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After some quick advice on timber size. We have an existing sun terrace/patio we wish to deck over, using it as support. What size timber would you use for the frame? All the sites Ive looked at give various from 2x4" up to 2x6" but they are all basing this on making a raised deck on the ground. As we are planning to use the existing patio for support what size should I go for? Can I get away with using smaller size timber?
Timber size is set by unsupported span.
Absolute minimum will depend on how much timber is needed to make a fixing.
Mine is built on 3x2 with 16” joist centres supported every 16” by plastic packers on top of a well laid patio. Works great.
Can't help on size but for planning purposes (assuming England?) you can only go up to 12" in height and 50% of garden under permitted development.
Also I really recommend you put in access points for rat / mouse baiting stations. We bought a house with 3 large decked areas and I would love to pull them up (can only take one up - but then need to dig out a patio that someone has decked over...)
Tanalised no matter what as it will all be subject to moisture/weather. Not sure if you can get 2x2 tan but that would be fine if well supported at 400 centres along it's legnth with as above plastic packers.
The joists will be laid on top of the patio, with slight packing to get level, so there wont be any areas un-supported. It is a raised sun terrace made of concrete with paving slabs on top
I'm not a joiner or any kind of expert in decking, but won't they rot if they're just put on top of the paving? Shirley they need raising off it.
You can buy "easy bearers" from wickes which slot together - very simple, probably 2-3 inch square.
We had these working as you are planning to do for a few years with no discernible rot when we took it up to build an extension. We had used plastic packing to raise the level a few mm as needed.
As mentioned above the point of the packers is to raise the timbers off the slabs, ptherwise they will rot very quickly tantalised or not. There’s also slim odds of getting the right fall off the deck so it can drain.
Even tannilised bearers will rot if laid directly onto paving slabs - use strips of DPC underneath and packers to get the fall. The problem with plastic bearers is the cost - they're like 3-4x the cost of wood, so unless you're expecting a 30+ year life, it's still cheaper to replace them.