Joists support floors and ceilings. They are the horizontal members that span two walls and/or beams. They have to carry the weight of the walls, people, furniture, appliances and anything else we place on them. When a load is applied to a joist, the wood fibers along the bottom edge go into tension and those along the top go into compression. But when you notch or drill a joist, you cut some of those fibers and reduce the joist’s ability to withstand compression or tension. And if you create a hole or notch too big or in the wrong place, you seriously weaken the joist, making it sag, and giving it an easy place to crack and fail.
If your home is more than 20 years old, your floor joists are probably solid wood 2x8s, 2x10s or 2x12s. One rule of thumb is to provide twice the distance between holes as the diameter of the largest hole. Also, notching the top of a joist weakens it less than notching the bottom, and it’s important to avoid locating holes and notches near loose knots. Engineered wood I-joists began to replace solid wood joists during the last 20 years. “Glue lams” are engineered beams comprised of wood chips bonded together with an epoxy resin then subjected to intense pressure producing a much stronger beam.
The rim joist that runs around the perimeter of your house often gets peppered with large holes because this space is the logical exit point for dryer and furnace vents, big electrical service wires and fireplace fresh air intakes. Since the rim joist is continuously supported by the walls or foundation it rests on, you don’t want to create a large hole or notch directly under a group of studs that support the end of a beam, or a window or door header. A steel lintel is sometimes used to span the open space.
The place where most people are tempted to create oversize holes and notches is under the bathtub, the part of the floor you can least afford to weaken. A cast iron tub, with water and occupant, can weigh more than 800 lbs. Care should be taken to insure extra support is installed under bathtubs.
If you’re building an addition or a new home, most notching and boring can be avoided by planning ahead to provide adequate pathways and space for pipes and ducts. The best thing to do is to drop your pipes and ducts down below the joists and box them in with wood-framed soffits. They’re the least obtrusive when run along an outside wall or center beam.
Call Quality Foundation Repair for advice and a free estimate regarding your joist work. They will save you many headaches and money as well. Quality Foundation Repair is in the top 5% of all contractors in the United States, and received Angie’s List Super Service Award for 2012.