In this article, we focus on end bearing, with the understanding that many other things can happen to joists and beams, including rot and insect damage.
House structures are a complex set of interconnected components comprised of different materials. There are many types of failures, but failures at connections are among the most common and the most serious. In this article, we will look at a specific connection—the end bearing of beams and joists. Beams are a critical component in that they typically transfer the large load of several joists to a foundation or column or post. They carry the live and dead loads of large floor areas. They also may carry the loads from additional floors above through walls or columns.
Many beams also have intermediate supports and although there are some special considerations for those, we will focus on bearing at the ends of beams and joists.
Joists are smaller and more numerous than beams and less critical, in the sense that they each carry a much smaller load than beams. Nonetheless, joists that lose their end bearing can lead to collapse, especially if several in one area all lose their bearing.
Steel or wood beams should rest on flat bearing surfaces that are at least as wide as the beam. The beam must project far enough onto the bearing surface so that the end of the beam or the bearing surface will not be crushed, and so that there is no risk that the beam will slip off the bearing surface with minor building settlement.
Beams and joists should rest on not less than 1 1/2 inches of wood or steel, or 3 inches of masonry or concrete. Steel beams only should be supported with steel shims. Steel shims should be welded to the beam so they will not move. Watch for shims that have slid out of position. Watch also for wood shims under steel beams. They often crush and creep. Shims often will move because the top surface of the foundation wall is not level.
Wood beams can rest on wood shims, although they should provide continuous bearing on the top and bottom of the shim. The shims should be secured in place with adhesive or mechanical fasteners. The shims should be of a wood at least as hard and dense as the beams.
Cedar, for example, is soft and doesn’t make good shims for beams.
Beams on masonry walls should rest on at least 3 inches of solid masonry. Many recommend that beam ends rest on solid masonry or concrete all the way down to the footing. In any case, a beam should not rest directly on a hollow concrete block. At least the top row of block should be filled with concrete. This can be difficult to determine during a home inspection.
The point at which the end of a wood beam is embedded in a concrete or masonry foundation wall is prone to rot. Best practice is to have a ½ inch of air space around the end, top and sides of the beam to allow the wood to dry. In some cases, the end of the beam is coated with pressure-treating chemicals to make the beam more rot resistant.
Causes of problems
If the bearing for beams is inadequate, it may be the result of one or more of the following:
- an original construction mistake
- deterioration of the foundation material or column
- settlement of the foundation or column
- sagging of the beam
- loss, movement or crushing of shims between the bottom of the beam and the foundation or column
Joists typically require a minimum of 1 1⁄2 inches of end bearing on wood or metal, and a minimum of 3 inches of end bearing on masonry or concrete.
At the points at which joists rest on beams, best practice is for joists to rest on the full width of the beam or wall rather than just 1 1/2 inches of it. Good practice also includes splicing joists that overlap from the opposite side of the beam. The joists can be nailed together with three 10d nails, or with metal or wood splices.
Joists can be supported in many ways:
- resting on sill plates on top of foundation walls
- being embedded in masonry or concrete foundation walls
- being on the sides of beams with joist hangers
- being on the sides of beams with ledger strips or boards (not less than a nominal 2 inches by 2 inches)
- being on wood stud walls with 1 -inch-by-4-inch ribbon strips and with nailing to the studs
- being in a mortise and tenon joint in which the end of the joist is trimmed to form a tenon and the beams have a mortise cutout to receive the tenon
- being notched at the end so that only the top part of the joist sits on a beam
- being on the bottom flange or top flange of steel beams as long as they have adequate and bearing
Different types of joist-end support
Joists may not have enough end bearing because of one or more of the following:
- original construction mistakes
- sagging joists
- differential settlement or heaving of the footing or foundation system
- bowing or leaning walls
- notched bearing arrangements
- rotted joist ends, sills, beams or foundations
At the points at which joists are supported on the bottom flange of a steel beam, it is good practice to have the joists slightly taller than the beam and a wooden splice across the top of the steel beam.
The bottom of the splice should be at least 1/2 inch above the top flange of the beam to allow for vertical shrinkage of the wood members. It’s not unusual to get 4% shrinkage across the grain in wood framing members. That’s about 3/8 inch in a 2 by 10 joist.
Notching weakens joists
One of the weakest configurations is when the joist is notched and only the top part of the joist rests on a beam or foundation wall. This reduces the strength of the joist and concentrates loads in the top part. Check for cracks radiating horizontally from the top of the notch. This can result in serious structural movement, especially if more than one joist shows this crack. The solution involves supporting the bottom of the joist, often with joist hangers.
A better (but still less than desirable) arrangement is to notch only the bottom inch or so of the joist and rest the majority of the joist on a ledger board or steel beam flange, for example. The joists are weakened slightly by such an arrangement. Again, watch for horizontal cracks running out from the notch.
Steel joist hangers can provide excellent support for joists on the sides of beams. This maximizes the head room in basements and crawlspaces because the joists don’t have to rest on top of the beams. However, joist hangers must be installed correctly to work properly.
Common joist hanger issues include the following:
- There are differently sized joist hangers for differently sized joists. A joist hanger designed for 2 by 4s should not be used on 2 by 10s.
- Joist hangers have lots of holes in them for nails. Generally speaking, every hole should be filled with a nail. It is very common to find that people have skimped on the number of nails.
- The right type of nails should be used. Some joist hangers have special nails made for them. Nails used with joist hangers need considerable shear strength and large enough heads so that the head won’t pull through the joist hanger. Many people mistakenly use roofing nails because of the large heads. Roofing nails do not have good shear strength and may not be able to adequately transfer the loads from the joists to the beam.
- Joists should rest squarely in the bottom of the joist hangers. When joists don’t sit on the bottom, they will settle to the bottom when loaded. This situation may cause cracking or sagging above it, and it may pull out some nails in the hangers.
- Joists should extend fully into the hanger to achieve adequate support.
- Perpendicular joist hangers should not be used when joists meet beams at a 45° angle, for example. There are special joist hangers for this application.
- Special joist hangers should be used to support doubled joists.
Ledger strips and boards
Ledger strips and boards need to be well secured to beams to transfer the loads from the joists to the beam. Watch for inadequate nailing—generally, you’re looking for two nails securing the ledger board to the beam below each joist. The joist should also be toe-nailed into the beam.
Mortise and tenon joints are rare in modern construction because they are labor-intensive and not particularly strong. The mortise and tenon joints are weak because the joist end is notched both at the top and bottom. Cracking along the bottom of the tenon is common. Extra support below the joist often is required to stop movement.
Where possible, look carefully at connections on beams and joists. Look for movement, including cracking and vulnerable conditions, that may lead to problems.
Carson Dunlop - Consulting engineering firm devoted to home inspection since 1978. ASHI@HOME Training Program, www.carsondunlop.com, Carson@carsondunlop.com.