Concrete for Garden Room - Advice please

[thebanishedone]

Hi,

I’m planning to put down a concrete slab tot be the base for a small garden room. I’m planning the room to be 4x5m.

I’m after some advice on how thick/deep a slab to pour and if I need to use a steel rebar mesh or not.

Thanks in advance for any advice,

Rob

 

[23vc]

At least 100mm, and I’d only put mesh in if there’s a reason eg trees nearby

 

[Diver Fred]

Depends on how firmly the sub-base is compacted. I'd be looking at 100mm of MOT1 compacted, minimum of 100mm insulation*, then 100mm of concrete. Mesh - I'd probably do it as a matter of course, may not be necessary but it would be my piece of mind.

*Insulation, an idea that is future proofing the build if you, later on decide to insulate the garden room.

 

[Ivor Windybottom]

What's the room to be built from? Ideally it needs to be on a foundation that reaches down beyond the topsoil - a good idea for a timber structure, utterly essential if it's brick or block.

 

[Notch7]

Hi,

I’m planning to put down a concrete slab tot be the base for a small garden room. I’m planning the room to be 4x5m.

I’m after some advice on how thick/deep a slab to pour and if I need to use a steel rebar mesh or not.

Thanks in advance for any advice,

Rob

Timber garden room or blockwork / brick?

 

[thebanishedone]

I’m not 100% sure on the structure at the moment. Originally was going with brick but I think I might do a timber structure so I can do more myself (OK with a saw and hammer, no skills as a brickie!!!)

 

[thebanishedone]

Would rather go a little over the top if that makes sense, go belt and braces approach.

 

[mrrusty]

I built a brick/block garage on a concrete raft and a garden room 6.5 x 3.6 on easypads. My 2p is that a concrete pad will mean you can keep the building a bit lower if you are trying to keep under 2.5m, but creates a lot of dug out soil to dispose of and makes a mess if you need to use a digger. Pads are very clean and easy as you only have to dig down in the pad area deep enough to get a good foundation - little waste and can be hand dug. However the substantial timber subframe required adds height and does restrict you to a timber frame construction. Have a look at SIPs for garden rooms www.simplysips.co.uk

For the foundation. Cost wise not that much in it if you factor in the digger, MOT, concrete v pads + timber subframe. For the structure cost wise not much in it between framing and SIPs, but SIPs are fast - if the base is very flat they go up like lego.

 

[Ivor Windybottom]

If you build anything on top of topsoil it will eventually move.

"Easypads" are basically just a development of the old method of building on top of paving slabs, they're not a miracle cure. They will sink over time, as worms, moles and decomposition inevitably allow them to sink over the years. You'll be congratulating yourself on year one, then you'll notice a creak in the floor, then that it's not level, then that the door doesn't shut. Eventually it will end up like our 20 year old shed built on slabs - a buckled wreck.

If you're installing garden rooms as a business, want it to last until the payment clears and are planning to go out of business before the small claims court summons arrive then go for easypads. If you're building for yourself and actually want it to last then build it properly, by digging down to the subsoil beyond the biology that inevitably causes movement.

You can't replace foundations after you find out it's failed. Do it properly or regret it, cheaping out will cost more as it will ruin everything built on top of it.

 

[mrrusty]

If you install easypads on top of just topsoil you would not only be pretty stupid, you would not be following the suppliers installation instructions. I think I said you dig down in the pad area far enough to get a good foundation. In my case about 400mm to be down to the firm clay layer under the top soil with wacked MOT on the clay layer under the pad to position the pad level with the ground level - all according to the manufacturers instructions.

Totally agree - you have to dig down below the top soil "biology" layer.

 

[Ivor Windybottom]

Here our topsoil is about 1m deep. That's a bit of an extreme example, but you don't know until you start digging. For my shed I have 40cm of concrete then 10 courses of bricks up to floor level. Easypads simply couldn't do this, I don't think they make extension poles for them, and if they did they'd be very unstable. You definitely don't want to use wood below ground.

Perhaps they could be OK where the topsoil is very shallow, where they may reach high enough to clear the ground level while being low enough to provide a decent footing.

 

[mrrusty]

Perhaps they could be OK

Of course they "could be OK". It's one thing to debate a method, which may or may not be suitable, but a commercial product has to be "OK" if installed as directed, or the company's product liability insurance would soon be used up!

In my case I gave easy pad people my dimensions, construction and ground conditions and they suggested the size of pad to use and the number required - in my case 2.5T pads in a 4x3 layout. My guesstimate is my building weighs in the order of 10-12T (and maybe less) so that's no more than about 1T per pad.

My clay is quite firm to stiff so from what I can see has a bearing capacity of a minimum of in the order of 100kPA which is in the order of 2.5T over the area of the pad. I'm no SE, but I'm confident it's a sound construction.

 

[Ivor Windybottom]

Well... they can cover themselves by saying they'll replace the products if they break, if anything goes wrong with the building then it's down to the installer. Their instructions are (deliberately?) pretty vague - they don't specify a depth to dig, how much rock or the degree of compaction. Ultimately, any company can just pull the plug and close if they sell products that result in a flurry of claims. Product liability insurance is not a requirement for trading.

Anyone who believes everything they see advertised will have handed their money over to a Nigerian prince long ago.

 

[mrrusty]

Their instructions are (deliberately?) pretty vague

they may be fairly superficial online, but they aren't if you talk to them directly, as I did - read my post