Timber roof structures built up by several trusses designed with structural timber members and jointed together by punched metal plate fasteners are considerably effective structures for long spans. They are considered cost-effective and can serve their purpose well during the lifespan of a roof. Lately, the limits of how long spans they can withstand for a normal loading situation have been pushed. Failures that often relate to lateral instability in the top chord or in other compressed members of the truss have been reported. The effect of various parameters on the load carrying capacity of the top chord is studied. Analytical solutions based on the method available in Eurocode 5 and a method based on the assumption of an elastic foundation is used in combination with finite element simulations. Results show that, in a Nordic context, the approach suggested in Eurocode is problematic due to the suggested bay length which is supposed to develop between lateral supports. Simulation results also suggests that load-displacement curves based on experiments performed on the connections involved should be used in simulations of the top chord rather than an elastic spring.