Advice Needed - Another Decking Question!

Discussion in 'Landscaping and Outdoors' started by SrD123, Mar 18, 2021.

  1. SrD123

    SrD123 Member


    I am just about to start back into my garden project, composite decking is being delivered in the next few days and I would like to get the frame built. I was hoping for a bit of advice on the best way to do it.

    The plan is to raise the decking to be at a level just under the patio doors, hiding the existing patio and making it all one level. I am going to run the boards along the 6.2m length to give the feel of it extending the room when the patio doors are open.

    The person who sold me the decking(and a few others who I got quotes from) all said that they would build on top of the existing pato without lifitng the flags, one of them also said that they wouldn't attach a ledger board to the side of the house because of the difference between the render and the brick.

    I was worried about the decking being above the dpc but they all said that this wouldn't be a problem as long as I left a small gap between the 2 walls and the decking boards.

    I've been doing a bit of research but I don't know what the best approach would be. I think I have 2 options:

    attach the ledger board to the brick with extra spacers to come out in line with the render and run joists across, then concrete posts into the garden area and frame it there.

    Or make 2 beams, 1m from the wall and 1m in from the garden running the 6.2m, then sit the joists on these.

    I have a few other questions:

    what size would you use for the joists?

    The joists have to be at 300mm centres for the decking that I've bought and they recommended that I use 6x1 for joists but others said 4x2.

    If I was to use 4x2 joists across suport this with a 4x4 post in the centre of each joist would that be ok?

    If I was to put posts directly onto the flags and just put down some dpm would that be ok or should I use a post shoe and drill it into the flag? Someone recommended I put a breeze block down on top of the flag and set the post on top of that.

    Most videos I have watch have the joists attached to a ledger board with joist hangers, though I have seen some that have them actually sitting on top of the ledger board instead, which way is best?

    Many thanks

    *edit: image links wouldn't work



    Last edited: Mar 18, 2021
  2. Severntrent

    Severntrent Screwfix Select

  3. SrD123

    SrD123 Member

    Thanks for this. From the link in that thread to here: I see that someone has basically done what I am intending to do.

    I should have asked in my post about notching out the posts to hold the joists and the bearer at the garden end? I see that most people use bolts or just screw but it makes more sense to me that the post is notched and the posts is taking the weight rather than a bolt?
  4. Severntrent

    Severntrent Screwfix Select

    The post still takes the load it just means it is transferred from the decking to the post by means of a bolt. The shear strength of a 12mm bolt is a minimum of 20 tons, many times greater than what they would be required to take but each to their own. You need to re think the spacing of your posts as the proposal to put one in the centre of every joists is excessive and also does not help in reducing the deflection of your 4 x 2 (c16 or c24?) which will be spanning 2.5m. You would possibly be better placed putting a 4x2 bearer at the 1/3 and 2/3 points of your joists and supporting the bearers at 1.5m post spacings but until the layout is finalised loadings and deflections cannot be ascertained.
    chillimonster and Jord86 like this.
  5. SrD123

    SrD123 Member

    I just got a price today, 4x2 c24 4.8m is £15 and 6x2 is 20 so for the sake of £100 I think I'll just go with the 6x2.

    If I were to attach the ledger board to the brickwork is it ok to use joist hangers have have the joists sit higher than the ledger board?

    If I was to build the bearer at the 1/3 and 2/3 points what would be the best way to connect these to the existing patio flags?

    Are these the sort of bolts you are talking about:

    Thanks again
  6. Severntrent

    Severntrent Screwfix Select

  7. Severntrent

    Severntrent Screwfix Select

    Forgot about using ledger boards at the sides so thats a 3 x4 grid of posts, Also in raised part of patio instead of support posts it may be feasible to support bearers off brick, block, tile or plastic spacers, the devil is in the detail.
  8. SrD123

    SrD123 Member


    Hopefully you can maybe help me with this bit too but I might post it as a new topic if you can't.

    For the ledger board, I think i'm going to have to space it out the 40mm so that it comes out from under the render, I need it to sit a bit higher up the wall and I think it would be easier if the joist was flush wit the top of the ledger board as I'm going to now run a picture frame around the entire deck.

    Do you think it would be ok to space it 40 mm from the brick? and what would you use to space it that sort of distance?

    ledgerBoardSketch (2).jpg
  9. Severntrent

    Severntrent Screwfix Select

    As per Jords reply or revert to post support at edges, the only reservation I have is spacer block will eventually rot due to rain running down pebble dash and possible rising damp problems if you breach DPC
  10. SrD123

    SrD123 Member

    The DPC is where the brick meets the pebble dash, this was a worry that I had.

    Would using posts instead of the ledger board work? I didn't think I would be able to do that if I was resting the posts directly on the existing patio as then the deck wouldn't be anchored down to anything.
  11. Severntrent

    Severntrent Screwfix Select

    With the self weight of the decking its not going anywhere, you could always put a metal bracket at each corner of the decking tying it to the existing brickwork if you feel the need.

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