Structural Queries Flat Roof

[Adrawk]

I'm building a rear extension that will produce a lower ground living space and a terrace, my back garden is about 1.8m below the street level.

After a couple of nudges the architect drew the inverted roof build up I requested. The engineer has detailed the roof as 50x175 C24 timbers at 400m centres and the span will be 3m.

I'm an electrician by trade so wood isn't really my thing. I've read a few details on inverted roofs and it appears 50mm thick pavers will be required to act as ballast for the insulation. A standard concrete 600x600 50mm paver is around 40kg, by my basic maths that's already 1.2kn/m2 for the pavers alone.

The engineers workings, according to the calculations are :

Flat roof -
Felt & chippings 40
Board & insulation 15
Joists & firings 10
Plasterboard & skim 15
Total 80

Dead 0.80

Live 0.75

I'm definitely not a structural engineer but that something doesn't tally up with me. The area will be a regularly used terrace with furniture on it, it'll be part of the back garden.

Any engineers here to give me a steer? Am I overthinking it?

 

[Notch7]

50x175 C24 timbers at 400m centres

That looks what You would expect for a standard floor joist loading of around 1.5 total imposed load….so prob not enough for heavy pavers

NHBC Standards 2011

 

[Adrawk]

Cheers.

I'll have to question the engineer again, it seems I'm constantly second guessing this engineer. He seems to have phoned in aspects of the design.

 

[Adrawk]

Just to give some closure to the thread. I sent the engineer the architects drawings, drew his attention to the discrepancy in the weight allowance.

He has confirmed he is happy with the original allowance.