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Composite decking joists

We're looking to build some decking, and given the location (north side of a building, Welsh borders), it's a fair bet that even thoroughly-treated timber would not have the longest of lives...

Composite decking boards are widely available from a whole stack of suppliers, and aren't hideously expensive. The sub-structure, otoh. Ouch. I've only found a couple of suppliers (Envirobuild Manticore, Millboard PlasPro), and the prices... <wince> 5x timber...? And I'm looking at a decent size - 5m x 6m, or so.

There's also steel and ally structures, but I'm a little more sceptical. They seem more suited to laying onto a concrete base with adjustable height pads. This one's going to be going over earth, and I was planning on a more "trad" structure - augered holes, then metposts into postcrete, before building up with posts.

Anybody done anything similar, got any thoughts or suggestions?
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Comments

  • Mickey666
    Mickey666 Posts: 2,834 Forumite
    1,000 Posts Photogenic First Anniversary Name Dropper
    Does it really have to be decking? 
    What about clay pavers (not those concrete ones used in filling stations and the like)?  I used them for a 'terrace' area of 105m2, which cost me about £8500 in total including 20 tons of MOT sub-base and 15 tons of sand, so about £2500 for 30m2 pro rata.  Apart from a quick pressure wash every 2-3 years its maintenance-free and should last forever . . . well, longer than me anyway.

  • AdrianC
    AdrianC Posts: 42,189 Forumite
    Eighth Anniversary 10,000 Posts Name Dropper
    "Have to"? No.

    But the amount of work involved in any paving would be substantial - it's a very lumpity-bumpity surface, and it's currently a chunk lower than we'd want the finished level.

    There's also a 700 x 1200 brick soakaway that we want to hide but retain access to...
  • weeg
    weeg Posts: 1,068 Forumite
    Part of the Furniture 1,000 Posts Name Dropper
    Use timber, paint it with bitumen all over. Should be good for 20 odd years.
  • weeg
    weeg Posts: 1,068 Forumite
    Part of the Furniture 1,000 Posts Name Dropper
    Mickey666 said:
    Yes, and good luck walking on it during hot weather.  A friend landscaped their garden with old railway sleepers that had been pained with some sort of bituminous/tar-like paint than went all runny and sticky when the weather was hot.
    Why would you be walking on  the joists?
  • Moss5
    Moss5 Posts: 371 Forumite
    Part of the Furniture 100 Posts
    https://millboard.co.uk/
    I'm late to the party, but I used this product and was happy to pay a premium to do it right and do it once.
    They do adjustable joist bearers, plastic joists, plastic decking and quality screws. Not cheap.

  • Moss5 said:
    https://millboard.co.uk/
    I'm late to the party, but I used this product and was happy to pay a premium to do it right and do it once.
    They do adjustable joist bearers, plastic joists, plastic decking and quality screws. Not cheap.

    Wow! That's nice! It might just persuade my bro to drop his hardwood option and go for this (depending on how much it costs...)
  • AdrianC said:
    We're looking to build some decking, and given the location (north side of a building, Welsh borders), it's a fair bet that even thoroughly-treated timber would not have the longest of lives...

    Composite decking boards are widely available from a whole stack of suppliers, and aren't hideously expensive. The sub-structure, otoh. Ouch. I've only found a couple of suppliers (Envirobuild Manticore, Millboard PlasPro), and the prices... <wince> 5x timber...? And I'm looking at a decent size - 5m x 6m, or so.

    There's also steel and ally structures, but I'm a little more sceptical. They seem more suited to laying onto a concrete base with adjustable height pads. This one's going to be going over earth, and I was planning on a more "trad" structure - augered holes, then metposts into postcrete, before building up with posts.

    Anybody done anything similar, got any thoughts or suggestions?
    Hi Adrian. Could I possibly be of some help? I hope so - after everything you have done/are doing for this forum.

    I'm also about to lay some decking and have settled on composite boards (Duradeck 'cos they have a 295mm wide boards which helps to reduce cost as well as giving a nice flat surface) over ali joists ('cos they are the strongest, allowing 1.2m spans and will last forever) sitting on adjustable pedestals (because it's easy and I'm lazy and Nan is paying for it all). Ok, ours is going down on a purpose built raised concrete patio area, so as easy as it gets.

    What would I do in your place? I think I would auger out bore holes (they don't have to be wide - 4 or 6") and then fill them up with concrete and any rubble to hand. Make shuttering boxes to sit over these holes to bring the height up to roughly what you will need for stands/pedestals/spacers, and then fill them up. You'll end up with lots of cast concrete posts sticking out of the ground at - hopefully - roughly the same height (you can be as precise as you want). If it's cheaper/easier, plonk in some concrete and short pre-made concrete stub posts - tap them down until they are at the required height.

    How high above this ground will your decking lie? (Oh, and don't forget weed fabric...)

    These posts can then take adjustable pedestals (they 'only' cost around £1.50 each - tho' you'll need lots), or else a dob of mortar that you lay and level your ali joists in (if you got your posts fairly level and at the required height in the first place...).

    Bottom line for me - I wouldn't use timber joists because I've had timber decking/pergolas/porches start to rot after only 5 years, and that is with 'treated' timber that's not even in ground contact; current 'treated' softwood is utter carp. I cannot bear all the bludy hours I've put in to building these things, only to watch them start to rot apart in next to no time :-(

    I'd consider composite joists, but they are not far off the price of ali and aren't as strong - so more supports needed. 

    Yes, ali is costly, but just think of the overall savings of time and money over decades. I'm going 'Duradeck' or BuildForLess for my 72mm ali joists because I haven't seen them much cheaper elsewhere, and Jouplast for the risers as it seems a good price: https://www.manomano.co.uk/p/adjustable-pedestal-40-65-mm-for-wooden-deck-jouplast-1512014?model_id=12813960






  • AdrianC
    AdrianC Posts: 42,189 Forumite
    Eighth Anniversary 10,000 Posts Name Dropper
    What would I do in your place? I think I would auger out bore holes (they don't have to be wide - 4 or 6") and then fill them up with concrete and any rubble to hand. Make shuttering boxes to sit over these holes to bring the height up to roughly what you will need for stands/pedestals/spacers, and then fill them up. You'll end up with lots of cast concrete posts sticking out of the ground at - hopefully - roughly the same height (you can be as precise as you want). If it's cheaper/easier, plonk in some concrete and short pre-made concrete stub posts - tap them down until they are at the required height.

    How high above this ground will your decking lie? (Oh, and don't forget weed fabric...)
    ...
    I'd consider composite joists, but they are not far off the price of ali and aren't as strong - so more supports needed. 
    Cheers, Jeepers.

    I'd seen the tinware frames, and sort-of-discounted them without even looking at the numbers. But if they're cheaper... Yep, it might well be a worthwhile approach.

    It's not going to be high above ground - it's not very level, but I s'pose somewhere around ~400ish above, on average.
  • Thinking about your starting point - the ground - two basic approaches come to mind. One is to dig/auger some bore-holes and fix a post or metapost in there using concrete or expanding foam, and the other is to go shallow but wide.

    (Have you consider expanding foam instead of concrete? Since these posts are simply designed to take a direct vertical load but not hold up a fence or anything like that, I bet foam will do the job to keep it in place. You'd need a non-weathering type, tho'. Sika actually make 'bags' of foam especially for fixing posts - only £35 per bag (one post's worth...))

    Is your ground fairly solid? Does it become marshy at all with heavy rain? If not, you could consider removing just the turf layer - 2 to 3 inches - in neat circles, and then casting concrete pads in situ. Eg. make up a ply collar of, say, 8" diameter and ~5" high and fixed with screws for easy removal. Plonk it on the ground and cut into the turf around it. Remove that bit of turf and tap the collar down until only a couple of inches sits above ground. Fill it with concrete. When set enough, undo the screws to loosen it and remove.

    You then have a choice of what to use for your support posts. Recycled plastic posts work out at about £9/m for 3x3s if bought in 3m lengths, so you are talking about £4-odd for each upright if you are after ~400mm heights. Now that you are off the ground using fully non-rotting materials, I guess you could risk going timber joists for the horizontal spans if you wish. Bear in mind (teaching egg-sucking, I know) that each cut will expose untreated ends, so get a large container - 25L - of Everbuild Lumberjack wood treatment - nice stuff to use as non-solvent - and soak them. Ditto for the actual decking board cuts. 

    Once the deck has weathered a bit and is ready for a fresh oiling, choose a good dry warm spell and fill a garden sprayer with Lumberjack and soak the whole bludy lot, allowing spray to go down between the boards and land on the joists below. Soak all screw heads. Soak all board ends. Soak the bludy lot.

    Let dry and then oil away. You'll probably end up oiling every year(?), but I think also worth Lumberjacking every, say, 5, and I reckon you should be ok. 
  • Doozergirl
    Doozergirl Posts: 34,021 Forumite
    Part of the Furniture 10,000 Posts Name Dropper Photogenic
    Don't forget your planning permission if you're above 300mm high...
    Everything that is supposed to be in heaven is already here on earth.
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