Shed base Concrete vs screen??

[kenny_david]

Hi All new to the forum. I have a new metal shed on top of a 4inch concrete base and need a final layer to waterproof and set the shed in place but I'm getting confused whether to put down a screed or just put down more concrete. The shed is 4x3 meters and I plan on putting down a DPM taping it to the walls then putting another final layer. If I use screed I was thinking of putting down 50mm but I'm worried that screed won't be strong enough. For the screed I was planning to make a 4 to 1 sand cement mix or should I just put down more concrete??? It needs to be a minimum height of 40mm to cover the steel base around the edges any advice would be much appreciate and I will be mixing with an electric mixer.

[Mr Punter]

If it is normal screed (sharp sand / cement mixed damp), 75mm is the standard thickness.

[kenny_david]

I've seen on websites that the British standard is between 50mm and 75mm so if I go with 50mm will it be too weak??

Or should I just do a concrete mix of cement, aggregate and sand to make sure the final shed base is strong enough?

really confused.

[Onoff]

"Taping it to the walls"...

 

Is the intent to sit the edge of the metal shed on the concrete base, then to lay out a DPM inside and tape it up the inside of the walls? 

 

Bad idea if so. 

 

[kenny_david]

hi Onoff my thinking on the "taping it to the wall" is only temporary while I put the final surface down I will cut away the excess DPM once it's cured, my thinking is that the DPM and extra layer of screed or concrete will stop any water coming in under the metal shed when it rains

[Onoff]

1 hour ago, kenny_david said:

hi Onoff my thinking on the "taping it to the wall" is only temporary while I put the final surface down I will cut away the excess DPM once it's cured, my thinking is that the DPM and extra layer of screed or concrete will stop any water coming in under the metal shed when it rains

 

Sorry, I'm a bit lost without a section drawing. 

 

One option could be 600x600x50mm council slabs laid atop the DPM...

[markocosic]

You're making a bucket.? "U" shaped DPM with screeed inside. Metal shed around DPM. Bad plan? Permanently wet?

 

- Concrete base not much bigger than the shed and higher than the surrounding ground level 

- Drainage ditch around concrete base (so that water doesn't stay on the concrete base) 

- Line drain across entrance door (in place of small ditch)

- Maybe a flashing around metal shed covering concrete (to direct water away from concrete and off the edge)

 

OR

 

- Concrete base

- One course of bricks around the shed footprint

- Metal shed on top of the bricks

- Screw down the shed

- Line drain outside front door

- Wooden "skirt" on the bottom of the front door

 

(allows the floor to drain into the line drain; bricks act as your "damn" to stop water running in otherwise' wooden "skirt" can easily be adjusted to fit door<>floor gap as things move)

 

OR

 

Concrete base

Metal shed

Screw down the shed

Treated 45x45 or 25 x 50 mm battens on concrete base

Wooden floor on top of battens

 

(allowing the water to go under the floor but lifting yourself up above where it's an issue)

 

 

 

[Onoff]

Option 1 = bad. Water / damp will drive in (red arrow) and rot your metal shed.

 

Option 2, not bad as the shed is sat on the DPM but a bit of a risk of water sitting between DPM and shed edge. With hindsight you could have cast a taper on the slab edge where the shed sits.

 

Option 3, fold and stick the DPM down the side of the slab. Saves it turning up and trapping water. Great if you can then flash from under the shed and down over the DPM.

 

4 ideally you want the "ground" 150mm below where the metal is. If less then a stone filled trench at least with drainage if necessary. 

 

[kenny_david]

52 minutes ago, Onoff said:

Option 1 = bad. Water / damp will drive in (red arrow) and rot your metal shed.

 

Option 2, not bad as the shed is sat on the DPM but a bit of a risk of water sitting between DPM and shed edge. With hindsight you could have cast a taper on the slab edge where the shed sits.

 

Option 3, fold and stick the DPM down the side of the slab. Saves it turning up and trapping water. Great if you can then flash from under the shed and down over the DPM.

 

4 ideally you want the "ground" 150mm below where the metal is. If less then a stone filled trench at least with drainage if necessary. 

 

 

 

Many thanks Onoff for explaining this so well it's much appreciated. I will be able to lift the shed up and do option 3, wish I had the advice before hand☺️. Would you do a sand and cement screed(4:1) on top of the pad/dpm or normal concrete with aggregate to keep the floor strong enough??

[Onoff]

7 hours ago, kenny_david said:

 

Many thanks Onoff for explaining this so well it's much appreciated. I will be able to lift the shed up and do option 3, wish I had the advice before hand☺️. Would you do a sand and cement screed(4:1) on top of the pad/dpm or normal concrete with aggregate to keep the floor strong enough??

 

I'd do "normal" concrete for strength personally or as I say 2" "council" paving slabs. But add others have said above the 50mm you say for screed/concrete is a bit thin imo.  Less mess, easy 'ish to lay (mind fingers). Depends how flat your slab is underneath. You can always bed on some sand. 

 

I wonder if you could even do away with the screed/concrete... Put down 50mm of pir or eps then 22mm t&g moisture resistant chipboard flooring over that.....keep your feet a bit warmer!

 

That extra flashing sketch to the right of 3 is good if you can do something like that.

 

You see so many metal sheds just sat on an often oversize slab and water pees in at the edges.

 

Devil's in the detail!