To ensure sufficient purlin bracing in steel buildings, multiple connections are required between the eave and ridge ends. A common building technique, sag angle, or strapping in basic aligned rows, will not automatically prevent breakdown and failure of purlin braces.

A row of purlin bracing should be anchored to the strong ridge angle or a channel at the ridge. For a double-sloped rooftop, this assists with reducing the compression introduced by the accumulated energy of bracing. It is not satisfactory to just place a sag angle along the ridge.

Parallel bracing is generally attached to the eave strut in one of two manners, either by means of a direct connection or through crossing the purlin braces. It can also be accomplished through the use of sag angles between the initial purlin and the eave strut.

Purlin stability is not easily achieved by a transfer of the purlin brace with the eave strut’s bottommost flange, due to the broad difference of the torsional opposition for the eave strut. If a crossed brace is included, it can considerably improve the reliability of the given purlin.

Superior design in blocking placement is achieved by the separation of the first “Z” purlin and then the eave struts. The application of blocking can provide great resistance to turning or twisting (torsion) as well as lateral buckling.  

In the case of an exceedingly wide all-steel building, the crossing method detailed above may also have to be attached to the angle braces of interior building bays to provide additional support.

Regarding sideways purlin bracing, a common assumption is that the eave strut is stationary and therefore an excellent location for attachment. However, the eave strut will indeed have movement, and may not provide much horizontal support for either the roof sheathing or purlins. But if the siding is connected with tightly patterned fasteners, eave struts can supply substantial torsional reinforcement for the purlins. Conversely, they provide only minimal support if purlin movements force screws to loosen or the eave strut is not even connected to the wall of a structure.

Another effective bracing scheme is the use of crosswise designed steel angles separating the top flange of one purlin from the bottom flange of the adjacent purlin. Crosswise purlin braces permit each purlin to form one side of a triangle, of which the remaining two sides are comprised of the roof and the crossways brace. This bracing technique is restricted to configurations using through-fastened rooftops; it does not work for standing-seam applications. This scheme is only viable if the rooftop has the ability to oppose compressive forces and is securely attached to the purlins.

Just like parallel purlin bracing, the diagonal brace method is heavily reliant on the use of ridge channels or angles to enable it withstand the numerous strains arising from a set of rooftop inclines. The structural soundness of any steel structure is greatly improved when the correct purlin bracing method is applied.