is designed to fit, at the cost of setting up for water, dust and torque.

This card owns the coring. The seal itself, for every kind of below-grade penetration, belongs to How to Seal a Below-Grade Utility Penetration, and silica health and the construction standard belong to Respirable Crystalline Silica and the Construction Standard. Cite both, and read the first before you choose a core size.

Before you set up: what the wall can do to you

• Something live in or behind the wall. Electric service conduit, a gas service and the water service often enter through the same wall you are coring. Locate every existing entry inside and out before you choose a spot. If a core does strike a gas line, everyone leaves the building, no switches, no phones inside, and the gas utility's emergency line or 911 is called from outside.
• Water and electricity together. Wet coring puts a stream of water at an electric motor on a basement floor. Power the drill from a GFCI-protected supply, use a motor the manufacturer rates for wet coring with its own in-line ground-fault device where it has one, and keep cords off the wet floor.
• Torque. A core bit that binds on rebar or a broken core can twist a handheld drill out of your hands or wrench a wrist. Above a small diameter, use a rig-mounted drill anchored to the wall or floor; on a handheld drill, keep the side handle on and use the clutch.
• Silica. Cutting concrete releases respirable crystalline silica. Water at the bit is the control for a rig-mounted core drill; a shroud on a vacuum is the control for dry handheld drilling. Dried slurry is dust again, so it is cleaned up wet.

Step 1: Confirm what the wall is and what is on the other side

Do: establish whether the wall is poured concrete or concrete block, and its thickness, from a window opening, a basement door, the top of the wall in the rim area or an existing penetration. Measure from inside to the grade line outside to know whether the hole will be above or below grade. Check whether the exterior face is exposed (an excavation, or above grade) or will stay buried.

Why: a block wall cores through hollow cells and needs those cells filled around the penetration before it will seal; a poured wall does not. A below-grade hole facing undisturbed soil can only be sealed from inside, which limits the seal you can use.

What breaks if you skip it: you core a block wall like a poured one, the seal grips the two face shells with nothing between them, and water runs down the open cell around it.

Step 2: Locate what is in and around the wall

Do: inside, trace every surface-run conduit, pipe and cable on the wall, and look at where each utility enters. Outside, any digging to expose the wall comes after a call to your state's one-call center (811), because state one-call laws require notice before excavation, with the waiting time set by each state. Ask the customer about anything buried that the utilities do not mark: a sprinkler line, a drain tile outlet, a private gas or electric line to a garage.

Why: a foundation wall is where utilities enter, so it is the one wall most likely to have something next to where you want to be. Private lines are not marked by the one-call locate, which covers the utility's own facilities.

Step 3: Scan for reinforcing steel before you mark the center

Do: scan the area with a cover meter or ground-penetrating radar, or have a concrete scanning firm do it, and mark the bars on the wall. Choose a core center that clears every bar by at least the core's radius plus a margin.

Why: most poured foundation walls carry vertical and horizontal bars, at spacings and positions that vary with the design and the builder. A core through a bar cuts it, and cutting reinforcing steel is a structural change, not a plumbing decision.

Route at the fork: if the only workable location cuts a bar, or the hole is close to a corner, a beam pocket, the footing or another opening, stop and ask the builder's or the owner's engineer before coring. Walk in with the scan marks, the core diameter, and the location measured from the nearest corner and from the floor. Slabs are a separate case, and a post-tensioned slab is never cored without an engineer; see Post-Tensioned Slabs and the Tendon You Must Not Cut.

Step 4: Size the core to the seal, and core rather than hammer

Do: choose the seal first: a modular mechanical seal (a ring of rubber links bolted between plates that expands to fill the gap between pipe and hole), a cast-in or grouted sleeve with a seal, or a compound seal the sibling card covers. Each seal's manufacturer publishes a chart pairing the pipe's outside diameter with the hole diameter the seal needs. Core to that diameter.

Why core and not hammer: a hammer drill and chisel break concrete by impact. They crack the wall around the hole, leave an irregular bore, and blow out a cone of concrete on the exit face. A diamond core bit grinds a clean cylinder to a known diameter, which is what a mechanical seal is built to fit.

What breaks if you skip it: the seal gets fitted to a hole that is too big in one place and too small in another, and leaks where the gap is largest.

Step 5: Set up for water, slurry and torque

Do: anchor a rig-mounted core drill to the wall with the anchor or vacuum base its manufacturer specifies, square to the wall face. Connect the water feed. Build a slurry dam on the floor below the hole with a water-control ring, a bucket, or towels, and have a wet vacuum ready. Plastic sheeting goes on anything nearby the slurry could reach.

Why: water cools the bit, flushes the cut and is the silica control. A drill that is not anchored square wanders, binds and twists.

Hazard at this step: the anchor hole for the rig is drilled with a hammer drill, which makes silica dust: use a shroud on a vacuum for it.

Step 6: Drill steadily and read what comes back

Do: start at low speed with light pressure until the bit has cut a full ring, then bring it to its rated speed with steady, moderate feed. Watch the water coming back: gray slurry is concrete. Listen and feel for change. A sudden metallic grind, a reddish tint in the slurry, a sharp drop in progress or steel particles in the water mean the bit is on a bar. Stop.

Why: you can stop on a bar before cutting it through, and that leaves you a choice. Pushing through leaves you none.

If you hit a bar the scan missed: back the bit out, record the depth, and go back to Step 3's fork. Do not finish the cut until someone with authority over the structure has agreed to it.

Step 7: Break through without blowing out the far face

Do: as the bit nears the far face, ease off the feed so it grinds through rather than punching. Where the far face is exposed, you can core partway from each side using a pilot hole to align them, which leaves both faces clean.

Why: the last inch of concrete breaks away in a cone if pushed. A cone on the exterior face is exactly where the exterior waterproofing and the seal's outside plate need a flat surface.

Step 8: Clean, inspect and measure the bore

Do: remove the core, flush the hole, and look along it with a light. Measure the diameter at both ends. Look for honeycombing (voids in the concrete), cracks running from the hole, and any exposed steel.

Why: a mechanical seal holds water only against a sound, round bore. A void in the wall at the hole is a path for water around the seal, and it gets filled with non-shrink grout before the seal goes in.

Step 9: Seal it

Do: install the pipe or conduit and the seal to its manufacturer's instructions, including the bolt tightening sequence, and restore the waterproofing on the exterior face where it is exposed. The details, by seal type and by water condition, are in How to Seal a Below-Grade Utility Penetration.

Worked example: a new water service through a basement wall

A plumbing shop is bringing a new 1-1/4 in polyethylene water service into a basement. The excavation outside has exposed the wall, and the service enters about 30 in below grade.

Step 1. A basement window opening shows an 8 in poured concrete wall. The exterior face is exposed in the excavation and carries a black damproofing coating.

Step 2. The electric service enters through a conduit on the opposite wall; the old water service is on this wall, 4 ft to the left of the planned entry, and will be abandoned. The one-call locate was done before the excavation, and the customer confirms there is nothing private buried on that side.

Step 3. A cover meter finds vertical bars at about 16 in and a horizontal bar about 6 in above the planned entry height. The planned center sits 2 in from a vertical bar. The shop moves the center 6 in to the right, putting it 8 in from that bar and 8 in from the next.

Step 4. The pipe's outside diameter is 1.660 in. Say the seal's chart, for that pipe OD, calls for a 3 in core; that is the figure the shop reads off the chart, and the reader's own seal will have its own. A 3 in core has a 1-1/2 in radius, so the nearest bar sits 6-1/2 in from the hole's edge.

Step 5. The rig is anchored with one expansion anchor, drilled with a shrouded hammer drill on a vacuum. Water feed on, slurry ring on the wall below the hole, wet vacuum beside it, drill powered from a GFCI-protected outlet.

Step 6. The bit cuts steadily for about 7 in with gray slurry and no change in sound.

Step 7. The plumber eases the feed for the last inch. The exterior face still spalls a ring about 1/2 in wide around the hole, chipping the damproofing.

Step 8. The bore measures 3 in at both ends, no voids, no exposed steel. The spalled ring on the exterior is patched flush with non-shrink grout so the seal's outside plate seats on a flat surface. That patch is done from the excavation: at 30 in it is under the 5 ft depth at which 29 CFR 1926.652(a)(1) requires a protective system, but a competent person still inspects it under 1926.651(k), and any sign of the face moving or cracking keeps everyone out of it.

Step 9. The service is pushed through, the modular seal installed and tightened in sequence, and the damproofing restored over the patch and around the pipe before backfill.

What would have changed the plan. On a block wall, the cells around the hole would be filled with grout before coring so the seal has solid material between the face shells. With a high water table, the seal and its pressure rating would be chosen for the head of water, which is the sibling card's call. And had the scan shown no workable spot between bars, the next move was the engineer, not a harder push.

How to verify you got it right

The core clears every scanned bar and no steel shows in the bore. The bore is round and measures what the seal chart called for at both faces. The exit face is flat enough for the seal plate, or patched flat. The slurry was cleaned up wet, with nothing left to dry into dust. After backfill, and after the first heavy rain, the inside face around the seal is dry.

References

• OSHA respirable crystalline silica in construction, 29 CFR 1926.1153, Table 1 entries for rig-mounted core drills and handheld drills
• Your state's one-call (811) law for notice before excavation
• Core drill, bit and modular seal manufacturer instructions and sizing charts
• See related: How to Seal a Below-Grade Utility Penetration
• See related: Respirable Crystalline Silica and the Construction Standard