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Thread: How can I find out/work out what bolts I need for my decking?

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    Question How can I find out/work out what bolts I need for my decking?

    Given that I have a large suspended decking (only about 600mm above ground), which will be constructed using 6x6 posts with 3x8 main joists (4.4m) supporting 2x8 cross joists (4.2m), and where the cross joists are resting on top of the main joists, how do I know what bolts to use to attach the 3x8 joists to the 6x6 posts?

    One additional question, if it's simple. Would 2x12 main joists be better than 3x8? Both should be sufficient strength, but it seems 2x12 is that bit stronger.


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    Re: How can I find out/work out what bolts I need for my decking?

    Go into B&Q, find some massive bolts and fire them through. Make sure they're hot-dipped galvanised though, otherwise you'll end up with rust and other problems, also ensure that any washers and nuts are also galvanised.

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    Re: How can I find out/work out what bolts I need for my decking?

    Quote Originally Posted by DaKid View Post
    Given that I have a large suspended decking (only about 600mm above ground)
    Not sure about the bolts, but if decking is more than 30cm off the ground you technically need planning permission for it, which a lot of people don't realise. https://www.planningportal.co.uk/inf...cts/11/decking
    Just something to bear in mind if you've not thought of it already!

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    Re: How can I find out/work out what bolts I need for my decking?

    Quote Originally Posted by Bagnaj97 View Post
    Not sure about the bolts, but if decking is more than 30cm off the ground you technically need planning permission for it, which a lot of people don't realise. https://www.planningportal.co.uk/inf...cts/11/decking
    Just something to bear in mind if you've not thought of it already!
    I wasn't aware, thank you. Seems a little ridiculous though.

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    Re: How can I find out/work out what bolts I need for my decking?

    Quote Originally Posted by DaKid View Post
    how do I know what bolts to use to attach the 3x8 joists to the 6x6 posts?
    i'm not sure if you intend to use bolts with nuts on, or coach screws which are like screws but with a bold head that you tighten with spanners or sockets. I'd go with those, so that you only have one side to hold. Nut and bolt needs both sides holding and that's tricky in some places.

    I'm no builder. I'd use minimum 10mm diameter and you likely need 70mm long, but you need to measure the wood and make sure they go deep but not out the back. Drill the holes for them a bit smaller and get your ratchet socket set out... they can then come out long after you're put them in if you need them too, as you cant burr over the head if you use a good socket set. The reason I'd use those and not a bolt through, it also that they grip the wood, where a bolt can't grip, it can only squeeze, needing washers etc

    Like thse
    https://www.screwfix.com/p/easydrive...-10-pack/9147t

    but you choose size please.

    Quote Originally Posted by Advice Trinity by Knoxville
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    Re: How can I find out/work out what bolts I need for my decking?

    Quote Originally Posted by DaKid View Post
    I wasn't aware, thank you. Seems a little ridiculous though.
    in one way yes, but in another way no. Various reasons, from overlooking and privacy concerns, but also drops more than 0.5m need handrails/balustrading to BS6180 and proper access designs.

    My 10p worth:

    NB for the sizes specified the grade of timber should be min C24 (not C16) to comply with BS5268-2 or BS EN 1991 for timber structures in external environments under standard domestic loading. NB most basic construction timber is only C16. The cost difference is not that great for smaller size projects like this - don't skimp. Get C24.

    200x50 joists in C24 timber are at capacity at 400c/c for 4.4m span - so space them at 300-400c/c max.

    Swapping the primary timbers - beware lateral buckling. 3x8 (75x200mm) has much lower buckling slenderness vs 2x12 (50x300). The bending capacity of the latter can be higher but only if you adequately restrain it against Lateral Torsional Buckling. Which the adjoining joists could be used to achieve - but the current detail could be greatly improved in that regard:

    To both lower the decking height + resist buckling consider lowering the secondary joists to the same level as the primary timbers. Affix 75x50 battens to the side of the primary timber beams with regular coach screws / nails and then notch the joist ends so they sit onto the bearers, and tight against to the beam itself. Then skew-nail the joist to the beam for good measure. Provided you leave approx 2/3 of the joist depth intact the shear capacity should still be sufficient. NB I've not run calcs here, but I'm quietly confident. Add blocking noggins between the joists at each end and at 1/3 span points to prevent rotation/racking under loading (and they will)

    If you want to use 300x50 2No timbers back-back to make a 300x100 net beam - you can, but my gut feel is the numbers would show that to be overkill. NB to bolt at max 600c/c to ensure they act as one member. Min M12 bolts for that.

    Bolts - either use sheradized, Coteguard or hot-dip galv (and the hot-dip bit is important). min 50mm square washers recommended to prevent pull-through/local crushing at the bolt site. You can also use stainless steel. Grade A2-70 or A4-70 should see you right enough.

    For clarity black bolts, bright zinc plated (BZP), general chrome passivated (CP), general brass and the like are not recommended. Some marine brass grades can be ok externally, but most readily available brass fixings are lesser grades, and finding out what kind is not always straightforward. I'd stick with steel fixings.

    NB to ensure the timbers are fully pressure treated (tanalised) and all cut ends coated with min 2 coats of ensele end-grain protector prior to staining/coating with your chosen finish.

    You haven't put a height on the image/description that I can find but you should check the planning requirements for permissible heights (and where the thresholds lie in terms off triggering the need for planning applications - including overall height, % of garden area affected, proximity to neighbouring buildings/boundaries) etc. You should also check with your local authority building control which regulations they would ask for with garden deckings - so you know which part of the building regs to allow for.

    Lastly two things I would also encourage you to think about in the design:

    1) Overall stability - the lid looks a bit unstable to me, not all directions appear adequately restrained/braced against collapse.
    2) Foundations, and how the frame connects to them. There's a good hood for catching the wind and taking off going on there. So the foundations need to work in the upward direction too. Something to think about...

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    Re: How can I find out/work out what bolts I need for my decking?

    and in answer to your original question:

    the maths goes like this:

    area * load/ m2 = force required
    then check against the bolt:timber joint capacity - which is tabulated in BS5268-2 (to save you the 9 individual calcs required in BS EN 1995)

    so (4.4/2*4.28/2) * 1.5kN/m2 = 4.7m2 * 1.5 = 7.1kN SLS load. That's on the high side for timber.
    to save you a lot of misery trying to find non-available documents:

    table 71 of BS5268-2 and the revelant modification factors suggest that you need several screws - probably too many to be practical, (so I gave up trying to look at whether the spacings would work). I imagine, if you were to go into details that a better answer would be along the lines of 2No steel bolts M16 diameter per post. And positioned on the post centreline spaced min 80mm apart vertically, and the top bolt set at least, IDK, 75mm below the top edge of the horiztonal beam (this being the heavily loaded edge) in that connection.

    You could also benefit from putting in midspan stub-posts directly beneath the primary beams to reduce the span and forces at the front and back post connections. You could even add midspan props to the joists too for similar gains (and a less bouncy deck)

    Edit: NB this suggestion is for BOLTS not coach screws. You can recess the nuts and get driven heads if required to achieve a better visual at the end.

    But anyway - if you really feel out of your depth just get an engineer to do some checks. It will be quick work for someone practising and relevant experience in the field, and a modest time basis charge will prob only be a few hours (depending on how much you want them to look at)
    Last edited by ik9000; 25-07-2018 at 05:37 PM.

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    Re: How can I find out/work out what bolts I need for my decking?

    DaKid, I think you owe this man a beer ^

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    Re: How can I find out/work out what bolts I need for my decking?

    Quote Originally Posted by Hoonigan View Post
    DaKid, I think you owe this man a beer ^
    You are so right! Ok ... will go get a nice cup of tea, so I can set about replying to the man. I may be some time ...

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    Re: How can I find out/work out what bolts I need for my decking?

    not got a clue about what bolts, but from seeing next doors decking and zip-line play house structure, I do have a a little bit of advice.

    weather treat the wood before sticking it together. so you get all the sides of the wood and the faces that are going to be next to each other.
    where you drill the hole for the bolts, treat inside the drilled hole as well or water will get down the hole and rot the wood from inside.
    wooden posts down into the ground/soil is not a good idea.

    I would also go and tell the neighbours what you are planning. especially the one that's the other side of the fence in the diagram. Depending on which way the sun shines, they might end up with a large decking shaped area of dead grass. you might aswell.

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    Re: How can I find out/work out what bolts I need for my decking?

    Quote Originally Posted by ik9000
    don't skimp. Get C24
    This makes perfect sense, and my local timber merchants always coughs up the good C24 stuff for me, so this shouldn't be a problem!

    Quote Originally Posted by ik9000
    To both lower the decking height + resist buckling consider lowering the secondary joists to the same level as the primary timbers. Affix 75x50 battens to the side of the primary timber beams with regular coach screws / nails and then notch the joist ends so they sit onto the bearers, and tight against to the beam itself. Then skew-nail the joist to the beam for good measure. Provided you leave approx 2/3 of the joist depth intact the shear capacity should still be sufficient. NB I've not run calcs here, but I'm quietly confident. Add blocking noggins between the joists at each end and at 1/3 span points to prevent rotation/racking under loading (and they will)
    I'd come to a similar conclusion about getting them at the same level, although I'd been thinking about just using joist hangers to put the secondary joists in between the primary ones, and to swap the primary beams to the outside of the posts in order to allow the decking to have a complete enough surface to be laid on. Do you see any problems with doing that??

    Quote Originally Posted by ik9000
    <lots of stuff about bolts and timber>
    Thank you!

    Quote Originally Posted by ik9000
    You haven't put a height on the image/description that I can find but you should check the planning requirements for permissible heights (and where the thresholds lie in terms off triggering the need for planning applications - including overall height, % of garden area affected, proximity to neighbouring buildings/boundaries) etc. You should also check with your local authority building control which regulations they would ask for with garden deckings - so you know which part of the building regs to allow for.
    Will check this. I know the neighbours are fine with it already, but I'll double check the regs.

    Quote Originally Posted by ik9000
    Lastly two things I would also encourage you to think about in the design:

    1) Overall stability - the lid looks a bit unstable to me, not all directions appear adequately restrained/braced against collapse.
    2) Foundations, and how the frame connects to them. There's a good hood for catching the wind and taking off going on there. So the foundations need to work in the upward direction too. Something to think about...
    1) I hadn't added them to the design yet, but I was intending diagonal cross-braces (probably in 2x4?) at each post/top-beam junction, for a total of 12 braces. Any comments on minimum size? Was just going to mitre the ends and then use outdoor screws to attach them.

    2) My plan was to bore out and fill cylindrical holes with concrete (or similar ... post-crete, perhaps), and to then attach the posts to it. Something like this ...



    What depth/diameter would I need to ensure this dual-direction "strength"?

    Finally...

    Thank you! Most helpful post ever!!!!

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    Re: How can I find out/work out what bolts I need for my decking?

    Quote Originally Posted by stevie lee
    weather treat the wood before sticking it together. so you get all the sides of the wood and the faces that are going to be next to each other.
    where you drill the hole for the bolts, treat inside the drilled hole as well or water will get down the hole and rot the wood from inside.
    wooden posts down into the ground/soil is not a good idea.
    Thank you. That makes sense.

    Quote Originally Posted by stevie lee
    I would also go and tell the neighbours what you are planning. especially the one that's the other side of the fence in the diagram. Depending on which way the sun shines, they might end up with a large decking shaped area of dead grass. you might aswell.
    He's fully aware of the plans, and I'll be showing him the plans before I kick off the project anyway, to make sure he's happy. The other side of the fence is chicken run that used to be fully shaded by the hedge I just removed; a shed; and his roofed gazebo area, so I don't think it'll be a problem!

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    Re: How can I find out/work out what bolts I need for my decking?

    Quote Originally Posted by DaKid View Post
    I'd come to a similar conclusion about getting them at the same level, although I'd been thinking about just using joist hangers to put the secondary joists in between the primary ones, and to swap the primary beams to the outside of the posts in order to allow the decking to have a complete enough surface to be laid on. Do you see any problems with doing that??
    Sounds reasonable to me. Hangers will be fine, but two things to consider 1) appearance - they'll forever be that shiny glint of distraction through the slats 2) most galv hangers are intended for internal use. They'll corrode faster outside. How long you get will vary, but well, it's something to bear in mind. For the short-mid term perfectly reasonable if you don't mind the look. Prob not worth the cost to go stainless steel. From outside the deck you'll notice it most if you use flanged hangers, so maybe go for face-fixed ones like the simpson-strongtie SAE (and SAI for the corners).


    Quote Originally Posted by DaKid View Post
    Thank you!
    anytime - it's kind of basic, but so annoying when you don't know where to look for something so simple.

    Quote Originally Posted by DaKid View Post
    Will check this. I know the neighbours are fine with it already, but I'll double check the regs.
    As anything in life, assumption is the mother of all stuff-ups so best to check before the event.


    Quote Originally Posted by DaKid View Post
    1) I hadn't added them to the design yet, but I was intending diagonal cross-braces (probably in 2x4?) at each post/top-beam junction, for a total of 12 braces. Any comments on minimum size? Was just going to mitre the ends and then use outdoor screws to attach them.
    Corner braces in two orthogonal directions? Sounds like a plan to me. Sizes? Most stuff will work tbh. It's about stiffening the corners so they can't rotate, then it forces the posts to act in bending. Without doing any sums I guess I'd start with something half the thickness of the post, and since you're using 200x75 joists... You want to make a reasonable 45 degree triangle - if it looks right to the eye you've probably nailed it. Speaking of which, skew nail intensively or better still double-skew (from each side) using coach screws. Set one side higher than the other so they don't clash and get good embedment through the brace into the wood itself. It's hard to describe in text.
    Quote Originally Posted by DaKid View Post
    2) My plan was to bore out and fill cylindrical holes with concrete (or similar ... post-crete, perhaps), and to then attach the posts to it. Something like this ...



    What depth/diameter would I need to ensure this dual-direction "strength"?
    It's about balancing the weight vs wind uplift. and for that, how sheltered is it, where in the country is it, how far from sea/fields, altitude topography etc etc etc. You need more than you think to stop it taking off in storms - so think significantly more than just a fence post base (in terms of volume of concrete). It's an interesting one, what is actually sensible for domestic pergola? One to mull on that.

    Right, my dinner's ready. That's me signing off for the night.

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    Re: How can I find out/work out what bolts I need for my decking?

    got a picture of my neighbours wooden 'thing' so you can have a look at what it may end up looking like in the flesh, so to speak. might help to visualize it a bit better seeing what one looks like so big and in plain wood look, and maybe how to do the roof.




    that's a 10ft brick wall behind it.

    the fence is to stop the St Bernard climbing up it and jumping over the wall (again)

  22. #15
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    Re: How can I find out/work out what bolts I need for my decking?

    Quote Originally Posted by stevie lee View Post
    got a picture of my neighbours wooden 'thing' so you can have a look at what it may end up looking like in the flesh, so to speak. might help to visualize it a bit better seeing what one looks like so big and in plain wood look, and maybe how to do the roof.




    that's a 10ft brick wall behind it.

    the fence is to stop the St Bernard climbing up it and jumping over the wall (again)
    Not seeing anything?

  23. #16
    boop, got your nose stevie lee's Avatar
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    Re: How can I find out/work out what bolts I need for my decking?

    Quote Originally Posted by DaKid View Post
    Not seeing anything?
    odd.. its uploaded to hexus.albums the inbuilt picture hosting for hexus. must be a bug at hexus'es end then.

    direct link ? https://forums.hexus.net/members/ste...zippy-zebo.jpg


    edit: done a bug report post https://forums.hexus.net/hexus-sugge...sting-bug.html found out Chrome doesn't show pictures linked from the hexus.my albums for some reason..
    Last edited by stevie lee; 25-07-2018 at 09:56 PM.

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