How do I calculate roof loadings for a flat roof?

I am considering building a roof terrace on top of my kitchen extention, which is a flat roof construction, approximately 2.5M by 3.5M.





I am aware that in order to do this, I need to be able to demonstrate to the local council planning department that the construction will be strong enough to carry the additional load of people %26amp; garden furniture on the roof.





1. What is the weight that I need to allow for (presumably based on the maximum number of people that could be assumed to be on the roof at any one time)





2. How do I calculate whether the existing joists and board are strong enough?





Existing joists run across the shortest run, and are 150mm x 55mm spaced approx 35cm intervals.





Roof board is particle contruction, however as the current felt roof has deteriorated and is now leaking I am planning to replace the boards with marine ply roofing board (18mm?)





Any comments / advice appreciated.

How do I calculate roof loadings for a flat roof?
I would start by getting a building engineer, to look at your roof 1st.. They will tell you, the weight per square foot allowed for your roof. Have your plan ready for him, he/she will then inform you on your next step on your project.
Reply:Flat roofs are constructed basically with joists spanning the gap between two walls, covered by sheet timber or other material. In the case of timber a further, waterproof cover is required. The weight of these last two coverings, together with the span, form the basis of the calculation for the size of joists required. A further part of this equation is discovered in Approved document A of the Building Regulations (Structure) where the possible snow loadings, depending on area of the country and height above the ordinance datum, are taken into consideration.





It is therefore important, before building your structure that you have the desired building regulation approval. There are 8 “maximum span” tables in the regulations, which deal with the loading and span of the roof together with any slight pitch over 10 degrees that it may have to dispose of rainwater. These tables and rules are designed to protect your home
Reply:don't worry about a structural engineer ..the loading will be approximately the same as for a normal floor ..obviously the walls below must be strong enough and the joist must span full the inside coarse of block work or brickwork ..to comply with building regs the joist must measure ..half the span plus two...so if your span is 2 point 5which is 100 inches ..half is 5 plus 2 ..so you would need 7 inch joist this is with 400 centres ..if in doubt ring your local building control
Reply:You need to hire a Structural Engineer in your area.





Depending on the value of the project, most Authorities Having Jurisdiction will not grant you a permit unless you submit to them drawings that have been stamped by a Professional Engineer.





Also, there are more factors involved, other than the capacity of people, that need to be calculated when determining your roof load. Most of these factors depend on where you live because each area has different requirements. For example: if you live in California your seismic requirements are much different than if you lived in Texas. Or if you live in Florida you have wind loads from hurricanes to factor. Same thing goes for snow and rain. Also, for your existing roof, you would need to know what species of lumber was used to build it since different species have different tensile and compressive strengths.





A Structural Engineer from your area would be familiar with these requirements and can point you in the right direction. To save you some money, many Engineers will be happy to review your drawings and stamp them for a small fee. Or they can do the entire design.





Good Luck! Hope I was helpful.