In flat roofs, with a slope of from 1:3 (18,5 %) do 1:10 (5,7 %), the most commonly used are claw-purlin structures. Tongue structures are not suitable for use in flat roofs, because the smaller the slope of the roof, the greater the force spreading the side walls.
A typical flat roof truss above a building with a span of approx 8 m consists of a ridge purlin supported by poles and swords, and a pair of clamps bracing the pole with two rafters.
With a larger span, the only modification is to add more purlins. In a roof with a span of approx 12 m, the truss consists of two intermediate purlins, with a span of approx 16 m, the truss has two intermediate purlins and a ridge purlin.
With very flat roofs (about the slope of the roof 1:10) The forceps may not cover the rafters, but the poles themselves.