Can I Drill Through a Doubled Floor Joist?

Issues

• Only two primary reasons to drill holes in joists exist: plumbing and electrical work. The concern when drilling through a joist is weakening the joist, creating problems with the floor warping or caving. When a joist is drilled properly, those problems should not be an issue. Consult your location's building codes carefully if you plan to undertake a drilling project to connect pipes or cables. The codes may have specific rules regarding placement of drill holes.

Hole Size

• The larger the hole you make when drilling, the more you endanger the joists. Keep holes in doubled joists to no more than 1 1/2 inches in diameter. Building codes vary by location on acceptable hole sizes; so look at your area's building codes for guidelines. If you have any doubt, hire an inspector or construction expert to drill the holes for you. A competent electrician or plumber should know the correct hole size limits based on your home's doubled floor joists and their position in your home. Although drilling doubled floor joists is allowed, it should never be done if it threatens the structure's stability.

Uses

• Holes in doubled floor joists allow electrical wiring to pass through the thick layer of wood to reach other rooms. Although electrical wires are well-insulated, they should not be grouped tightly together in enclosed spaces such as holes in floor joists. The wires should fit loosely in joist holes. Jamming a collection of electrical wires through a single hole creates a fire hazard. Avoid that problem by drilling multiple holes if you have many wires to guide through doubled floor joists.

Advantages

• If a structural stability problem prevents you from drilling a single floor joist board, then a doubled floor joist may be the solution. Because of the extra strength that a second floor joist board provides, you may be able to drill through a doubled floor joist to connect plumbing or electrical work where you could not with a single floor joist.