I need to calculate the loads on a lintel for a 2 m opening in a 10" ( brick+ cavity+ 100mm block ) ground floor gable wall through to a sunroom at the side.The opening will start about 1.2 m from the external corner of the bungalow building gable. The gable wall is a bungalow so outer wall is normal bricks triangulating from internal ceiling height to the apex of the roof. Load on the outer gable lintel will be the bricks above. Is it acceptable to use a lintel with a safe working load (SWL) of 20 kN. If I assume a column analysis for the bricks above the opening and in worst case assume 24 KN/m3 for the bricks I get 19 KN load. The inner wall is more complex. It is block but carries two purlins which support the pitched roof. I am told the roofing tiles are of a Marley concrete _ 25mm thick . There is no first floor loading other than ceiling joist load. In the worst case can I assume say 2/3 rds of the roof weight is supported through the purlins on that gable wall? Loading say 47 kg/m2 and there is area of 6 x 5 m = 30 sq m so end bearing is 470x30x0.66 = 9.3 kN for the 2 purlins. Obviously this load is shared/ dispersed to the 100mm concrete blocks of the inner wall. One of the purlins lies to one side of the intended opening while the other is almost half way above the projected 2 metre opening. I am going to assume that 50% of load from the roof is being wholly supported by the purlin lying centrally above the opening i.e. 4.6KN. Again I have the weight of the blocks which say is 24 KN/m3 so again 19KN load + 4.5KN from roof = 23.5 KN total load for an inner lintel. The design load spec for a 153mm hight lintel is below this calculated design load so I am tempted to go for a steel lintel 175mm height. Would this be acceptable? I am unsure of the offset loading factors i.e. UDL kN 19:1 Does this look ok? If it is a lot more complicated I'll get a structural engineer etc. Thank you.