How to build composite deck over concrete patio?

[aaronsnoswell]

Hi folks! First time posting.

I'm preparing to DIY a small, low deck to extend an existing concrete patio at my house. I would like to use composite decking, and I would like to remove the existing patio tiles (already underway), and extend the composite decking boards over the existing concrete once the tiles are removed.

Where the patio joins the house door, there is 30mm vertical clearance from the cement patio slab to the top of the door lintel (see photo). I understand the top of the decking needs to be lower than this to prevent water draining into the house. Most composite decking I can find seems to be about 25mm thick boards, which means I haven't got much extra vertical 'wiggle room' on the cement patio.

Can I fix my composite decking directly onto the cement patio slab? All the information I can find online says you absolutely must use some kind of raiser or joist beneath decking (but I don't have the vertical space!). E.g. ModDeck has a guide that says minimum 50mm above cement. Another page I found says you need a wood base frame so you can get a level surface and for drainage, and to make it easier to secure the decking. A Victorian company has a guide that says 40mm minimum if you have 'adequate drainage'', or 90mm otherwise. Ekodeck says the same.  Why is this? It seems to be to prevent water pooling and mold growing on the cement / underside of the decking?

Can anyone suggest a possible way forward here? Could I raise the decking boards on 5mm window packers regularly spaced to create some room for ventilation and drainage? Or is there some kind of ultra-thin joist or raiser or shim product out there that would let me elevate the decking off the cement by 5mm?

Or could I use some more water resistant decking product like Moistureshield? Or maybe cut drainage grooves into the cement to aid runoff?

Another consideration - I was planning to have the decking run parallel to the house edge, but will this inhibit water runoff? Should I instead put the decking running perpendicular to the house edge?

Sorry for all the Q's! Hope they all make sense, and thank you in advance!

[JacobZ]

Hi @aaronsnoswell,

A warm welcome to the Bunnings Workshop community and thank you for your question.

Unfortunately, no composite decking producer would offer any kind of warranty or recommendations for installing decking directly to concrete as it creates an environment where mould and rot will take hold and cause premature failure of the decking. 

Regardless of your efforts to increase drainage, there is not enough space here for composite decking to be a viable option. Unfortunately, 5mm is not enough clearance.

The only way I can see where composite decking will become a viable option is to remove the concrete patio, so you have enough room to install a timber frame.

Allow me to tag @Dave-1, @Noyade and @Nailbag for their thoughts.

Jacob