Fixing an aluminium pergola to composite decking

[renfrew999]

Hi. I have purchased a 3m x 4m aluminium pergola which I am going to fix to an already installed composite decking. The decking is fixed to a timber frame which sits on a slabbed patio.
My idea is to create 4 x concrete footings. Which will sit flush with the top of the timber frame, and bolt the legs through the composite boards and into the concrete. Firstly, does this sound like the most sensible way to do it, and secondly, is there any issues with creating the concrete footings direct onto the slabs? I was going to create a timber box and pour the concrete in and leave to set. Does this sound reasonable, or am I missing something? Any suggestions/advice would be appreciated.

 

[DAZB]

My preference would be to screw, from beneath if you can, or if not where a leg will cover the screws, a 3.6m long 6 x 2 timber beneath the decking where the pergola legs will sit and then drill through the decking and the 6 x 2 and use M10 Coach bolts a nice square washers to bolt the legs down through both deck and joist. That would spread the load over numerous deck planks and with it being composite will not flex and give like timber would so will be a very stable surface to fix to.

 

[Mr Chibs]

How large are these 'timber boxes' going to be? if the depth is only something like 150mm then think you'll need to dig down a bit. You need a bit of weight.

If it was mine I'd be wanting to make a concrete cube around 400 or 500mm square for it to sit on. Bolting through the composite is OK but use meaty coach bolts.

I recently removed a composite deck, and a lot of the screws had sheared off (just wood screws, not my work!), presumably due to the decking weight.