Ground prep for logcabin shed - hardcore whackery?

fullerand · 26-06-2020 6:51pm #1

Hi there, I've ordered a logcabin-style wooden shed for the back garden and am hoping to get some advice on the ground preparation please. Am new to this!

The shed is 4m x 3m (this one) and will be used as an exercise room so I'm keen to ensure it's solid++. As part of the installation the shed company will install a block & timber frame foundation (4in blocks, then 4in solid timber frame, then 2in floor joists), but I've to do the ground prep myself.

I've started digging down about a foot deep to get the levels right (see photos attached) and I'm wondering if I need to put down a layer of hardcore and then maybe level it with a whacker before the foundation goes in?

Asking because the soil seems pretty compact after digging down (it was under plastic and gravel for 16 years), so wondering if the hardcore/whacking (ooer) is overkill. The soil drainage is pretty good at present, would whacked hardcore risk the possibility of a water pool under the shed in future?

AF1QipNVz6wvUiqzVb0xUsqhy8qKS2pMNY1Yr-bPke5i?key=ZjRlSThadEpoNnFWUWxXWWdEb1hpWFgyRXBOeTFn

standardg60 · 26-06-2020 8:39pm

The compacted soil won't go anywhere so no need to excavate further.
You could put in a couple of cm of quarry dust to level it off perfectly.

Lumen · 26-06-2020 9:56pm

What have the shed company asked for exactly?

The standard approach is to replace the vegetative topsoil layer (which is a different colour and texture and only a few inches deep) with something that's stable regardless of seasonal variations. You'll know yourself when you're deep enough because it will feel like hard work.

That and whacked hardcore is critical for a stab under a block building because if it's not properly supported it will crack, but since this is timber frame on blocks it's not going to crack if there's a bit of movement.

If you leave it like that it'll just fill up with water, which will soak through the blocks into the timber (although they should put DPM-type material between them to stop this). I'd just fill it with crushed stone to bring the level up and tamp down a bit with whatever you have to hand.

fullerand · 26-06-2020 10:44pm

Lumen wrote: »

What have the shed company asked for exactly?

Thanks Lumen. They've been a bit vague TBH but in fairness they haven't seen the site since I started digging it out.

I didn't mention it above but I gather the joists sit onto steel risers so that moisture doesn't rise up from the timber frame below, I guess that replaces the need for DPM.

Lumen wrote: »

I'd just fill it with crushed stone to bring the level up and tamp down a bit with whatever you have to hand.

Cheers. Just so I'm clear - you reckon I've probably dug deep enough for the soil to be solid, so the main reason for the crushed stone would be to isolate the blocks from moisture coming up through the soil rather than for stability, is that right? So I guess it can be loose enough to be able to "bed" the concrete blocks down into it a bit.

(Oh, and is crushed stone the same as hardcore?

)

Lumen · 26-06-2020 10:52pm

fullerand wrote: »

T
Just so I'm clear - you reckon I've probably dug deep enough for the soil to be solid, so the main reason for the crushed stone would be to isolate the blocks from moisture coming up through the soil rather than for stability, is that right? So I guess it can be loose enough to be able to "bed" the concrete blocks down into it a bit.

(Oh, and is crushed stone the same as hardcore? )

More a case of it's easier to get the area level by raking stone than by moving earth, and you presumably the need ground to be raised back up a bit or you'll be building a shed in a hole.

It's a big tricky to guess without knowing the construction details like how much the thing will weigh and what the pressure will be under the blocks.

Levels are important. Like for instance, if you have wooden cladding that's close to the ground then you will get rain splash rotting the bottom of the boards. And if it's too high then the door threshold(s) will be out. Maybe they can compensate for ground levels with the risers, I have no idea! It's guesswork.

This is why the company should be giving you specific instructions about site prep.

What's traditionally called hardcore or "clause 804" is a mix of different grades of stone and dust that, when whacked with a vibrating plate compactor, will be perfectly stable to take a slab. By crushed stone I mean coarse angular gravel with little or no dust that might drain better.

Hurrache · 26-06-2020 10:54pm

I've used them at another site and a bigger unit than yours. Unless they've changed requirements we didn't need to do any prep at the site.

Their crew came and dug several holes into the soil and laid concrete into them, on which the frame was subsequently laid upon. It didn't need a full sized concrete base like you would with a metal shed to which you'd anchor it to.

If each concrete pour is not level they use wood to level it beneath the frame.

fullerand · 26-06-2020 11:26pm

Thank you both - very very helpful. I think it's best if I ask the company to come have a look at the site before I go further.

jt69er · 10-04-2021 12:38pm

Hi, just wondering how your prep and install of log cabin went?

fullerand · 10-04-2021 3:55pm

jt69er wrote: »

Hi, just wondering how your prep and install of log cabin went?

All good thanks, was happy with the job. In the end I didn't need to put down hardcore or anything like that - just needed to get the ballpark levels correct and they did the rest like what Hurrache said above. Photo attached if it helps.

milout · 10-04-2021 4:55pm

I'm thinking of doing something similar, just wondering about the void left under the timber / stone frame.

Is this infilled at all, if not is there a potential issue with rodents?

Cheers

Reckless Abandonment · 10-04-2021 6:06pm

I would be inclined to fill with hard-core. I didn't and now I've a fox living under the shed

Ground prep for logcabin shed - hardcore whackery?

Comments