Discussion in 'Builders' Talk' started by Marcin88a, Sep 21, 2017.

  1. Marcin88a

    Marcin88a New Member

    Hi all
    Please excuse me, but I am not a builder, so not sure if I will use correct wording...
    I would like to hear your views on the best method of creating a suitable base for a summerhouse as well as some other questions I need help with. The summerhouse is to be used as a gym and storage (will be divided by a wall). It will be located at the back of my garden, which is about 35m from the street. The plan is to insulate it, so we can use it in winter, with (fan) heater installed (there is an existing power supply to a shed which will be demolished).
    I am not sure how long we will live in the house, so potentailly I would like to add value by this addition.
    Outer dimensions will be approximately 6m (wide) x 5m (deep), gym 4m x 5m, storage 2m x 5m. I have decided to build the floor from 2x4 treated timber (I am aware of the span and spacing requirements) and use the same for all of the walls and pitched roof (low pitch).
    I would like to insulate the floor withn 50mm celotex sealed with aluminium tape, supported by battens (so that insulation sits flush with top of the joists). On top of that ply or osb? Can I use rockwool slab insulation instead of? If I can, what would be other pros/cons and requirements? How will this affect teh base design?
    Walls structure plan (from external): cladding (which is the least maintenance and will not fade over time?), breather membrane, insulation (celotex or rockwool?), internally boarded with osb. Did I miss something?
    I have not yet decided about the roof structure yet. Any suggestions are welcomed... I would like it to be lighweight. Judging by the dimension of the building (5m deep) I suppose I will need a ridge beam as the roof if not flat will be of low pitch.
    Anyway, the base...
    There is an existing concrete slab (or rather 6 smaller slabs, adjoining and in a brickwork overlap pattern). The depth is between 2" and 3" and is set on a shallow 1" - 2" compacted hardocre. The base dimensions are 3m x 6m, so is too small anyway. The individual slabs are not level, which makes the whole thing even more awkward. Two of them have a light(?) slope, no more than an 1"/3m.
    I am considering:
    1. Diging up a trench for the foundations where there is no existing concrete base. Following on from that I would lay a course of brick or blockwork around the perimeter of the footprint and where the joist span would require to make it all level; or
    2. Use railway sleepers (I can get my hands on about 30 free of charge :), which have been stored in a dry warehouse), lay them over existing concrete slab and level the surface as I lay the floor joist. Where there is no slab, dig up about 4" - 6", fill and compact with crushed hardcore (or shall i use shingles?) and put the sleepers down.
    3. Break the base, dig up for the full foundations and where the joist span will dictate (I assume this will be the most pricey, due to concrete pum required).

    Any other suggestions, or points which I should consider?
  2. Dr Bodgit

    Dr Bodgit Super Member

    You may want to look at my project thread here

    Timber base all the way for me, snappyfish is also building (and now demolishing) his log cabin which has a concrete base. But concrete bases attract and puddle water with nowhere for it go, so the timber rots.

    There are many ways to get a good firm and level base, key is that any timber can breath and dry out should it get wet, which generally means keeping it off the ground.
  3. KIAB

    KIAB Super Member

  4. Dr Bodgit

    Dr Bodgit Super Member

    Reading your post a little more carefully, I'd probably do what I've done but make use of the existing base. I used 150mm x 150mm treated timber as legs to support the sub-frame which allows for differing heights; these can stand on the existing concrete base with DPC under and where there isn't concrete, build a padstone or similar like I did.

    If the areas of the concrete base are level, you might use this as a reference and use blocks or bricks to raise the subframe (as snappyfish is now doing); elsewhere use timber legs to make up the level.

    I also use 4" x 2", doubled up around the outside and for joists, well supported on the inside.
  5. Marcin88a

    Marcin88a New Member

    Hi. thank you for your link. I read it just briefly, will go through it with more detail tonight. I will probably go with treated legs design similar to yours, although my main worry is if the slab is of sufficient depth to support the structure? Also when making your slabs, what depth did you dig up for the concrete?
  6. Dr Bodgit

    Dr Bodgit Super Member

    Each of the 20 pads is:
    - 1"-2" of compacted hardcore (which I had left over)
    - 1" chicken wire
    - 4" concrete
    - 1"-2" bedding mixture
    - paving slab

    So a bit more than you've got. I reckon each leg is supporting about 200-300kg. Obviously the more supports you have the less weight there is on each. I guess you're concrete base has been there for a while so its condition should be fairly obvious.

    Is your structure going to be like a log cabin where the walls are built first so much of the weight is around the perimeter, or like a shed where the base goes on first and then the walls sit on the base with the load spread more evenly?
  7. Marcin88a

    Marcin88a New Member

    I thought of building like a shed, base first, walls afterwards. Is there any difference? I do not mind supporting at lesser spans where the existing slab is, so that the overall weight gets more distributed.
    As for the existing slab, the condition is good, so that is does no crumble. Some minor deterioration where the indivudually poured slabs adjoin, probably good for the water draining :) The slab has been there for at least 10 years, originally having 2 sheds on it. Both rotten , so one was removed 2 years ago, second one is on its last legs... These were 10 x 6 each.
  8. Dr Bodgit

    Dr Bodgit Super Member

    A shed like construction means the weight of the walls and roof is spread more evenly over the area of the base, so there's less pressure on the outside of the concrete. Sounds to me like your existing base would be up to the job.
  9. Marcin88a

    Marcin88a New Member

    Good to hear that!
    TBH the only reason the existing sheds have failed, was lack of damp proof underneath. They were sitting directly on the slab and rotten from the floor. I would have bought another shed(s) to replace, but want something we can use all year round for exercising.
  10. Dr Bodgit

    Dr Bodgit Super Member

    Just keep the base off the concrete so air can get in and dry things out should it get wet - which it will. Doesn't need much of a gap. And use DPC too.
  11. snappyfish

    snappyfish Active Member

    100% sound advice, read @Dr Bodgit project page and take a look at my concrete issues. Concrete is ok if already there if you can get the whole base of the concrete on piers/blocks of some sort.

    I have sealed, painted and sealed some more, water will find a way in then no real way to dry out
    Dr Bodgit likes this.
  12. Dr Bodgit

    Dr Bodgit Super Member

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