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Institution Forums > General > Roof Load calculation with purlin View modes: 
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Site User - 02/01/2018 14:36:50
Roof Load calculation with purlin
I’m doing structural calcs for a lintel over a significantly enlarged window opening, and would appreciate any comments on the way I plan the calculations.  The calculations will be given to the BCO for a final check before starting the work.

My diagram here:
The upper floor window is in a room that has a partially sloped ceiling (ie the rafters form a 45deg section between the wall and ceiling of the room).  I plan to calculate the load and then use a pre-designed lintel that can take that load (rather than doing the lintel beam calcs myself). 
The rafters are supported by a purlin whose lower surface is coincident with the top of the joists forming the attic floor / room ceiling.  My uncertainty is over what proportion of the roof load will reach the leaves of the cavity wall.  My thought is that the purlin (assuming it is in good condition) will take the load from all the roof lying above it, and I only need consider the short length between the purlin and the eaves.  Also, if that is OK, whether to consider the roof load acting vertically.  My calculations are below, any comments would be appreciated.  (I copied the DL and LL values from another example quoting “BS6399-1:1996 Loading for buildings”, hopefully they are consistent with current standards)
Roof Dead Load DL   DL = 0.85 kN/m2 (?: clay tiles)
Roof Live Load LL     LL = 0.85 kN/m2
(Roof pitch              PR = 42.0 Deg)
Lintel Span              LS = 3 m
Length, purlin to eaves LPE  LPE=1.5  m
Load on lintel:  = LPE * LS * 0.5 * (DL + LL)   ( + weight of bricks above lintel)
(factor of 0.5 because load distributed between purlin and cavity wall)
                                = 1.5 * 3 * 0.5 * (0.85+0.85)
                                = 3.825 kN
From this it seems as if a standard cavity lintel L=3300 (specified to should do the job:

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