concrete block N7, and RSJ question

B+Q sell standard concrete blocks (known as breeze blocks in UK?) and they have N7 in the description. I've a feeling the N stands for Newtons, but what does this force mean in this context?
I'm about to build a single storey outhouse using said blocks and will be building the roof to a very strong spec so it will hold the weight of soil and "green roof" plants. Will a single thickness concrete block wall hold the weight?
The roof will have 2 ridges at right angles to each other, making 4 sloping planes, one ridge joining the centre of the other to form a big T shape. Each ridge will be 4m long. Each plane will be 2.5m wide. What size timber would I need to use for the ridge beams, or would RSJs be a safer bet? I've been quoted 178 x 102 x 19kg/metre 2 @ 4.000 forming "T" with angle brackets 250.00 + VAT = about 300 for RSJ. How much would equivalent strength (if available / possible) timber cost?
The total weight of the RSJs above would be about 160kg. Would 3 concrete blocks support this weight? (one at each end of the T)
Big questions but being a layman / beginner builder I'm seeking some informed answers as I haven't been able to google any!
Many thanks, Jim
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