Steel shed concrete base: looking for the easiest way

Calahonda52

In the attached picture, the timbers are 5" deep and the drop off in the slope from front to back leaves the back requiring 12" of fill to top of timber.
The base only need to be 4" concrete so looking for ideas to simplify the work.
Need to finish the shuttering first of course

1. Hard core not really an option as the back 12" needs to be solid and vertical when finished.
2. Ready mix not an option either as is only wheel barrow access and my guess is 1.2 m3 [3 by 2 by .2]
3. What about using a dry mix of cement and gravel as the base layer, leaving 4" for concrete?
4. If doing that, how long would the dry mix need to be left to "go off"?
   
   [*}Can borrow a mixer

All suggestions welcomed.
Thanks as always

Ronney

Bring it up in lean mix to bottom of timbers or just pour it all in concrete

Calahonda52

Ronney wrote: »

Bring it up in lean mix to bottom of timbers or just pour it all in concrete

By lean do you mean dry or just a weaker wet mix?

Ronney

Calahonda52 wrote: »

By lean do you mean dry or just a weaker wet mix?

A dry mix, would usually be about 20kN in strength

Calahonda52

Ronney wrote: »

A dry mix, would usually be about 20kN in strength

So in a mixer
8 to one by weight gravel to cement?

Lumen

Calahonda52 wrote: »

Ready mix not an option either as is only wheel barrow access and my guess is 1.2 m3 [3 by 2 by .2]

How far to wheel barrow?

I did about 0.75m3 of ready mix for my oil tank base, it only took a few minutes to wheelbarrow (although the distance was only about 10ft).

Could have easily have done double before the concrete man's patience would have run out.

Whole job took about 15 minutes.

Calahonda52

100 feet, narrow passage.
Also screeding such a large area, being in 12" of ready mix in wellies is not easy!

Lumen

I guess the question is whether it's less hassle to build a foot high concrete wedge or level it.

There's only a couple of m3 of soil to move around.

foxy farmer

Cut some plywood to close off your formwork. Screw or nail it to the 6x2. A few battens fixed to the edge running along the ground will stiffen the ply. Drive a few stakes or steel bars to keep the ply from bursting out when concrete is poured. If you can get some rubble or filling then build up a "platform" about 100mm + in from the shuttering. This will allow concrete down to the ground and support the floor along the perimeter. A base of 100 to 150mm thick will be quite adequate for your job. No need for huge concrete wedge.

Lumen

You could use 2 part PU foam for the infill, then you'd have an insulated floor (apart from the cold bridge under the walls).

Like a ICF, except above ground.

So maybe...dig a couple of narrow trenches levelled with aggregate, into those build two unmortared piers of 100mm precast lintels to rough level, bridge with two more lintels, line the inside with poly sheet and duck tape, pour the foam to top of piers, top the bridging lintels with spaced A142 mesh, build the formwork about 100mm outside the lintel frame (much easier as the pressure will be lower due to infill), pour the concrete.

That should limit the concrete effort, effectively just locking the lintels into position and levelling the top.

hesker

Buy 3 wheelbarrows and get 2 mates to help.

3 of us moved 2m3 of concrete up a hill through a house and out to end of back garden a couple of years ago. Took about 20 minutes I think.

Could really do with a 4th person to rake the concrete and level.

Calahonda52

hesker wrote: »

Buy 3 wheelbarrows and get 2 mates to help.

3 of us moved 2m3 of concrete up a hill through a house and out to end of back garden a couple of years ago. Took about 20 minutes I think.

Could really do with a 4th person to rake the concrete and level.

Thanks, easier said than done as I am only a few weeks in this new house in a new town so don't know anyone yet.

Anyway, thanks for insights.
Case closed

my3cents

What I have done in similar situations (couldn't mix enough concrete on my own in one go) is to build a blockwork wall around the edge as part of the foundation then infill with hardcore before finishing off with about 4 inch of concrete.

Calahonda52

Ready mix plus a few lads with 3 wheel barrows coming this week, rather permitting to pour it:)

my3cents

Well that isn't going to subside any time soon.

Add some rebar and it will survive a nuclear attack

recedite

The only problem with a wedge like that is that you have created a "slip plane" so there is a tendency for the whole thing to move backwards over time. That's why wedges are not used generally in foundations or slabs. A foundation on a slope would be stepped instead, so always a horizontal surface.
Anyhow, it won't matter for a shed. Worst thing that can happen is the shed gradually moves back towards the patio by a few mm, but its too small for any cracks to appear, so it would all move at the same time.

Lumen

recedite wrote: »

The only problem with a wedge like that is that you have created a "slip plane" so there is a tendency for the whole thing to move backwards over time. That's why wedges are not used generally in foundations or slabs. A foundation on a slope would be stepped instead, so always a horizontal surface.

You could dig a rough channel across the slope in the middle before the pour to create a "rib" on the underside of the slab.

Calahonda52

recedite wrote: »

The only problem with a wedge like that is that you have created a "slip plane" so there is a tendency for the whole thing to move backwards over time. That's why wedges are not used generally in foundations or slabs. A foundation on a slope would be stepped instead, so always a horizontal surface.
Anyhow, it won't matter for a shed. Worst thing that can happen is the shed gradually moves back towards the patio by a few mm, but its too small for any cracks to appear, so it would all move at the same time.

Thank you for that very insightful observation.

The yellow clay is very very hard, so I will take Lumens suggestion and dig a key

edit 18th

What about driving in lengths of rebar down at the deep end?

Calahonda52

Three lads, two barrows: was not even here

updated picture added

Four Phucs Ache

Fair play, that looks like glass.

mengele

Calahonda52 wrote: »

Three lads, two barrows: was not even here

updated picture added

How much for the concrete?

recedite

Calahonda52 wrote: »

What about driving in lengths of rebar down at the deep end?

Just saw this now, so obviously too late to reply. But anyway, you'll notice that people are using galvanised rebar and mesh now, whereas a few years ago they threw in rusty steel. Basically you have to careful not to damage the coating on the steel, otherwise you are potentially creating a weak point/ time bomb in your concrete. Its the reason a lot of motorway bridges around the world are collapsing, including that one recently in Genoa.

The bridge was constructed using reinforced and pre-stressed concrete about 50 years ago. There are a large number of reinforced concrete bridges in Italy, Europe, USA, and Canada with the same age, which are suffering from corrosion of reinforcement and or pre-stressing tendon."

Anyway, a shed base will be fine, it can't fall anywhere
Looks like a nice job.

Calahonda52

Thanks: its 2mm off square in 3.385m

Calahonda52

Am going to put a timber floor in it, have access to 40mm MDF [it's FOC:D ]

So, will have to line it with plastic.
What I was thinking was of "sticking" say 6" or 9" wide dpc onto the concrete before they come fit the shed, and then seal the bottom of the shed against the dpc with say tec 7 or similar.
Then cut the plastic to suit and glue it to the dpc.
then put pressure treated timber lathes on plastic and lay the mdf which should be heavy enough to not need fixing to concrete..
Any thoughts?

Shed not due till end Nov

[How would I stick DPC to the concrete? ]

recedite

No need to stick it. Try to get your hands on some radon barrier, its extremely tough and it works as a DPC too.
The shed will presumably be fixed to the slab with rawlbolts or similar, so let these pierce down through the DPC. A few holes in it should not matter, just blow out any concrete dust and squirt in some silicone or Tec 7 so no moisture wicking can occur from the holes later. Then you can seal around all the edges afterwards, as per the plan. Your battens won't need to be pressure treated because they will be dry.

Lumen

Don't see the point in using a strip of DPC since it's only €65 for a 12.5x4m sheet of 2000G DPM sheeting which will give you two layers with plenty to spare.

So maybe one sheet flat over the concrete (under the shed walls), and then another inside the shed, lapping up the walls, maybe use a perimeter batten to tidy it up allowing expansion gap with the MDF.

I'm not sure it's worth sealing anything since water from the outside is going to have to climb a couple of inches of battened DPM to get anywhere near the MDF.

recedite

If using a radon barrier, it would be one big sheet, the size of the slab. So fairly easy to hold in place without any "sticking" while the shed is being fixed. Its easy to tear polythene while walking around on it in heavy boots.
IMO you'd be better of squirting the sealant in below the shed frame, (and not trying to have the DPC rise up the inside wall) because it will stop water and dirt pooling under the steel frame. Also because mice and insects might try to get in. OP claims 2mm accuracy of course, which is smaller than a mouses skull, but we'll see what gaps appear under the steel when its laid out on top of the concrete slab/DPC

Lumen

recedite wrote: »

IMO you'd be better of squirting the sealant in below the shed frame, (and not trying to have the DPC rise up the inside wall)

Yeah, that's putting a lot of faith in sealant though.

What about detailing at the doorway? Is door opening in or out? Can water get under it to the flooring?

Roen

I know you got the MDF free but any reason for not just leaving it bare concrete?

recedite

It starts with the mdf flooring.
Before long, its oak paneling and wallpaper.

Idbatterim

I’m thinking of using ground screws for my next project. Way quicker, no mess. The catch is cost obviously ...

Calahonda52

Thanks guys for the suggestions/comments:

1. the 2mm was the squareness test across the diagonal: not how level it is.:)
2. Door is above the bottom frame which goes all the way around.
3. Not leaving it concrete as it has no dpc in it.
4. How is radon barrier sold? sheet/roll?

Calahonda52

They came yesterday: 40 minutes!

Its 3m by 2m and eaves are c 2m

Roen

Steeltech?

Calahonda52

Roen wrote: »

Steeltech?

Yep, one downside with sliding door is it is not mouse proof

10-10-20

Is that double-glazed?

Calahonda52

10-10-20 wrote: »

Is that double-glazed?

yes doubled glazed
It was 350 euro
The whole unit was 2,000 and 350 for the concrete base, I made and stripped the formwork, 56 euro in timber which was full recovered, the rest came from skips.

Roen

Great sheds, should last a lifetime!

10-10-20

Calahonda52 wrote: »

yes doubled glazed
It was 350 euro

Flash! I went for the plastic roof-light and I also selected the non-drip 'insulation', which in my cases is just a very light wooly fabric coating.

I assembled mine so that the shed sat on a similar plinth, but I put the DPC down only in the interior and then poured a floor with the DPC coming up the inside of the bottom L-support. I noticed that some of the walls are wicking rain water off the L-support/plinth. I'm not sure how to seal this off, or if it's even possible. Maybe a good bead of exterior silicone sealant will do it, but I don't want to create a water-trap.

How are you planning on doing it?

Lumen

10-10-20 wrote: »

I noticed that some of the walls are wicking rain water off the L-support/plinth. I'm not sure how to seal this off, or if it's even possible

Remove a strip of the fabric where the wall meets the plinth?

10-10-20

If it's damp, the lower section will rust as it's plastic coated and either galv or SS. Maybe I need to check that.

tradesman

Calahonda52 wrote: »

They came yesterday: 40 minutes!

Its 3m by 2m and eaves are c 2m

The plastic should have been on the inside so water will not get in

Calahonda52

tradesman wrote: »

The plastic should have been on the inside so water will not get in

That is what is recommended if I was pouring an additional slab inside

I will be sealing the radon barrier to the bottom of the shed with industrial strength "flash-band" and then putting in a ventilated timber floor.

Calahonda52

Lumen wrote: »

Remove a strip of the fabric where the wall meets the plinth?

No easy to do as the bottom section of the frame comes up a few inches inside

Calahonda52

base: 2.6 cubic meters of ready mix: 350 euro delivered and screeded, [all shutter timber that was purchase fully recovered; 56 euro]
Shed steel tech 3m by 2m: 2k euro [most level base they had seen ]

Floor supports: 45mm by 35 mm pressure treated timber at 300mm centres, 6 no 4.8 m lengths, 1m waste: black tape to hold them in place.50 euro
floor: 18mm OSB: 2 sheets, 2 saw cuts, little waste: 56 euro

Any thoughts on treating the OSB?
will put Alu checker plate at the door.

BrokenArrows

Calahonda52 wrote: »

Any thoughts on treating the OSB?

What are you using the shed for? Storage or constant work use?

Calahonda52

Storage and light DIY, no floor mounted machine tools.
When I get my bench made I will cut through the osb to have it supported on the concrete.
Will have gravel outside the door

lgk

Calahonda52 wrote: »

Any thoughts on treating the OSB?

You could get an off-cut or end of roll of lino, super easy to clean then.

recedite

Calahonda52 wrote: »

Any thoughts on treating the OSB?

I'd go with the assumption it will remain dry and does not need protection against rot, and also bearing in mind you want to avoid toxic fumes in a confined space wherever possible.
So seal it with something basic like varnish or even simpler;polybond/PVA/ Evo-Stik glue.
PVA sealer keeps it fairly clean and spill resistant, also less chance of splinters loosening from the OSB boards. One dilute coat followed by a stronger coat makes a nice floor seal.

Calahonda52

recedite wrote: »

I'd go with the assumption it will remain dry and does not need protection against rot, and also bearing in mind you want to avoid toxic fumes in a confined space wherever possible.
So seal it with something basic like varnish or even simpler;polybond/PVA/ Evo-Stik glue.
PVA sealer keeps it fairly clean and spill resistant, also less chance of splinters loosening from the OSB boards. One dilute coat followed by a stronger coat makes a nice floor seal.

Thanks, it is in fact full ventilated along the eaves due to the roof profile but you point is well made

jack of all

I've had good results using dilute PVA to seal the ply-lined walls in my small shed/ workshop, however I wouldn't recommend using it on a floor. Regular PVA is not waterproof and wouldn't stand up to wear and tear from wet foot wear in a flooring situation, especially in heavily trafficked areas (think about just inside the door). Far better to seal the OSB with something like a oil based floor varnish or a polyurethane floor paint (Castle Paints in Tullamore do a good value product that would be ideal). It might be a bit expensive for what you want, but OSB is inclined to delaminate or splinter so a good coating would go a long way towards preventing this. In addition a good coating would make the OSB easier to keep clean by sweeping or wipe down.