Herringbone Strutting
/ ˈhɛrɪŋbəʊn ˈstrʌtɪŋ /
Also known as: cross-bracing, strutting, joist bracing, lateral restraint
Definition
Herringbone strutting is diagonal cross-bracing fixed between timber floor joists in an X-pattern to prevent them from twisting or rolling sideways under load. The pattern resembles a herring skeleton. Required by Approved Document A for joist spans over 2.5m (one row at mid-span) and over 4.5m (two rows at third-points). The alternative is solid blocking (solid noggins fitted between joists at the same depth). Both provide lateral restraint; herringbone allows services to pass through the floor zone, while solid blocking is quicker to install but obstructs the cavity.
In practice
Herringbone strutting is commonly omitted in domestic construction and in repairs to older floors - a frequent cause of springy, bouncy, or squeaky floors. When lifting floor boards to investigate a bouncy floor in an older house, absent strutting is often found. Retrofitting herringbone strutting requires access to the joist bay, but solid blocking cut to fit between the joists can be retrofitted from below (by screwing up through the sub-floor or pushing into place from an accessible under-floor space) more easily than herringbone, which requires working from above or from the void.
End blocking at the bearing walls is as important as the mid-span strutting - without it, the whole row of strutting can rock laterally without providing effective restraint. The Building Regulations span tables (Approved Document A, Section 2B) specify where strutting is required for each joist size and span. Modern engineered I-joists typically have their own manufacturer's requirements for web stiffeners and lateral restraint that differ from traditional solid timber requirements and must be followed separately.