A lintel is nothing more than a stone beam, and the same formulae apply to stone as to wood, with the exception of the quantity representing the strength or "modulus of rupture" of the material. The following formulae give the strength of lintels under distributed and concentrated loads, the only cases likely to occur in practice :

Distributed breaking load = 2 X breadth X square of depth / span in feet X C.

The breadth and depth should be taken in inches. C is one-eighteenth of the average modulus of rupture, and may be taken as follows:

Granite, 100; marble, 120; limestone, 83; sandstone, 70; slate, 300; bluestone flagging, 150.

These formulae give the breaking strength of the lintel. If the load on the lintel consists only of masonry, and is not subject to shocks or impact of any kind, the safe load may be taken at one-sixth of the breaking load. If there are any unfavorable circumstances the safe load should not exceed one-tenth of the breaking load.

Nearly all laminated stones are stronger, as beams, when set on edge, and where the full strength of the stone is required, they may with advantage be set in this way and be protected from the weather by placing a moulded course above set on its natural bed.

Floor beams, or any construction carrying a live or moving load, should never be supported on a stone lintel. The above formulae apply to a slab as well as to a lintel, although if the slab has a bearing on all four sides the strength will be considerably increased.

Example. - What is the safe distributed load of a granite lintel, 6 feet opening, 20 inches high and 8 inches thick ?

Answer. - Breaking strength = 2 X 8 X 202 / 6 X 100 =106,666 pounds.

One-sixth of this gives 17,777 pounds for the safe distributed load.

Example II. - What is the safe distributed load for a bluestone flag 4 feet clear span, 4 feet wide and 4 inches thick ?

Answer. - Breaking load = 2 X 48 X 42 X 150 / 4 = 57,600 pounds.

As the load on a flag would very probably be a live or moving load, we will make the safe load only one-tenth of the breaking load, or 5,760 pounds.