The Risks of Drilling Into an Outside Basement Wall

Wall Types

• Poured concrete reinforced with steel makes the strongest wall commonly used in underground construction. Reinforced concrete withstands settling pressures from earth fill outside of the basement and safely supports the building above it. In older buildings, basements lined with masonry walls use a different system to resist outside pressure. The curved walls of old basements compress under load and withstand forces that would collapse a straight masonry structure. Drilling into old masonry construction can crack the mortar and cause a weak spot in the wall. Older types of mortar heal when cracked, but modern mortars lack the flexibility of the old formulas.

Drills

• Masonry bits fracture the wall ahead of the bit instead of cutting with rotational force. A rotary drill with a masonry bit has little effect on concrete, but as the bit of a hammer drill turns, the drill chuck slams forward repeatedly, chipping the concrete away with the bit's leading edge. Diamond core bits cut with a matrix of tiny diamonds bonded to the drill's tip and cut through concrete and embedded steel. Both types of drills create fine concrete dust that can cause respiratory irritation. Additionally, concrete dust can scratch your corneas and cause permanent eye damage. Wear a mask and goggles to protect yourself from dust and flying chips.

Water Damage

• Boring through a basement wall punctures any liner installed to prevent water from seeping into the building. Modern basements might depend upon an exterior waterproof barrier and a layer of insulation to deflect groundwater. Punctured liners cause seeps at the base of the wall. Because all pipes expand and contract at different rates than concrete and stone, sealing the pipe requires a flexible caulk that also bonds securely to the wall. Any dust or dirt in the hole can prevent a waterproof seal, and the small movements of the pipe gradually open gaps.

Other Damage

• Hammer drills can cause cratering on either face of the wall being drilled, but the greatest damage occurs when the bit nears the outside surface. The impact of the drill bit can punch out a crater much larger than the diameter of the bit. If the crater lies above the frost line, ice freezing and thawing in the broken area can increase the damage to the wall. Striking a piece of reinforcing steel while drilling can snap an expensive diamond drill. By using a metal locator, similar to a stud finder, you can eliminate costly mistakes by locating steel reinforcements before drilling.