Joist size for a log cabin

[pejoli]

Afternoon,

I've purchased a Tuin Laura log cabin (4.3m^2) https://www.tuin.co.uk/laula-log-cabin-DG.html, and will be going with a wood frame foundation to avoid the need for a slab - the ground is fairly uneven.

To keep the height down, I'd like to use 75x100mm joists, but trying to figure out the distance between supports that will require. The standard span tables are geared around floors where the weight is spread - for this, the perimeter joists will be taking all the weight.

Anyone got any guidance on what spacing I should use ? And is 100mm going to be pushing it - should I be looking at more like 150mm ?

Thanks

 

[ktuludays]

I'd go with 6x2 timbers spaced around 400 centres. No point cutting corners on the base, if you go 4x2 and its not strong enough then you have a huge headache once it's fitted.

 

[HERTS P&D]

I would use 6x2 and go for 300mm centres, less bounce and only an extra 4 lengths.

Andy

 

[pejoli]

Thanks for your replies. But for this sort of building, the floor is essentially a separate consideration from the perimeter foundation beams (in terms of load bearing - the weight of the structure is taking on the outside beams in their entirety, the other floor joists will just follow the usual thinking process for any floor - they are only added as essentially a floating floor at the end)

The current plan was to support the joists every 1.33 metres or so (ie four points of support over the 4m). But no way of validating what thickness wood I need. I’ll be up against the 2.5m height limit (and some grumpy neighbours) - hence keen to keep it at a minimum. So if four points are not enough, would five suffice (every metre)? Can’t figure out how to find out.

I do hate a bouncy floor though - I suffered from a loft conversion from some cowboys that needed rectifying for that.

 

[ktuludays]

Thanks for your replies. But for this sort of building, the floor is essentially a separate consideration from the perimeter foundation beams (in terms of load bearing - the weight of the structure is taking on the outside beams in their entirety, the other floor joists will just follow the usual thinking process for any floor - they are only added as essentially a floating floor at the end)

The current plan was to support the joists every 1.33 metres or so (ie four points of support over the 4m). But no way of validating what thickness wood I need. I’ll be up against the 2.5m height limit (and some grumpy neighbours) - hence keen to keep it at a minimum. So if four points are not enough, would five suffice (every metre)? Can’t figure out how to find out.

I do hate a bouncy floor though - I suffered from a loft conversion from some cowboys that needed rectifying for that.

Support the joists at your planned spacings then see where it bounces and add more support as necessary.

I've just built a summer house base and used 6x2 timber and needed 25 supports on a 4.3m x 2.9m base. Noggins in between joists at 400 centres to tie it all together. It's solid as a rock.

 

[Mike13]

I had a slope to contend with, levelled the base area then used an auger to make a bunch of 6-8" holes at corners and several along each edge to about 1m deep, needed a lot less cement than a full slab. I then used composite beam to keep the base plate dry. that would be a lower height then you are currently planning.

 

[pejoli]

Thanks, I went with the 6x2 as advised - we’ve moved it to 2m from the boundary,s o height no longer an issue.

Next questions then - we’re reusing an existing 2m slab over part of it, so don’t have to create footings in that area. There is a fall on it, so not usable as is, so the corner of the frame will rest on a paver on top of it. what’s the best way/material to add a relatively small (less than 2cm) shim at the other support points?

and given there’s now a small air gap between the base and the concrete/earth - will that give sufficient ventilation that a DPM is not required? (There’ll be DPC between the joists/pads)