Used Log Cabin

47x100 regularised is about 45x95 as far as I know. So 5mm difference to existing internal bearers.

 

Get some new 2x2 as internal bearers and use the old bearers around the roof?

 

I think it will.

 

Well there is that. Needs more planning!

 

If you can find a really shallow block or brick you might end up making a sub frame like mine

 

Mine is pretty strong with 4x4 around perimeter. Got 5 legs on long side and 4 on short.

 

Quite a few at different spacing to miss the legs, most are 350mm I think. But its easier for you to support them at regular intervals so you can do 600mm I reckon. Wonder if a paving slab cut into quarters would be viable.

 

The joists need supporting along their length then they won't budge, hence my thought about a slim brick or block, place a few under each joist.

 

One every meter should be fine I think.

 

I don't think so as the perimeter timbers are 4" wide and the bottom of the celotex is above that level. Cant really see how spalsh back could get in there. May be there's a tad right at the edges but I can't see it's anything to worry about. I can take some photos to give you an idea.

 

Gives you an idea https://www.flickr.com/gp/djmclodge/5Z581c

Plus I've got 20mm gravel underneath which helps a lot.

 

Right, did a drawing to sketch out what your base might look like along the lines of mine using 4x2s, attached.

Your cabin is nominally 5.5m x 4m, right? With 100mm off each end gives you 5300 x 3800 and then another 10mm all round for overhang giving 5280 x 3780 as outside dimensions of the sub-frame. The two shorter timbers along the widths are 3780 and 3690, long timbers are 5190 and 5100 assuming 2x4s are 45mm thick. Inside you have 7 joists, 600mm apart except for one which is 585mm. These spaces where the celotex goes are 3600mm long, really convenient as that's exactly 1.5 sheets long, so 6 sheets should do it nicely.

For the "feet", I'd reckon on 9 along the length as there are 9 joists include end bearers, and at least 5 across the width (1m spans). That's 45, 3 to a slab gives you 15 of those thin slabs.

For the roof, I worked out you'll need 9 sheets of celotex so 15 total. 10 sheets of OSB:
- on the 1830 side, cut each sheet to 1200 x 1830, 5 across, gives you 5 offcuts of 570x1200
- on the 2550 side, use the 570x1200 offcuts and supplement with the other 5 sheets cut to 1200 x 1980, 5 across (end sheets cut to width, 900mm wide). Stagger the 2 different sized sheets top/bottom.