Laying decking over an old patio

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We're now planning to lay decking over an old patio, but don't have much height allowance. The old patio is solid pavers, on an old solid base that will be a pain to remove (me and my partner want to do it all ourselves).

what options do i have please on laying decking over slabs?
 
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What about the joists sitting in puddles of water when it rains (over time)? Even treated timber
And whats the smallest height of timber joist i could comfortably use as I am restricted on overall height
 
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I wouldn't use a big sheet of dpm, the water would just sit on top of it, could mkke matters worse.

I would cut up some old car tyres, 4" squares of tread. Use these as packing between the old patio and the joists (nail them to the underside of the joists to stop them moving). The rubber and that natural curve of the tread should also help to reduce any knocking noise when you walk on the new decking.

You will need to use joists under the decking, decking boards need to be screwed down to stop cupping and warping. You may get away with using a 3x2 on its side?
 
Thanks guys

Munroist good idea with the car tyres. Also, you say I could possibly get away with a 3x2” frame on its side, do you mean the 3” sides in contact with the old pavers and the underside of the deck boards?

Am thinking of lifting the old pavers to get an extra 30-35mm allowance, but then the solid base I’d have for my frame to sit on wouldn’t be as level
 
Thanks guys

Munroist good idea with the car tyres. Also, you say I could possibly get away with a 3x2” frame on its side, do you mean the 3” sides in contact with the old pavers and the underside of the deck boards?

Am thinking of lifting the old pavers to get an extra 30-35mm allowance, but then the solid base I’d have for my frame to sit on wouldn’t be as level

I still think the whole thing should be anchored down to posts set in the ground, as a free floating raft I would be concerned that the whole thing could warp out of shape. We tried something simillar at home, one side was attached to posts and the other was to be free floating resting on the driveway, I eventually had to secure part of that down to combat warping..
 
It's up to you of course, bear in mind that every (and I mean every) single deck on a commercial building or block of flats say with external decking has the decking just sat there under its own weight, nothing is allowed to penetrate the waterproofing layer so it has to just sit there, your deck will be fine and will settle down with no bounce. It's either fixed all over or not at all. Been there got the T-shirt.
 
Thanks guys, one part of the decking that will sit directly onto the old patio is 25m2. Was thinking of using composite joists on here, as I could lay them straight down on the pavers (the joists are 30mm x 50mm dimension, id use the 30mm height as very restricted)

How do you join 2 x joists (they are 3m in length) to cover a 5m run? And also, how would you make a 'T' join? (at 90 degrees to one another)
 

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