waterproof deck/structural members

Hi,

I am in the design phase of a renovation that includes a 4m x 5m compressed fibre cement and tile deck with a habitable area beneath, (1:100 fall on the shorter span). The structural engineer has come back to me with 3 options - I have discounted one option but am tossing up between the other 2 and have some questions re: ease of building, economy etc.

Q1 Do you NEED a 30 mm reinf cement bed, or can you just tile onto waterproof membrane then to F.C sheet (james hardi had a detail for this calling it a liquid membrane) how ever most indicate a mortar bed min 25mm?

Q2 Option 1 was 250UB on the 5 m span (its picking up another member holding up the existing adjacent floor) then a 150 UC parrallel with 130 x 36 LVLs (450 crs)notched in picking 2 m up each side of the UC (bearing on exterior wall and the UB) these a layed to the correct fall!

Is notching in and making sure there all at the right fall a pain in the @@@@ for building, I do like it as i worry about flexing from the deck load maybe causing dramas in the future due to egress of water & another steel member seems solid???

Option 2 250 UB 37 only picking up 170x45 LVS at 300 crs layed at the right fall spanning the full 4m width. This option seems easier as you are laying half as many members and notching (as there not being cut in half as in one) allthough there is more obviously, due to reduced spacing. However I worry about flex in the future?

Q3 The min stock length seems to be 9m for structural members - any way around this or just suck it up there is going to be spare steel??? (note: plannig to get builder to do it so maybe they will buy bulk but doubt it??

I can supply a dwg if not explained properly.

Note: steel members are kept high as possibly causing notching so as to reduce excavation below.

Regards
exdraftee


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