Hi - we've demolished and rebuilt a large deck (About 8m across and 5m out although the roof eave covers about 1.3m at the house end) which extends from 1st floor living room out over the driveway. There's room to park two cars under it making it an effective carport. The original 60yo deck had galvanised metal suspended underneath it to run the water off to spouting. I think the metal was actually re-purposed from concrete flooring formwork. 

The new deck has nothing underneath it, so currently the rain (and kwila bleed) is just pouring through and I need to construct something to catch it and feed it into the stormwater drain.

My initial idea is to screw polycarbonate roofing sheets to the underside joists. The joists run out from the house, and I plan to fix battens across them to create a fall, and fix spouting to catch it bear the house end.

If I can find it I'd probably use opaque sheets as it will probably get dirty and not look great underneath if it's clear?

Will probably wait until after the whole renovation has received it's CCC before I do this to avoid complications with the council - although I will run the water into the stormwater drain the old deck used to flow into.

Has anyone done this or have any advice? How much fall should I allow? Are there any risks with capturing dampness under the deck?

Thanks in advance,

JohnO