lateral restraint of roof purlin

huangyhg

lateral restraint of roof purlin
consider a pitched roof on a house.
the tiles rest on battens which rest on timber rafters.
the rafters span from eaves to apex and are supported by steel purlins at mid span. the rafters are bird mouthed over the purlin and "may" be nailed to a timber runner bolted to the top flange of the purlin.
the purlins are supported on the top course of blockwork walls rising from the floor below.  the amount of building-in is negligible.
question: reference clauses 4.2.2 and 4.3.3 in bs5950 can we consider the purlins as having "full lateral and torsional restraint" ?
i consider them to be unrestrained and to have an effective length = 1.2l + 2d  is this too conservative ?
i also know of many circumstances where the effective length has been taken as l.  these structures have not experienced any problems that i am aware of.  is this just good fortune ?