extending concrete base with a concrete trench, bad idea?

[g8dy]

Hi all, ive been busy reading up on the many great shed projects here! My current plan is to build a 4m x 1.8m shed. Ideally I'd like a 4mx3m shed. Now to save me digging up a large section of the garden (I don't have the budget for a skip and all that extra concrete. I was wondering if it possible to just dig up a 1ft trench and lay concrete so that I can then lay down some concrete/wood bearers. Or even just concrete pads every few feet, Is this a bad idea? Im thinking that the section of bearer that is elevated slightly above the paving stones could crack under stress, but I guess I could shim that part to reduce the load.

Lastly, Ive been reading up on Mike's guide to building shed, but I couldn't find info on the spacing of the concrete lintels?

 

[Jameshow]

You could - I would drill into the existing concrete slab and resin in threaded rod / rebar every foot or so.... 6" into slab 6" into new concrete. Or better still to 4" of edge and a length of rebar tied to it going along edge of slab.

 

[DeanH]

Spacing of the lintels will depend on the span of the timbers you presumably lay perpendicular over the top. If you know the dimensions you can look them up on span tables.

https://www.timberbeamcalculator.co.uk/en-gb/span-table/floor-joists?load=1.5&class=C24

 

[g8dy]

You could - I would drill into the existing concrete slab and resin in threaded rod / rebar every foot or so.... 6" into slab 6" into new concrete. Or better still to 4" of edge and a length of rebar tied to it going along edge of slab.

Thanks for your input, im a bit confused by your suggestion. Ive seen the method of connecting new slab to old slab with epoxy and rebar. My plan was to lay a separate trench of concrete that would have the current paving slabs in between the trench and the old slab. How would you connect that via rebar.

Sketch to make sure I'm understanding correctly

 

[DeanH]

Also bear in mind that a shallow concrete trench could be susceptible to heave if laid on clay. It doesn't have the same overall mass as the larger slab so can't resist the movement as much. It'll probably be ok though.

 

[Jameshow]

Thanks for your input, im a bit confused by your suggestion. Ive seen the method of connecting new slab to old slab with epoxy and rebar. My plan was to lay a separate trench of concrete that would have the current paving slabs in between the trench and the old slab. How would you connect that via rebar.

Sketch to make sure I'm understanding correctly

Sorry I miss understood you!

Obvious my idea wouldn't work!

Why not use pillars instead of the trench or simply lift the slabs and put a 6" depth if concrete under those that will be load bearing??

 

[g8dy]

Sorry I miss understood you!

Obvious my idea wouldn't work!

Why not use pillars instead of the trench or simply lift the slabs and put a 6" depth if concrete under those that will be load bearing??

By pillars, do you mean something like my sketch below?
Honestly, I'm looking for the easiest option to remove as little soil as possible. This will be a workshop with wood and metal working machines (probably the heaviest being a metal lathe (300kg). I can always place the heavier machines on the back wall so they are sitting on the main concrete slab.

Would you just dig a hole, compact with hardcore, and then pour in the concrete, using the earth as the form? Would you bother with DPM between the hardcore and concrete for a small set of pillars like this?

UPDATE: Upon further research, I've seen a plinth method of compacting hardcore and placing concrete blocks for plinths. Seems like this would be the easiest method of them all. Would something like this be ok?

 

[Jameshow]

By pillars, do you mean something like my sketch below?
Honestly, I'm looking for the easiest option to remove as little soil as possible. This will be a workshop with wood and metal working machines (probably the heaviest being a metal lathe (300kg). I can always place the heavier machines on the back wall so they are sitting on the main concrete slab.

Would you just dig a hole, compact with hardcore, and then pour in the concrete, using the earth as the form? Would you bother with DPM between the hardcore and concrete for a small set of pillars like this?

UPDATE: Upon further research, I've seen a plinth method of compacting hardcore and placing concrete blocks for plinths. Seems like this would be the easiest method of them all. Would something like this be ok?

View attachment 162430
View attachment 162431

Yes like that DPM between block and you shed base.

 

[ChaiLatte]

Why dig at all?

The crucial question in this is how good are the existing slabs? Lift one to find out on what they are bedded.

What will kill the slabs if you place the new building on them is local point loads. The more weight you put in a single place, like the pads you propose or the individual concrete blocks the more likely you are to have problems with settlement.

If you spread the weight of the new shed over the full area of the slabs it covers, the surcharge is negligible.

Consider your 300kg lathe as 12 bags of cement. If you put those 12 bags on the existing slabs covering an area the same as that covered by the lathe, would the slabs complain? If you put the whole lathe on just one slab, that might lead to problems.

As well as the line of support in the same place as your proposed trench or line of pads, add a couple more inboard of it. If the levels work out, I would consider a U-shaped frame to form the perimeter of the new shed and then lay Roofmate directly on the slabs with your floor (constructed as a membrane - e.g. t&g chipboard) on top of that.

Another option for load spreading is the fibreglass grids you see on the railways (search GRP grating). They are very stiff, do not rot and provide good ventilation to any timber item placed on top of them. A few of those (4 x 1m squares) on the slabs and then traditional shed bearers on top of them would distribute the load into the patio.

 

[g8dy]

Why dig at all?

The crucial question in this is how good are the existing slabs? Lift one to find out on what they are bedded.

What will kill the slabs if you place the new building on them is local point loads. The more weight you put in a single place, like the pads you propose or the individual concrete blocks the more likely you are to have problems with settlement.

If you spread the weight of the new shed over the full area of the slabs it covers, the surcharge is negligible.

Consider your 300kg lathe as 12 bags of cement. If you put those 12 bags on the existing slabs covering an area the same as that covered by the lathe, would the slabs complain? If you put the whole lathe on just one slab, that might lead to problems.

As well as the line of support in the same place as your proposed trench or line of pads, add a couple more inboard of it. If the levels work out, I would consider a U-shaped frame to form the perimeter of the new shed and then lay Roofmate directly on the slabs with your floor (constructed as a membrane - e.g. t&g chipboard) on top of that.

Another option for load spreading is the fibreglass grids you see on the railways (search GRP grating). They are very stiff, do not rot and provide good ventilation to any timber item placed on top of them. A few of those (4 x 1m squares) on the slabs and then traditional shed bearers on top of them would distribute the load into the patio.

I understand what you're saying regarding load. So maybe I do not need to do anything then . If I just build across the paving section would I just shim up the concrete lintels as much as possible to spread the load on the paving stones? What would be best to use for shims in that case? Or lay a strip of mortar the entire length of the lintel and lay it on that so the mortar takes up all the deviations of the paving stones?

Ive dug up a section next to the paving area and it looks like the paving slabs are sitting on 50-60mm of mortar, the paving slabs themselves are 20mm. Would this be a sufficient foundation?

 

[okeydokey]

Perhaps square paving slabs laid on a good bed of 6inch concrete would do the job. Slabs positioned so that the shed floor bearers sit centrally on them.
BTW I cant see the point of tying the old concrete slab into new If you went that way of extending