The rule of thumb for the depth of common joists is to take half span in feet; to this number add 2 for the depth of the joist in inches.
E.g., For A Span Of 18 Feet
Half this is 9, add 2, which gives 11 inches for the depth.
With the same quantity of timber, the deeper the joists can be made the stiffer and stronger they are. The depth can be calculated by the rules given above or those in Part IV., or obtained from the table, p. 143.
Joists should not be less than 2 inches wide, or they will be split by the nails holding the boarding, especially at the heading joints where four nails come together. In a trenailed floor (see p. 78) the joists should be wider. They should never be more than 3 inches wide if they are themselves to carry a ceiling (without the intervention of ceiling joists), as the lower surface of the joists causes a blank space behind the ends of the laths, which interrupts the key for the plastering.
Joists sometimes have a slight curve or "camber" in their length, due often to seasoning - in laying them this should be placed upwards to allow for the "sagging" or drooping which will take place after fixing - any knots should be kept uppermost i.e. in that part of the joists that will be under compression when they are loaded (see Part II.) The whole floor should be laid a little higher in the middle than at the sides of a room. This, however, is difficult to arrange.