Yet Another Foundation Question (sorry).

[panhapticon]

My Perfect Plan had a brush with reality today, and I didn't like it. I'd like to garner some more opinions before I get to crying for the night.

At the moment, the site is mostly covered with a two wooden sheds on a concrete slab, about 45 years old, which is starting to crack and crumble. My Plan is for a single storey, 4½m x 3½m, brick-and-aerated-block with 1:4 mortar, warm EPDM roof, sitting on a slab comprising 20cm hardcore, 2-3cm blinding sand, DPM, and 20cm of C35 concrete reinforced with either A142 or A98 mesh. I estimate the total above-ground structure weight to be 10t empty, and close to 11t fully furnished - let's say 12t. Back-of-the-envelope calculation shows the heaviest wall will exert 40kPa == 0.04N/mm² on the slab.

From what I've read on here and other forums, this slab should be fine for a single storey structure.

However, I recently bumped into a distant neighbour who's in the trade, and he shook his head disapprovingly when we discussed the foundations. In his opinion, this will not last. He suggests at least a 60cm - but ideally 1.2m - slab depth, which would be 40-100cm of concrete. While I could see that 20cm of concrete might be insufficient, is 100cm really necessary??

Has anyone here built anything similar on a slab? What did you use, and how is it lasting? What would you suggest for a structure lifetime of at least 40 years, while keeping costs to a reasonable minimum?

Most of the slab foundations discussed on the forum are either timber or single-skin brick... which leads me to think I'm attempting something dumb.

Please tell me I'm not dumb.

Please.

(Edit: This is an outbuilding garden gym)

 

[stuart45]

Why don't you use strip foundations?

 

[noseall]

My Perfect Plan had a brush with reality today, and I didn't like it. I'd like to garner some more opinions before I get to crying for the night.

At the moment, the site is mostly covered with a two wooden sheds on a concrete slab, about 45 years old, which is starting to crack and crumble. My Plan is for a single storey, 4½m x 3½m, brick-and-aerated-block with 1:4 mortar, warm EPDM roof, sitting on a slab comprising 20cm hardcore, 2-3cm blinding sand, DPM, and 20cm of C35 concrete reinforced with either A142 or A98 mesh. I estimate the total above-ground structure weight to be 10t empty, and close to 11t fully furnished - let's say 12t. Back-of-the-envelope calculation shows the heaviest wall will exert 40kPa == 0.04N/mm² on the slab.

From what I've read on here and other forums, this slab should be fine for a single storey structure.

However, I recently bumped into a distant neighbour who's in the trade, and he shook his head disapprovingly when we discussed the foundations. In his opinion, this will not last. He suggests at least a 60cm - but ideally 1.2m - slab depth, which would be 40-100cm of concrete. While I could see that 20cm of concrete might be insufficient, is 100cm really necessary??

Has anyone here built anything similar on a slab? What did you use, and how is it lasting? What would you suggest for a structure lifetime of at least 40 years, while keeping costs to a reasonable minimum?

Most of the slab foundations discussed on the forum are either timber or single-skin brick... which leads me to think I'm attempting something dumb.

Please tell me I'm not dumb.

Please.

As above - strip foundations and a GBS (ground bearing slab).

 

[mrrusty]

I'm presuming this is an outbuilding and not under building regs? I think your friend is talking Bollax. Here's a couple of projects I've built recently. The garage is on a raft with a 150mm reinforced concrete central pad, over 100-150 wacked MOT with 300x450 reinforced perimeter toe. When I showed my C.Eng MICE son and asked him if he though it was OK (it was my design) he laughed and said it wouldn't be going anywhere anytime soon. 5 years on, it hasn't. The garden room is SIPs and Easypad founds. The SIPs are fast, quite cheap, and of course provide structure and insulation in one product.|

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

https://community.screwfix.com/threads/garage-work-in-progress.222282/

 

[panhapticon]

Why don't you use strip foundations?

It'll be used as a gym - heavy weights and impacts - so I'll need a thick central foundation. I just presumed that if I needed to dig deep for strips, and not-quite-so-deep for the slab, it'd be simpler to lay a thick slab and call it a day.

Looks like I might be heading back to the drawing board.

 

[stuart45]

For a home gym it's more about picking the correct mats for the job. For garage floors when heavy equipment is going to be used a 6 inch slab is recommended.
Steel is only really needed if the ground below the sub base is soft.

 

[Notch7]

Equally it's an extremely complex area where the Gov and local authorities have a duty but rely on 3rd part HA or the private sector. I am not a fan of State control, but in this case I don't see any unifying plan or delivery model to bring all the different aspects together without a degree of leadership from the Gov.

your difficulty with a slab is the damp arrangement -have a think about where the DPM will go and how you stop water ingress where the walls sit on the slab

its hard to make a flat slab / raft damp proof

its easy to make a trench foundation and separate slab damp proof

 

[stuart45]

your difficulty with a slab is the damp arrangement -have a think about where the DPM will go and how you stop water ingress where the walls sit on the slab

its hard to make a flat slab / raft damp proof

its easy to make a trench foundation and separate slab damp proof

Think you pressed the wrong quote button Notch.

 

[panhapticon]

(Sorry, thought I'd posted this, but apparently not!)

I'm presuming this is an outbuilding and not under building regs?

Yes, I forgot to mention, it's a garden gym.

I like the look of your garage. My room will probably turn out to be similar in weight, so I might pilfer your design!

 

[Notch7]

At the moment, the site is mostly covered with a two wooden sheds on a concrete slab, about 45 years old, which is starting to crack and crumble. My Plan is for a single storey, 4½m x 3½m, brick-and-aerated-block with 1:4 mortar, warm EPDM roof, sitting on a slab comprising 20cm hardcore, 2-3cm blinding sand, DPM, and 20cm of C35 concrete reinforced with either A142 or A98 mesh. I estimate the total above-ground structure weight to be 10t empty, and close to 11t fully furnished - let's say 12t. Back-of-the-envelope calculation shows the heaviest wall will exert 40kPa == 0.04N/mm² on the slab.

the damp arrangement is difficult to do on a flat slab / raft -have a think about where the DPM goes and how you stop water ingress where the walls sit on the slab

one way to make it work is with a rebate in the raft, but that needs formwork to achieve

 

[Ivor Windybottom]

The thickness of the concrete does matter, but what's under it probably matters more.

If it's built on topsoil then it won't last. It will crack or tip eventually. You can't stop the worms and various other animals burrowing and organic matter decomposing.

We have 1m deep topsoil. I had to dig about 1.4m deep to get a good clean footing. The floor is a slab, which does have some topsoil under it, I've wackered the life out of it with a thick layer of crushed rock but it will probably sink eventually. Hopefully the fact it's 180mm thick and steel reinforced will mean it spans between the walls rather than cracks, despite the soil shrinking down away from it.

I did consider using a concrete block and beam floor, but they have their own issues and you have to draw the line somewhere. My shed's already better built than the house.

We have 1950s floors in the house, all have cracked and sagged. They'll get screeded over one day.

 

[stuart45]

1000mm of topsoil. You must be growing some prize winning veg.

 

[Ivor Windybottom]

It's bloody lovely soil. If you're a gardener and don't want to build anything. Sandy fluffy topsoil, it's like slicing cake to dig. Under it is bright yellow sand, just like a beach. All plants (and weeds) love it. I should grow some ivy to cover the cracks in everything.

It's in the middle of a field that was farmed for centuries before it was built.

 

[panhapticon]

Alright, I'm convinced I'm heading for failure here. I will do my own research, but can anyone recommend 'Strip Foundations & Slabs - An introductory course for backward children' on YouTube? Please don't reply

It's easy! Just groop the furgs, and slarble the quux! Ensure that fweng is greater than 3.2, but only when Zorg is ascendant. Bosh! Proper job!

because that will only fill my heart with confusion and fear.

 

[panhapticon]

the damp arrangement is difficult to do on a flat slab / raft -have a think about where the DPM goes and how you stop water ingress where the walls sit on the slab

Please be gentle, because I'm self-taught, but : A DPM under the slab, a DPC in both leaves, water-resistant batts in the cavity, and liquid DPM applied to the inside floor and walls up to DPC, in addition to drainage channels around the outside of the wall that would prevent rain sitting by the wall, wouldn't be enough?