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The Forces in Roofs Simon Course, a Graduate Member writing from Salisbury, has explained why calculations for tie forces are often wrongly derived: Consider a simple roof construction, as shown in Fig 1, with all three joints pinned and a rafter UDL. When checking submissions for Building Regulation approval, it seems that many engineers derive the tie force (and hence the load for the rafter/tie connection design) by using the expression T=R/tanè. As tanè=2H/L, this gives T=RL/2H. This is in fact incorrect.