Hi i'm building a summer house with a green roof , the Green roof dead load with be 45/800 kg m2 half pitch roof 2.5m to 2m clear span 4.7m was going to us 145x45 (6x2) with 240 Center's to keep celling hight and cost down However the Span table for C16/C24 dose not go below 400 Center's so to work i out i have times the span by 2 for exmaple 600 centers span is 2079 span therefor i assume 300 centers would appx be 4158 0.75 - 1.25kN/m2 Centers 200 225 240 300 400 450 480 600 Spa 5286 4986 4828 4158 2643 2493 2414 2079 am i right any help would be appreciated
There's a massive difference between 45kg/m2 and 800! Usually round 80-100kg/m2 As for your centres, I don't know,
You are way off beam Blu. (Pun intended!!) There isn't a linear relationship span to load. The span is multiplied by itself 4 times in the calculations so a 10% span increase requires 40% extra strength in the beam. Beam depth is also non linear. What you can do from tables is use widths of timber to tweak centres. For instance 75 wide timbers at 400crs is equivalent to 2.5x75 = 187mm/m so if you used 47 wide timbers you would need 187/47 = 4/m or 250 crs. You definitely need to pin the load down though as said above
By a half pitch do you mean a roof with a ridge in it ? Because either side would have to be longer than 2.5m to have a span of 4.7m. If it is a ridge, then a lot will depend on whether you have trusses as the span tables are different. A jumbo / grab bag weighs typically 850kg and the soil should cover 4 sq m at between 150/200mm depth. Plus wet weight, roofing etc. So you could be looking at 300 kg per sq m. Your walls will obviously have to be capable of holding this weight.
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