Garden room foundations

[OxfordBlue26]

I'm currently doing some ground work for a garden room self build that I hope to do later in the year. I'm planning on an 8m x 4m timber build with a flat roof (warm roof, rubber coated). The site where I will constructing this is ~2 metres from a line of smallish trees/bushes, and 4-5 metres from a couple of much larger trees (one in our garden which I think is a leylandii.... debating whether to get rid of this first!). The ground has a slight slope from front to back (100-150mm across the 4 metre depth of the garden room). The top layer of soil is quite rocky (there was an old shed base here which had bits of old breeze blocks/bricks/concrete etc underneath), and a foot or so down, it is quite heavy clay. Also of relevence is that the site is ~100m from the road, and any material will need to be wheelbarrowed from there...

I am trying to decide what the best option is for a foundation for my garden room? I was initially planning on a concrete pad, but concerned that over time this might be impacted by tree roots? Other options I've considered are concrete piles or ground screws, but I was concerned that having a void under the building might provide an opportunity for pests like mice and wasps (we appear to be particularly plagued by wasps in our garden!). However, I'm sure there will be some way of preventing wasps getting under the building should I go down the ground screw/concrete pile route.

Any advice would be very much appreciated!

 

[Tetrarch]

Oh - a dilemma indeed

Normally, MOT and concrete base would be a no-brainer for precisely the reasons you offer.

However - 100m !

I'm not sure it's exactly scalable, but it took me 4 hours to move 6 tons of MOT 20m up my drive

Regards

Tet

 

[OxfordBlue26]

Yeah, there's no getting away from it, it will be hard work! If necessary I'll take a week off work and do it over several days. I'll hopefully be able to draft in at least one helper as well! If I go down the concrete pad route, I'm hoping concrete could be pumped that distance?..

 

[Ivor Windybottom]

You could probably get concrete pumped that far. Your back will definitely thank you.

I think I paid about £300 for the pump truck for the day, on top of the £1000 for the mixed on-site concrete. An utter bargain.

But I'll warn you, once you start thinking about it it's hard to stop evolving your plans. I went from wood shed, then concrete base, then thinking I don't fancy watching a shed rot away and continually daubing it with stinking chemicals when I'm old, the rats that lived in our old wooden shed. Long story short, I'm building a brick shed with cavity walls. Basically a miniature newbuild house.

In which case you'll need concrete twice - first for the wall footing, then for the floor. I got pumped concrete for the footings. For the floor I found a local quarry that has a shorter truck which reversed into my drive and tipped the ballast not far from where it needed to be. I set up the mixer next to it and barrowed it in myself, the missus grabbed one end of the plank as I tamped it.

Dig through anything black to proper clean subsoil and it will last for ever. Build on topsoil and it will sink and crack.

 

[mrrusty]

Here's what I did for similar reasons - I didn't want the disruption of a digger and huge amount of spoil.

https://community.screwfix.com/threads/garden-room.256842/

Like you we have heavy clay about 400mm down, so I have wacked MOT and easypads on top. The timber frame is clear of the ground and I have rat mesh all the way round. I had some trouble with wasps during the build when all the nooks and crannies in the SIP construction were exposed - they seemed to be attracted to it, but since it is built this seems to have gone away.

A potential advantage of easypads, which I would hope never to have to use is that they can be adjusted at any time with a big spanner, so although you hope you never get any ground changes, theoretically you could adjust at some point in the future.

 

[OxfordBlue26]

Those easypads look like they could be a simpler solution for me (especially given distance from road and uneven ground!). Did you go with one of their standard kits? I see they sell an easypad kit for an 8m x 4m timber garden room. How long has your building been up? Presumably you are happy with how the easypads have performed?

Thanks for the suggestions!

 

[mrrusty]

I sent easypads my plans and they came back with what they suggested - mine are the 2.5T pads except the ones on the little deck extension which are 1.5T

So far so good! My building has been up a bit over 1 year now and has not moved a smidgen - the interior is skimmed on to plasterboard and zero movement cracks.

Mind you, with the substantial timbers I used for the frame, and a SIP construction which is extremely rigid, I suspect you could almost jack one end of the building up and it wouldn't distort! The SIPs are both glued with adhesive foam and nailed on every spline/joining timber and are immensely rigid and strong.

 

[Notch7]

Other options I've considered are concrete piles

I would do this, the ground can then move around due to tree roots without causing any issues.

You can use a fence post spade to cut out a hole, fill with concrete and have a stainless steel threaded bar sticking out (it needs a 100mm square steel “washer” on it to grip in the concrete.)

Just do a strip of concrete around the perimeter and some blocks or something to stop animals getting in.
Cap off the ground with a thin layer of concrete (35mm or so) or some terram and some concrete crush

 

[seanious]

You could probably get concrete pumped that far. Your back will definitely thank you.

I think I paid about £300 for the pump truck for the day, on top of the £1000 for the mixed on-site concrete. An utter bargain.

But I'll warn you, once you start thinking about it it's hard to stop evolving your plans. I went from wood shed, then concrete base, then thinking I don't fancy watching a shed rot away and continually daubing it with stinking chemicals when I'm old, the rats that lived in our old wooden shed. Long story short, I'm building a brick shed with cavity walls. Basically a miniature newbuild house.

In which case you'll need concrete twice - first for the wall footing, then for the floor. I got pumped concrete for the footings. For the floor I found a local quarry that has a shorter truck which reversed into my drive and tipped the ballast not far from where it needed to be. I set up the mixer next to it and barrowed it in myself, the missus grabbed one end of the plank as I tamped it.

Dig through anything black to proper clean subsoil and it will last for ever. Build on topsoil and it will sink and crack.

Hi Ivor,

I'm looking at doing the same. Can I ask, how did you work out what trench foundation depth you needed. I've asked a local design company to give me the foundation calculations and their starting price is 750 +vat.

Kind regards
Sean

 

[Ivor Windybottom]

£750? For doing what exactly? Bet they don't drive a Dacia and have soft hands. You'll be into multiple thousands if that's their starting price.

Get your spade out. Dig a hole and keep digging. See what's there. You need something that's not black, need to dig into it enough to ensure it's clean, i.e. no random bits of topsoil.

Measure the depth you end up with, add a bit, there's your answer.

Now flatten out the ground where you want it. Strim/mow or dig/flatten so you have a blank canvas. Spray paint some lines, ensuring it's all carefully measured, straight (use a string line) and square. I like blue as it shows up well. I previously used red, the digger driver complained that this means don't dig here, said he had a mental block with digging through a red line. So don't use red.

Get a bloke and a digger for the day. Perhaps a self-drive dumper truck too if it needs moving to where you want it or where it can get picked up from.

Watch while he's digging, if it's not all clean at the bottom then ask him to dig deeper.

Then phone a concrete company. Get the mixed on site type, then you only need a guesstimate the absolute worst case maximum to ensure they bring enough. They'll stop when they get to your markers, and only charge for what's actually needed.

 

[seanious]

Thanks for the reply Ivor.

They want 750 + vat to carry out the calculations at tell me how deep to go. As I've never done this before I wanted a firm idea on how deep to dig the trench. My initial thought that this cost was going to be around £300 but 750 was a surprise. My guess is they might use software, tell the software that I live in Market Rasen to know the soil type and condition, let the software know I'm building brick on the outside and concrete block on the inside with a cavity, and from there it would spit out how deep I need to go.

My soil is clayee and dense. My thoughts were that I need to go down at least 450mm for the frost protection, an the maybe a further 1m for the concrete. I was going to make the trench 600mm wide so I guess that's going to be loads of concrete which I don't mind if that is where I need to go.

The summerhouse is going to be 7m x 4.2m x 2.5m high with a sloped roof.

I'll start off as you say with a dig and see exactly what I have.

Again, appreciate your input, its valuable to me.

Sean

 

[mrrusty]

My soil is clayee and dense. My thoughts were that I need to go down at least 450mm for the frost protection, an the maybe a further 1m for the concrete

IMO you're over thinking this. Timber garden room? how much is it going to weigh. I reckon mine at 6.5 x 3.5 with a slate roof weighs somewhere in the order of about 12T. It's supported on 12 easypads rated at 2.5T which are bedded down on wacked MOT to the same clay layer, which is about 400mm under the top soil, that my brick semi stands on on shallow brick corbelled footings. In 2 years it has not moved a smidgen - no cracks on the skimmed plasterboard internal walls.

 

[simplesteve]

Very interested to see an update from @OxfordBlue26 ( O.P. In January)

 

[Ivor Windybottom]

Thanks for the reply Ivor.

They want 750 + vat to carry out the calculations at tell me how deep to go. As I've never done this before I wanted a firm idea on how deep to dig the trench. My initial thought that this cost was going to be around £300 but 750 was a surprise. My guess is they might use software, tell the software that I live in Market Rasen to know the soil type and condition, let the software know I'm building brick on the outside and concrete block on the inside with a cavity, and from there it would spit out how deep I need to go.

My soil is clayee and dense. My thoughts were that I need to go down at least 450mm for the frost protection, an the maybe a further 1m for the concrete. I was going to make the trench 600mm wide so I guess that's going to be loads of concrete which I don't mind if that is where I need to go.

The summerhouse is going to be 7m x 4.2m x 2.5m high with a sloped roof.

I'll start off as you say with a dig and see exactly what I have.

Again, appreciate your input, its valuable to me.

Sean

Honestly and bluntly, how do you think a supposed expert with a computer could possibly know more about your soil than you could find out for yourself with a spade?

Use common sense, just look at it for yourself.

Have you ever seen the first thing that happens before farmland gets houses built on it? They dig trenches, all over the place, to see what's there. You can often see them on Google Earth, they look like caterpillars all over the field from above. Here's a photo of a field from 2007 with the present day roads overlaid on it...

If you're building one storey high then you don't need to worry too much. Remember that builders commonly build 3 or 4 storeys using the same technique. A lot of it is common sense rather than some scientific calculation, you have loads of safety margin.

The one and only thing that will guarantee failure is if you build on anything black. Only build on clean subsoil, not on anything that has any biology going on in it.

I used 40cm of concrete in whatever grade the concrete company said was OK for foundations, I can't even remember the grade. A builder would have probably used less depth. You also have the option of just filling it to just below ground level. It costs a little more than building up with brick or block, but it saves massive amounts of labour and gives you a ridiculously strong foundation, way beyond anything you could ever calculate. I didn't want to as mine includes a number of pipes passing through, so I needed to carefully set them out and build lintels over them. Otherwise I would definitely have fully filled it, it was horrible scrambling around in the trench building up to ground level.

Once you get the base for the walls you'll need to dig out the floor. In my case I just dug out by hand, with a wheelbarrow and ramps. It really didn't take long. I didn't have enough space to get the digger back. For houses they normally clear the entire site down to floor base level first, then dig further down for the walls. But this won't make sense for a shed.