Portable cabin siting

[Pete D]

I have just bought a portable cabin, with 4 jack legs. Its like a Portakabin but I cant call it that as made by someone else.
My question is
Has anyone experience of siting one of these and can advise me the best thing to stand it on.
The area where it will stand is just mud / grass at the moment. Will 4 slabs be enough to stand it on and adjust the legs to get it level.
I wanted to have it off the ground, say 18 inches or so to stop things living under it.

Cheers

 

[oliver90owner]

Depends on how big the cabin is (like weight!), how big the feet are and how thick the slabs are. If 50mm garden paving slabs, I would guess and say no. Paving slabs bedded on adequate volumes of concrete would be OK.

RAB

 

[Pete D]

28'x10', about 2 ton, on four jack legs with feet about 4'' square.
I had in mind decent slabs with maybe some solid building blocks under them and a bit of cement / concrete.

 

[oliver90owner]

Compressive strength of mostly any concrete is more than enough. Problem is tensile and cracking, not crushing, for paving slabs.

Add another couple of tonnes for contents? Your problem is a firm, stable foundation for the slabs, particularly for seasonal ground movement for your area I would think. You don't want one corner taking the load and causing twisting of the structure, so good stable footings are better for longevity - and proper door fitting!

Is your house built on footings or on a concrete raft?

RAB

 

[alanf]

28'x10', about 2 ton, on four jack legs with feet about 4'' square.
I had in mind decent slabs with maybe some solid building blocks under them and a bit of cement / concrete.

I've seen these placed on building sites. Slightly different because they are there for a few months so you want foundations that can be removed - not like a dumper of concrete. Partly depends on the soil type but as you know the basics are to spread a total ton or more a leg over an area which won't sink in or tilt. Surprising how much a BS paving flag can sink into asphalt and crack over a couple of years. On softer earth I've seen two or three BS concrete slabs stacked under each leg on top of compacted brick rubble, the flags separated with a dryish mortar. Probably more robust but it might still need a jack tweak after a few months.

You're probably not loading it heavily but the jacks are unlikely to be designed for the leverage of full extensions all round on an exposed site. The longer extension is more to deal with slopes. On a long term site you might have to consider the mounting points deteriorating, it wouldn't be the first cabin with blocks where the jacks used to be.

 

[Pete D]

Compressive strength of mostly any concrete is more than enough. Problem is tensile and cracking, not crushing, for paving slabs.

Add another couple of tonnes for contents? Your problem is a firm, stable foundation for the slabs, particularly for seasonal ground movement for your area I would think. You don't want one corner taking the load and causing twisting of the structure, so good stable footings are better for longevity - and proper door fitting!

Is your house built on footings or on a concrete raft?

RAB

My house is on a 'raft'.
The legs are 6ft in from the ends so it looks as though a bit of prep is needed to make a base for each leg prior to delivery.

 

[Pete D]

I've seen these placed on building sites. Slightly different because they are there for a few months so you want foundations that can be removed - not like a dumper of concrete. Partly depends on the soil type but as you know the basics are to spread a total ton or more a leg over an area which won't sink in or tilt. Surprising how much a BS paving flag can sink into asphalt and crack over a couple of years. On softer earth I've seen two or three BS concrete slabs stacked under each leg on top of compacted brick rubble, the flags separated with a dryish mortar. Probably more robust but it might still need a jack tweak after a few months.

You're probably not loading it heavily but the jacks are unlikely to be designed for the leverage of full extensions all round on an exposed site. The longer extension is more to deal with slopes. On a long term site you might have to consider the mounting points deteriorating, it wouldn't be the first cabin with blocks where the jacks used to be.

I note that for long term installations they say remove the legs and sit it on a base. hmmm

 

[oliver90owner]

Much larger cabins - school class rooms per eg - would be jacked up and supported on 'axle stand' type supports at intervals along the bearers. Crawling access required for those, as later adjustment (jacking and shimming) may be required.

What starts out as a simple job suddenly escalates into a major building project ...

RAB

 

[wightbees]

How about using sleepers ?

 

[Pete D]

How about using sleepers ?

Considered this, did think that eventually they would possibly rot and compress, but would be a long while. The one at our association apiary is on sleepers, just 1 under each leg.
Wanted to lift it up and stand it on its legs by about 18'' just to stop things living under it.

 

[ratcatcher]

I used four slices of oak log around 18" dia to put a caravan on in our woods, and all seems ok,

 

[wightbees]

Would last years at a guess,I have them under my shed and that's been eight years now.

 

[Dishmop]

Dig some holes, bung some big rocks in and top off with cement. Or old OSR honey.

 

[user 6228]

Much larger cabins - school class rooms per eg - would be jacked up and supported on 'axle stand' type supports at intervals along the bearers. Crawling access required for those, as later adjustment (jacking and shimming) may be required.

What starts out as a simple job suddenly escalates into a major building project ...

RAB

Everything depends on the ground... At our current location un disturbed ground requires almost no preparation for a building ,(high ratio of gravel to clay) in fact its so hard any preparation is difficult. At the sister - in -laws deep soft clays they need 4m+ of piles with a ring beam to put a garage up!