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    • CommentAuthorCrawfW
    • CommentTimeSep 2nd 2020
     
    Hi,

    A first-post from a very-long time lurker. The material here was indispensable during the renovation of my cob house - i just regret i was too knackered and stressed to join here properly.

    As time marches on, i am now joining the rest of the world in trying to build a garden studio - something 6.6*4.6 – timber-framed, off block piers.

    I thought I knew what I was doing, then had a look on YouTube and got myself confused!

    This chap made something 6.5*5 by breaking it into six equal size boxes (obv. about 2*2.5 each). He first screwed the joists to the headers, braced the boxes diagonally to square them up, and tied them all together by adding an additional external header. Then he went back and added hangers to the screwed joists. And bolted it all down.

    The fact that (almost) every timber in sight is doubled – and that he uses needle-nose pliers to hold the nails – makes me thing this approach is a little over-complicated……

    My own idea had been to set out the four sides, hang joists at 600mm centres, and square up with noggins. And then angle-bracket/bolt it all to the piers. Too simple?!

    I can see benefits to his approach if I was in a factory. But, as I am in my garden, it would seem easier to be to build in-place and not create something I cannot lift…….

    Comments welcome and appreciated!
    • CommentAuthorowlman
    • CommentTimeSep 2nd 2020
     
    Hi Crawford, it may be me being thick but I don't understand your dimensions 6.6*4.6 etc.?
    Do you have a sketch of what you are trying to achieve, block pier spacing etc.?
    If it's just a timber floor for a garden structure to to sit on, from what I read it sounds a complicated method.
  1.  
    Posted By: CrawfWsomething 6.6*4.6 – timber-framed, off block piers.

    Posted By: owlmanbut I don't understand your dimensions 6.6*4.6 etc.?

    A bit confusing
    either 6.6ft x 4.6ft - a bit small
    or 6.6m x 4.6m in which case a decent size
    Given that the joists proposed are at 600mm centers it is probably the latter. (Edit to say I've just seen the other post which quotes 30m2 so I guess 6.6*4.6 is metric)
    Posted By: CrawfWi am now joining the rest of the world in trying to build a garden studio

    AKA a deluxe shed !!

    CrawfW
    A bit more info about the joists, e.g. what are you going to hang the joists from? how many piers etc. - As above a sketch would help
  2.  
    If I understand the description of the youtube video, it sounds about right. Essentially making up timber frame panels/cassettes.

    I would go for;

    - 3 panels of 2.2m x 4.6m (assuming you have someone who can lift the other end)
    - make these panels by nailing with a paslode or 4" galv wire nails
    - fit the additional joists (in the 2.2m direction), adding jost hangers. The thing about holding the nails with pliers I suspect is to do with this stage, as the joist hangers use twist nails as fixings, which are short chubby nails, that can be a bit tricky to hold into the corners. You get better quickly, when you smack you thumb nails a few times.
    - nail 9.5mm OSB sheets over (having squared the panels first of course)
    - staple a breathable membrane over the OSB
    - flip the panel over, and set it on the piers (DPC etc)
    - fix the panels together, fill with insulation of your choice now or later when roof is one.
    - remember to add additional timbers/noggins just in from the outside edge, to carry the edge of the flooring (chip board or OSB sheets).
    - the panels will be held together securely once you fix on the bottom runner for the walls, and sheet the floor
    - to hold the whole structure down, use steel tie straps / holding down restrains (ideally set into the blockwork as you build the piers, or screwed into the face of the piers after). These straps rise up the pier, over the floor edge, and up the walls.

    That's how I build them, but there are lots of ways to skin a cat. If I were concerned about snakes, I wouldn't set it on piers, rather a simple strip found around the edge perimeter, with a couple of dwarf walls at the panel joints.

    What ever you decide, draw it all out, build it on paper first. I've done enough of these to know following drawings saves a lot of "off cuts":shamed:
    • CommentAuthorCrawfW
    • CommentTimeSep 2nd 2020
     
    Thanks for all the immediate replies - and apologies for not including units. Especially unforgivable given my day job.

    They are indeed metres, with the plan being to have piers every 2.2m.

    Green Paddys comments will need a little more thought - but are very much appreciated. The immediate question though is - given that i don't have much by way of help to move let alone flip - is there a reason not to build it in-situ in one piece (and the right way up)?
    •  
      CommentAuthordjh
    • CommentTimeSep 2nd 2020
     
    Posted By: CrawfWis there a reason not to build it in-situ in one piece (and the right way up)?

    If you build it right-way up, how do you nail the OSB to the underside of the joists?
  3.  
    Well GP mentioned, there's >1 cat flaying methods... I would go for thicker insulation than just the depth of the (short/thin) floor joists, so would consider suspending rockwool between and below the joists on netting or membrane.

    We also live in a rural place and accept that mice etc are a major link in the food chain round here, and they will eventually find their way into any construction that has been built by peoplekind. We draw the line at mice inside our living space, but welcome them in the outdoors, as without them there'd be no buzzards/stoats/owls/kestrels (or snakes?)

    We recently had them digging into some EPS insulation through a gap where two ply sheets had warped apart, they made a mess of polystyrene chippings. On another thread, GBF people thought that fine (<6mm) steel mesh would keep them out, although IME they have great powers of teleportation to get into unexpected spaces. Other materials they have wrecked are: squirty foam; PVC cable insulation and flexible ducting, clip on pipe insulation.

    We definitely have them under the floors and in the loft where they will presumably be living in the mineral wool insulation, they don't seem to do any lasting damage to that, their tunnels close themselves up when vacated.

    So your options are: to build so to exclude your local wildlife (OSB won't keep them out, use steel mesh or concrete) or else choose materials which are less damageable.
    • CommentAuthorowlman
    • CommentTimeSep 3rd 2020 edited
     
    Whatever construction method you choose, either insulated cassettes which are then inverted. Or built directly from the piers and then insulated by either suspending something onto netting from above. Or fixing rigid insulation on to your boarded joists and then overlaying with a finished floor covering.
    All these techniques are possible but I'd take a good look at your shed footprint and if possible, try to some extent, to plan it within the constraints of standard regularised timber metre lengths e.g. 2.4, 3.3, 4.2, 4.8, for 47mm (2") and occasionally 5.4 and 6 for larger sections (72mm ( 3"). that way you'll obviously keep costs to a minimum and not end up with lots of odd useless offcuts. I'd also go looking for a good timber yard with a decent selection of stock sizes, treated and untreated, rather than the DIY sheds.
    Use the same criterion with flooing 1220 x 2440mm sheet or 2440 x 600 sheet T&G; these sheet sizes can vary slightly depending on the supplier mills. All that may have a knock-on effect on your pier and joist spacing.
    Pesrsonally I wouldn't go for 600mm spacing for the sake of a few extra joists.
  4.  
    Another thought..

    - drape a breathable membrane over the piers, like a table cloth
    - build your base in one go (using 45x225 treated timbers, joist hangers etc)
    - pull the membrane towards each joist, and gather it up the vertical sides of the joist say 50mm to allow it to be stapled, working across the floor. It will be a bit wrinkly, but doubt the mice will complain.
    - lay 200mm wool between joists

    Following on from Owlman's comments about standard timber lengths, would a base of 4.8m x 6.0m work for you, setting your piers at 1.2m cntrs on the long side? Could be 2.4m on the short side. The joint at 4.8m on the 6.0m side will then land on a pier. That standardisation will also carry through up to the walls (assuming they are to be timber frame too?). You'd need to double up the external frame timbers if you're spanning the 2.2m between piers you mention above, where they carry the joists, walls, and roof.
    • CommentAuthorEd Davies
    • CommentTimeSep 4th 2020
     
    What I've done with my house, which has a similar construction, is nail sarking boards (19 or 22 x 150mm rough-sawn treated timber) under the joists. Mine has, just about, enough crawl space underneath to do that with a nail gun. I'd imagine a garden studio wouldn't be quite so high but I'm wondering if you could build your floor structure in place just temporarily jacked up 300 or so mm while you nail cladding under it. Lifting and dropping it in place might be a bit time consuming but would be a lot easier than turning it over, I'd think.

    https://edavies.me.uk/2018/03/some-flooring/

    Regarding standard sizes, getting 7.2m lengths isn't much trouble in my experience though they'll need to be ordered in. Longer than that is a bit tricky as that's the length of the standard curtain-sided wagon used for deliveries.
    • CommentAuthorCrawfW
    • CommentTimeSep 5th 2020
     
    So, i've put together an illustration (with units!) and (hopefully) attached it to this post. It shows an aerial view of the garden office, which has a total internal surface area of 30m2.

    The walls have a total thickness of 140mm (11mm OSB, 120mm structural timber, 9mm plasterboad).

    Tables show that C24-graded timber of 140*45 can have a span of 2.34 at 0.25-0.50 kN/m2, so the proposed pier locations seem fine. However -

    a) Would it be worth moving the piers in rows A and B slightly so that the nearby joists actually rests upon them?

    b) Should the joists be single 4.5m lengths supported by piers at half-length, or should they be 2.3m joists supported along the mid-line by another central joist?

    Comments and suggestions welcome, and very much appreciated.
    • CommentAuthorJonti
    • CommentTimeSep 6th 2020 edited
     
    Posted By: CrawfWSo, i've put together an illustration (with units!) and (hopefully) attached it to this post. It shows an aerial view of the garden office, which has a total internal surface area of 30m2.


    Hi Crawfw,

    the attachment appears to be missing.
  5.  
    Your drawing may well clarify, but consider the timber "ring" around the outside, to which you joists are attached, and your walls and roof will load onto.

    These are for simplicity lintels. That's a very different "load table" to joists, hence my comment about doubling up on 45x225 timber at 2m span, and the benefit of reducing that span in one direction.

    Plus deeper floor structure allows for more insulation, and more rigid joist for the dynamic loads and any other dead loads, which are not always accounted for in "load tables", eg, work bench, tools, piles of timber off-cuts, all the junk you think will be useful one day and bare to part with (sorry, that's my garage :sad:)
    • CommentAuthorCrawfW
    • CommentTimeSep 6th 2020 edited
     
    Hopefully the illustration is attached this time!

    The labels i add for rows A and B don't seem to be showing. They are the middle pair.
    • CommentAuthorCrawfW
    • CommentTimeSep 6th 2020
     
    <blockquote><cite>Posted By: Ed Davies</cite>What I've done with my house, which has a similar construction, is nail sarking boards (19 or 22 x 150mm rough-sawn treated timber) under the joists. Mine has, just about, enough crawl space underneath to do that with a nail gun. I'd imagine a garden studio wouldn't be quite so high but I'm wondering if you could build your floor structure in place just temporarily jacked up 300 or so mm while you nail cladding under it. Lifting and dropping it in place might be a bit time consuming but would be a lot easier than turning it over, I'd think.

    <a href="https://edavies.me.uk/2018/03/some-flooring/" rel="nofollow">https://edavies.me.uk/2018/03/some-flooring/</a>

    Regarding standard sizes, getting 7.2m lengths isn't much trouble in my experience though they'll need to be ordered in. Longer than that is a bit tricky as that's the length of the standard curtain-sided wagon used for deliveries.</blockquote>

    Thanks for the suggestions, and link to your blog. Very interesting reading!
    • CommentAuthorCrawfW
    • CommentTimeSep 6th 2020
     
    Right, i have now just myself thoroughly confused! So much so that what follows probably won't make much sense - apologies in advance.

    I want the interior to be 6.6m by 4.54m, to maximise the space on the site and get right up to the 30M2 limit. (Other routes to 30M2 don't fit).

    Given that the walls are 140mm thick this means i need to increase the spacing of the piers by this amount, giving an overall size of 6.88m by 4.78m.

    However - even when i double the headers as suggested by GreenPaddy - i am still left with something that doesn't accommodate 600mm centres for the joists.....

    The spacing (almost) works if i measure off the internal edges, not the centres. Is that OK?! Seems a bit bodgey! The only other thing i can think of is to add a third header timber for each pier, but that seems a bit excessive.

    What am i doing wrong?!
    •  
      CommentAuthordjh
    • CommentTimeSep 6th 2020
     
    Posted By: CrawfWI want the interior to be 6.6m by 4.54m

    FWIW, your drawing shows internal dimensions of 6.6 m x 4.5 m (not 4.54 m)
  6.  
    So, are you thinking that there must only be 600mm cntrs for every joist? It would be quite unusual for a frame to work perfectly like that. Start at one end, set 600cntrs, and you get what you get at the far end.

    I think I mentioned previously, you'll need additional joist/noggins around the perimeter (internally), to catch the edge of the flooring, as the double ring beam will be covered by the wall bottom runner, so that will alter your setting out cntrs.

    Not sure the two heavy timbers in the middle (running up the page) are necessary. Just use joists across each half. I would add a row of full depth noggins (horiz on the page) between each of the 2.4m joist runs.
  7.  
    CrawfW - Are you going to insulate the floor? If so is the insulation is going between the joists, slab insulation (e.g. water and damp resistant EPS) comes typically in bits 50cm x 100cm so I would space the joists to accommodate these. Mineral wool of some sort also comes in 50cm batts. So the joist centers would be 50cm plus the thickness of the joist. Whilst this may finish up with an extra joist the saving in cutting and fitting would IMO be worth the extra. If you are going to use roll out mineral wool (glass or rock) arrange the joists for a half a roll width (over here the rolls are 125cm wide)
    • CommentAuthorCrawfW
    • CommentTimeSep 12th 2020
     
    Thanks for all the comments - very much appreciated.

    Peter_in_Hungary - My current thinking for insulation was putting 2.4*1.2 sheets of EPS70 (70 or 100mm) over the joists, and fixing OSB by screwing to the joists. Not too worried about height, and a useful bit cheaper than Kingspan etc, plus quicker. Mad?!

    (Can't seem to find anything about being able to walk on this stuff!)

    This would have the advantage that if i run at 600 centres and end up with a odd bit at the end, i don't have to spend ages cutting the insulation.

    GreenPaddy - Thanks for reminding me about the extra noggins, and the possibility of avoiding the extra heavy timbers.

    Anyone (!) - For all the helpful comments above, i still seem to have in mind something different to others. Can't spot what is wrong with this!

    Something like:

    1. Inside header to length, sitting on DPC on pier

    2. Add 2.4 joists at 600mm centres - immediately with a hanger, or nail first?

    3. Add noggins to adjust spacing, and at ends to support flooring

    4. Add outside headers; non-overlapping joints

    5. Angle bracket/bolt to piers

    6. Insulate over with EPS

    7. Board with OSB

    The key thing would be doing all this from above, never having to turn it all over when built and heavy.

    I suspect i am missing something about why i actually need to build them as separate sub-units that i square up and connect to each other.....

    Patient advice welcome and appreciated!
    •  
      CommentAuthordjh
    • CommentTimeSep 12th 2020
     
    Posted By: CrawfWMy current thinking for insulation was putting 2.4*1.2 sheets of EPS70 (70 or 100mm) over the joists

    Not doable. You'll need something on top of the joists and underneath the EPS to spread the load, as well as something on top of the EPS for the same reason. You can use the 1% deflection load case as a limit IIRC.
  8.  
    What target u-value have you set for the floor? In Scotland the legal limit* is 0.18 W/m2K with a target of 0.15 W/m2K, although many Green Builders feel existing standards are inadequate and need to be tightened.

    0.15 W/mK is equivalent to about 225mm of EPS or 125mm of PIR (celotex equivalent).

    *You can insulate the floor worse than the target value if you insulate the windows walls roof to higher spec than their respective targets, but that might be expensive.
    • CommentAuthorGreenPaddy
    • CommentTimeSep 13th 2020 edited
     
    making individual frames, and fixing together is just one way of doing it, but not optimal.

    Your drawing with a large outer frame, a heavy central spine, and the rest as joists is fine. Don't change that approach.

    What is the gap/space between the underside of the floor timbers and the ground (ie. top of pier to ground)?

    Also, what depth of floor frame timbers have you settled on?

    Once you've clarified that, flooring insulation methodology can be commented on better.

    Before deciding upon target Uvalues, maybe you could clarify your planned usage of the building/garden room? Is there value in trying to achieve a low Uvalue, if perhaps it's not occupied very much, for example.
    •  
      CommentAuthordjh
    • CommentTimeSep 13th 2020
     
    Posted By: GreenPaddyWhat is the gap/space between the underside of the floor timbers and the ground (ie. top of pier to ground)?

    FWIW, when I installed our garden shed, I supported it on timber bearers on flagstones rather than individual piers, just for simplicity. But what I wanted to say is that I deliberately made the clear space under the shed big enough for a cat to get get under as well as mice and rats. We haven't noticed any problems with creatures damaging/nesting/whatever under the shed. :bigsmile:

    Just the usual creepy-crawlies inside the shed. Mostly spiders but today we found a clutch (collective noun?) of peacock butterflies in the shed, presumably newly hatched/whatever. :bigsmile:
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