Garden Room Base

If you are building a garden room to a standard where it will be used for more than 10 years then I would want a better form of foundations. If the wood rots out it will be pretty much impossible to correct it once built.

I did mine with breeze block pillars but a common method is to concrete in big metal threaded bar to support the wooden floors. It needs more threaded metal bars but easy to get it all level.

The threaded bar method looks pretty good to me, the method in the picture above wouldn't fill me with confidence

I did a solid concrete base on mine. Partly due to issues with unstable ground and an old badger set. A friend has had a nightmare with badgers digging under his.

I'm toying with the same idea over the coming years...

I've literally just (well..yesterday) come across ground screws..

I'm planning a 5x3m base/room...and that would require 15 screws. With the fitting kit, it's about £650, which isn't too bad. You literally pile them in with a screw motion. They're 850mm long, and can sit proud 200mm, so leveling out should be fine...

DrP

DrP. They look a good idea, where did you get those from please?

That will rot well within 10 years, vermin will probably burrow under it autumn time

You'd want joists sitting on top of a block wall with a dpc  and ventilation underneath

That photo is probably blanking all the vents if the house already has a suspended floor

Joiners wanting to take on the job but out their depth and trying to cut down the cost

meandyuk - i saw a vid on youtube, then jsut googled "ground screws".
Tehre's lots of companies - take your pic (I've one a few miles from home).. groundscrews.co.uk i think is reasonably priced.

DrP

We had a concrete base put down and then built a log cabin on top. By the time everything was totaled up a proper base wasn't that much more expensive.

The 'problem' we've ended up with though is the concrete base is larger than the cabin because the outside dimensions of the cabin are nominal 12ft x 18ft. Which means the walls themselves are more like 10x17. Which means water pools under it.

And the cabin base itself is a rectangle of 3x2 treated timber with 3x2 joists running across to support he floor. Which means under the floor is soaked (almost broke my neck when I walked in and the impermeable rubber gym matting surfed across the floor because it was breathing through the boards and getting trapped under it!

Solution has been to lay impermeable insulation between the base and the floorboards. But really that's just trapping the problem where it can't be seen. Short of dismantling it though there's not a lot we can do. Best idea I can come up with is that if/when the treated base fails, buy 4x toe jacks, lift up the whole cabin (it's 2.6tonnes) and slide 15-20mm thick composite (recycled plastic) bearers under it to allow some airflow at say ~200mm centers, and re-lay new 3x2 base on top of that.

TINAS - that's frustrating... And what I reckon the ground screws could combat...

(BTW - I don't work for a ground screw company! I just thount they looked like a fab idea!)

DrP

meandyuk - for groundscrews, have a look at stopdigging.co.uk.  I work for the Yorkshire installer.  They are great for a cost-effective base for a garden room, depending on ground conditions.

If doing it with a concrete base or concrete slabs the tip is to make the base 40mm shorter than the building at all sides so the cladding overhangs the concrete and any water drips off

I built a 5x4m garden room. I went direct to a concrete pad with rebar.  This gave confidence that any ground movement would have minimal impact on the 'room'.  Also, I knew the land had previously been brown field site and had no idea what was under the ground.  Biggest challenge with a concrete base being getting the much in, if you can get the lorry to pour direct that saves a world of pain, if you need to barrow it in get a few friends to help.  Cement lorries normally allow 30min per delivery, you pay for anything beyond that.

Ground screws - I used them a lot on a Network Rail project with great success.  They provide a robust foundation and again (to date) I have not heard of any impact from ground movement.  I used them a lot when installing steps on embankments and would be supporting GRP stairs.  Biggest challenge from the install team was getting them in the ground i.e. a lot of leverage can be requried and can be impacted by subterreanean obstacles i.e. rocks/roots/sleepers/rail.

I would be very nervous of using a wood stake cemented in the ground.  Wood fence posts dont last so why should these?  As pointed out, if one rots you have a very difficult task to resolve.

I used these under my decking frame on top of slabs. Made the job simple and I don't have to worry about timber rotting in the ground. I'll use them again when I get a new shed this year.

https://www.wickes.co.uk/Jouplast-Plastic-Adjustable-Decking-Risers---80mm-to-140mm---Pack-of-6/p/207743

We had a partial concrete base in existence on the site of our prefab unit, but were unaware of depth etc. we used 6 treated railway sleepers as a base and craned in the unit.

If the railway sleepers ever rot out we can jack the unit up, slide them out and replace them.

I built the office I'm in now on concrete piles made using 150mm diameter packing tubes as formers. wooden frame (150x50) sitting on top and then built off that.

8 yrs later and its still level and standing...

tinas - why not just cut the excess concrete base off? rented angle grinder with a big disc should get through it

tinas – why not just cut the excess concrete base off? rented angle grinder with a big disc should get through it

Considered that, but:

* it's ~100mm thick and was nicely bordered by sleepers, so it'd go from looking nice, to looking shit and I'd have literal tones of concrete and timber to dispose of.

* It's assuming I could run a ~250mm masonry blade right up against the wall without a guard and not making a mess of my fingers, limbs or the summerhouse.

* If any lip was left it would defeat the purpose as water running down the wall would still find its way underneath

On balance waiting a decade or two and seeing if it is an actual problem worth solving seems the easiest solution. Especially as I'll probably have moved by then 😂

TINAS you want these:

Composite Profiled Foundation Beams for Log Cabins (tuin.co.uk)

jack the cabin up and slide it underneath.  My cabin is on an old slab which is slightly bigger than the cabin.  These mean the first log never gets wet.  I also then filled any gaps between the slab and the foundation beam with cement as the slab was a bit wobbly.

Inside is still completely dry, installed it myself 3 years ago

OP - if you follow that link you can see the various options tuin offer

I recently tried out gravel grids for my asgard.  Absolutely solid when installed.  Found it a bugger to get a completely level base for the shed though due to the nature of the stones poking out the top of the grid.  Also used them for water butts etc... anywhere where you want a stable base but also drainage.

Yea, I saw those tuin beams at the time but went with a different manufacturer that included things like the floor, UPVC doors/windows and an EPDM rood as standard. And did consider pointing in the existing base with cement.

At this point though fixing it before it breaks would cost the same as fixing it if it breaks. I may as well leave the exiting treated timber base in place for 10/15/20 years and then replace it with a composite one. I've got my hands full with enough projects (including another patio) without making work for myself 😂.

Inside is still completely dry, installed it myself 3 years ago

The inside probably would have been dry, it only became apparent when I put the rubber flooring down that moisture was breathing through the floorboards and getting stuck.

Our screw piles going in.  no arising , no cart away, no disturbance of the extensive tree routes in the garden.  After 6 year very little settlement.  All good would definitely recommend. NB you will probably need a gorilla or 2 on the end of the drill when installing

Hi,

I built a garden room & decking last year using ground screws. Very pleased with the ground screws as really didn’t want to pour concrete foundations.

You’ll need to get a pull test done to make sure the ground is suitable but it shouldn’t be an issue.

Our area was 8m x 4m and because we have a sloping garden we used 3 different length screws.

Happy to answer any questions.

As above, post directly into concrete is a bad idea as they will rot, so block piles / piers, concrete slab or grounds screws would be your best choices. Building up some concrete block piers on top of some concrete would be cheapest but can be the most awkward to get level & true dependent on the ground. Ground screws is the easiest and quickest but the most costly - when I got it quoted from stop digging it was about 50% more than the cost of a concrete slab. Concrete pad is fool proof and easy enough to do yourself, intermediate cost but the most work

+1 for the plastic decking risers. You can also get them in various sizes which help if your garden is on a slope.

I put them on 60x60 pavers and my frame on top of that. Means there's a reasonable step up in to my shed but it's safe(r) from flooding as we back on to a small river.

The quantity required for a summer house may mean you effectively need a continuous base underneath instead of the spot foundations I made - by which point you may have just done a slab underneath.

I got the concrete people to also bring a pump, it was super quick and not a back breaker. Shifting it all dry and mixing down the garden or shifting it wet. We don’t have wheelbarrow access so would have been difficult and slow.
Speak to your employer or accountant. Garden office can be contributed towards by employer as long as you declare it when you sell the house.

That will rot well within 10 years

Softwood in contact with the ground, will start to go in under 5 years. It's classed as 'non-durable'

That said, that is the formula for in contact with the earth, which is probably meaning in contact with moisture. But as its in concrete, moisture will form to some degree, so its only fair to suppose between 5 and 10 years is its lifespan.

Hardwoods, depending on their classification can last considerably longer.

Moderately durable is 10-15, durable is 15-25 and very durable is 25+ that being in some timbers easily over 50.

Oak for example is classed as moderately durable. Teak is very durable, and for teak you can expect 50 years as a minimum

Resurrecting this as we have now had a concrete slab put down.

The issue I have now is, when I measure it, it appears to be around 40mm lower on one side to the other - which I think is quite a drop!

I have a tuin charlie log cabin on order (with composite foundation beams), arriving in a few weeks, so I'm after any ideas on the best way to get it level as I have heard you can have issues with the logs splitting / windows and doors not fitting etc if it's all on a wonk.

Any help appreciated !

Shutter around your unlevel pad and run a screed over it? Maybe drill some holes bung some fixings in so the screed bonds with base?

Not ideal but you aren’t building a bomb proof bunker. I’m sure Google can tell you a minimum thickness for the thin side of the screed. Once your insulation and floor are on top I’m sure it will be fine. (Ianab)

Google decking risers. Plate sized plastic discs that can level out things like that. I built a garden room on them which was on a wonky comcrete pad. They have a capacity of 400kg or something each so are more than up to the job.

Having a slope sounds good to stop water pooling underneath. I wouldn't level the slab. Some kind of spacer or firring would be my preferred solution.  Depends what the base structure of the building is like.

Thanks for the suggestions. I don't really want to go to the expense of building an additional base frame if I can get away with it.

The base structure of the building is that all the weight is at the perimeter, so this is where it needs to be level. You put down a foundation beam around the perimeter, then build up the logs on top (they slot together)

Can you reshutter and use an exterior self levelling compound to flatten it off?
https://www.wickes.co.uk/Setcrete-Exterior-Floor-Levelling-Compound---20kg/p/206820