3m x 5m Decking help

Discussion in 'Carpenters' Talk' started by snky1987, Apr 7, 2021.

  1. snky1987

    snky1987 New Member


    I am attaching the decking frame for the 3m x 5m "Picture framed" deck.
    - Construction is low, 20cm high and the posts will be dug 50cm into the ground
    - Joists 145mm x 47mm (3m length)
    - Posts 100mm x 100mm
    - Boards: Yellow Balau 21mm x 145mm 5m (length)

    The upper side of the frame will be attached to the house with just 2 posts in the corners to make it easier on the corners. The joists will be attached to the 5m longboards with the Simpsons Joist Hangers and the 5m deck boards will lay horizontally left to right.
    I was trying to make add a post every 1.5m (that's why they are all in the middle) with some noggins to secure the joist from twisting.
    Noggins will be screwed at 45 angle to the joists using the wood screws and the joists will be attached using the coach bolts to the posts.

    Would the attached frame be ok or should I make it more robust (and expensive)?

    Attached Files:

  2. ejenner

    ejenner Active Member

    I've done quite a few decks before and unfortunately the bounce in the boards will be very similar to what you get when building ordinary floors inside a house. I say unfortunately because it is more expensive to fit more joists. It's more time consuming and I also found out the hard way. I tested the distance between joists by stepping on a board which was supported at a distance I was happy to work with and it hardly flexed at all. So I built the deck to that specification and when the job was finished the deck was bouncy to walk on. I didn't have to redo the job as it was 'ok' and we were selling the house. But next time I did another deck I did smaller spacing.

    So advice from an experienced deck builder is to stick with the standard 16-inch centres we all know and love. There's a reason why 16-inches is what is so commonly used.
  3. CosD

    CosD Active Member

    Personally I'd be adding more posts into the ground; I'd either stager another 5 each side to the centre line adding a post to every other joist, or instead of a central line on every post at 1.5m I would have two rows instead so at 1m and next row at 2mm.

    I prefer to go over than under to avoid that bounce.
  4. ejenner

    ejenner Active Member

    Looking at your drawing more carefully it looks as though you plan to install a large number of posts. I think you could save time and cost by installing fewer posts and using some left to right joists to hold the top to bottom joists. Even if you have to dig out a bit of trench into the ground to make room for the left-right joists I think you'd save yourself some cost and trouble.
  5. snky1987

    snky1987 New Member

    Thanks for all your advice. I am planning 40cm joist spacing which is 16'' I will attach another plan that looked a bit more stable. Still with a lot of posts but one friend manages to ask the gardener working in his garden and he said that it looks OK. Of course, I would love to hear your opinions.

    I think this one looks a bit better but is it really?

    Attached Files:

  6. ejenner

    ejenner Active Member

    Across a 5m span you could probably get away with 3 posts in the middle. One directly in the centre and two spaced evenly either side.

    Most important is to not use noggins but to put a supporting beam below which runs the full length as it'll help to spread load more evenly.
  7. snky1987

    snky1987 New Member

    Thanks, it definitely makes sense but... I guess it will rot over time when placed directly on the ground?

    Edit: I can even buy 2 or 3 x 5m beams to support the construction from the bottom but I am wondering if it will stay healthy for years. Or should I put it on the posts and have it approx 5cm above the ground
  8. Severntrent

    Severntrent Screwfix Select

    When you consider that if that was a house floor with those joist size and spacings they would be capable of spanning over 3m. Your decking is so OTT you could park a tank on it.Forget the noggins and as stated above and put a beam below the joists if you are going with posts in the middle area
  9. ejenner

    ejenner Active Member

    You would dig a trench for it with plenty of room for water to drain away. Also waterproofing of some kind should be applied. The timber you buy will most likely be treated additionally so it's not going to deteriorate any faster than the deck top or the posts you're going to put into the ground.

    Also, you have to do something to prevent weeds growing up through the deck. So you have to plan for some kind of treatment for the ground you're decking over. It's not good enough to leave bare soil under there as you'll have trees growing through your deck sooner or later!
  10. snky1987

    snky1987 New Member


    - Where would you consider the beam to be put?
    - Shall I get rid of all the noggins when I put the beam underneath?
    - Should I get rid of some of these posts?

    Simple paint over would be appreciated and I will adjust the drawing in the afternoon :)

    The total cost of the frame in the picture is around £500 according to https://www.estatesawmills.com/decking-calculator/ which is a company in my area. But if I could remove some noggins and few posts it could get to £400 easily which could be nice :) To be honest this is going to be a coffee area holding usually between 2-6 adults. I will have another, larger deck for BBQ and potentially heavier load.
  11. ejenner

    ejenner Active Member

    yeah, you could delete all the noggins. Have x2 left-right rails. The rails should be held up from the bottom by posts in the ground and you'll have posts in the corners and at the edges of the deck.

    For ground preparation / weed blocking I used black plastic on one deck. On another I used the rolls of weed blocking fabric which you can get from the garden centre. On another I used nothing but it had a roof so there was no way for sunlight or rain water to feed plants so took a risk with that one.
  12. Severntrent

    Severntrent Screwfix Select

    One central rail with posts as per your drawing and no noggins
  13. Hager the Horrible

    Hager the Horrible Active Member

    As above you have more posts than you need and more noggins. You are using hardwood boards too which will reduce the bounce significantly. Remember you will need the special screws for those boards and a step drill bit.

    The yellow balau boards are well worth the extra money compared to the softwood offerings. No need to treat them at all and they dry out within half an hour even after a week of rain!
  14. snky1987

    snky1987 New Member

    Thanks. I will upload a more detailed but also modified frame plan. I would appreciate your (and all other people here) opinion about it.

    Is it still too many posts on this updated plan?
    I know someone said 1 posts row strengthen by side to side double bar and I made 2 rows of those posts with double bars. I wonder if it's still overkilling or safe enough?
    I did the full quote with Estate Sawmills and the sum of materials is £590 which is not too bad, to be honest.

    Joist 150x47x2.4m x2 = £9.05 x2
    Joist 150x47x3.0m x2 = £11.30 x2
    Joist 100x74x4.8m x4 = £12.05 x4
    Joist 150x47x3.0m x16 = £11.30 x16

    Post 100x100x0.9m x17 = £4.72x17
    Postmix 20kg x17 = £4.79x17

    30x Coach Screw m10x180 = £0.79 x30
    10x Coach Screw m10x150 = £0.59 x10
    20x Coach Screw m10x200 = £1.06 x20
    16x Coach Screw m10x300 = £1.65 x16
    Washers M10 x 50 = £0.08 x50

    Simpson Joist Hangers 50mm x28 = £1.14 x28
    Groundtex Geotextile Landscape Fabric x1 = £13.80
    Decking Screws 60mm x600 = £7.92 x3

    Attached Files:

  15. Hager the Horrible

    Hager the Horrible Active Member

    I think your plan will produce a deck stiffer than a teenager with the entire Razzle back catalogue. If you are buying yellow balau hardwood boards you want Spax T25 5.0 x 60 mm decking screws and if installing smooth up boards get the Spax step drill for the pilot and countersink...




    Also worth getting a good quality TX25 bit and/or a few spares...

  16. ejenner

    ejenner Active Member

    Stainless steel fasteners are well worth the extra dosh. Depends on whether or not you'll ever have to get the boards up. Any drains or utilities under the proposed decking area?
  17. Severntrent

    Severntrent Screwfix Select

    If you can be bothered put decking in the search engine and you'll get a host of OPs on decking with with various layouts and advice. Personally I think that unless you have running sand as your foundation the number of posts, the sizes and depths will support a 2 storey house
  18. ejenner

    ejenner Active Member

    The problem is flexibility rather than overall strength. If you make the gaps too big you'll get bouncing and flexing. If you're trying to put a table with drinks on it in the middle and someone comes bounding out the backdoor of the house all the drinks spill if you've not got it right. Far better to be over rather than under in this context.
  19. Severntrent

    Severntrent Screwfix Select

    As m
    As long as the spans conform to TRADA tables deflection of the joists will not be a problem, Actual overall stability of the structure will depend on rigidity of the joints and posts.
  20. ejenner

    ejenner Active Member

    I've done a couple in houses where I've lived with welded steel frames. 50x50 box section. The most recent one I screwed the decking timber directly to the steel joists with self-drilling screws. No timber framework at all. Was an awesome experience. It think it's going to be there for a long long time!

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