Flitch Beam Span Table

How far a flitch beam (2x lumber bolted to a steel plate) can span, for 2x8, 2x10 and 2x12 lumber with 1/4, 3/8 and 1/2 inch plates and loads from 200 to 1,000 pounds per foot. Find your beam and load for its longest span, or use the size chart to pick a beam for a span.

The spans are for Douglas Fir-Larch No. 2 lumber and ASTM A36 plate, checked for bending, shear and deflection by allowable stress design (ASD) under the National Design Specification (NDS) for wood. The flitch beam design guide walks through the same checks in the free beam designer.

Flitch beam span table

Longest simple span for each beam and uniform load. The plain lumber rows show what the plate adds.

• Loads are service (unfactored) loads, not counting the beam's own weight, which the checks add. 70% of the load is taken as live load.
• Each plate is 1/4 in shallower than the lumber (7, 9 or 11 in), so it doesn't stand proud of the wood as the wood dries and shrinks. Two-ply beams have one plate between the plies; three-ply beams have a plate between each pair.
• Live load deflection is limited to span/360 and total load deflection to span/240. The beam is simply supported, braced along its top edge by the joists, and the plies and plates are bolted so they bend together.

Flitch beam size chart

The smallest flitch beam from the table above for each span and load: the shallowest lumber, then the fewest plies, then the thinnest plate.

For example, a 14 ft beam under floor joists that span 10 ft on each side holds up 10 ft of floor. At 55 psf (15 dead and 40 live) that's 550 lb/ft, so use the 600 lb/ft column: a 3-2x8 + 2 × 1/2 in plates. Where the chart shows a dash, a steel beam or an engineered wood beam is the better choice.

How a flitch beam is checked

Steel is about 18 times stiffer than Douglas Fir-Larch No. 2 (E = 29,000,000 psi against 1,600,000 psi). Bolted together, the wood and plate bend to the same curve, so the plate takes 18 times the stress of the wood beside it. The usual way to check one is to turn the plate into the wood that would do the same job: multiply its moment of inertia by n = E_steel / E_wood = 18.13 and add it to the wood's.

For a 2-2x10 with a 3/8 in × 9 in plate:

The plate more than triples the beam's bending strength and stiffness. Under 500 lb/ft plus the beam's own 18 lb/ft, bending limits the span to √(8 M / w) = 12′ 11″, which is the 12′ 11″ in the table. Deflection allows a longer span, so bending governs.

Flitch beam questions

How far can a flitch beam span?

Under a light 300 lb/ft load, a 2-2x8 with a 1/2 in plate spans 14′ 10″ and a 3-2x12 with two 1/2 in plates spans 27′ 10″. Past about 20 ft, a steel beam or a glulam or LVL beam is usually lighter and cheaper.

How thick should the steel plate be?

3/8 in and 1/2 in are the most common. A thicker plate adds strength in proportion to its thickness, but the beam gets heavy fast: a 1/2 in × 11 in plate weighs about 19 lb per foot, more than the lumber around it.

How are flitch beams bolted?

With through-bolts, often 1/2 in or 5/8 in, staggered top and bottom along the beam and doubled near each end, where the load passes between the wood and the plate. The bolts have to carry that load, so their size and spacing should be checked for each beam; this table doesn't cover them.

Why use a flitch beam instead of a steel beam or LVL?

A flitch beam is the same width as the wall and framing around it, takes nails along its sides, and can be carried in by hand in pieces. It's also a common way to strengthen an existing beam in place. For new work with long spans, compare it with a steel beam from the steel beam sizes list.

To check a flitch beam with your own spans, supports and loads, use the free beam designer and follow the flitch beam design guide.