Shed: Wooden Sill to concrete advice needed

18 May 2020
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United Kingdom
Hi All,

I have a concrete slab in my garden where the previous owners shed once stood. I now want to build my own "Posh Shed" there.

My issue is with connecting the wooden sill to the concrete in a damp free manner. The concrete slab is on a slope so ground level is higher at the back which means water will get in whenever it rains.

I need to raise my wooden sill off the concrete, how best to do it?

I could use solid dense concrete blocks but with that and my wooden foundation it is going to raise it by 1.5 feet on the entrance side which is already 10 inches off the down slope.

Any ideas on how to slightly raise my wooden base and protect it from water?




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Rebel-Rosi, good evening.

Sorry but I am confused [not unusual in these unusual times?]

From the image posted do you intend to utilise the existing concrete blocks around [at least] one side and the rear of the concrete slab? to rest the perimeter of the floor joists on to ?

Hi Ken,

The original plan was to use the concrete blocks all the way around to raise the floor joists off the slab which will get wet.
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Hi Catlad,

That looks really interesting thank you, I will need to do more research on these but I presume these can be bolted into the concrete as I am on a hill there are some pretty big winds here so I need to make sure everything is fastened down but that is a great alternative, cheers.
Knock your base/floor up first, then level by whatever means you chose and then fix a few large stainless steel brackets into the base and then into concrete.
Hi again.

Can I suggest you consider removing the existing concrete blocks and use as "Catlad" suggests?, these things bolt down to the Concrete and the devices are screwed to the floor joists, so in effect the structure should be moderately stable. If you then fix tanalised timber externally between the edge joists and the slab, they will stop a howling gale under the floor, but? leave some air gaps. similar to "Air Bricks" in a house wall, it will allow an air flow, not a gale, final trick would be to fix some Chicken wire over the "gaps" to halt rodents, this combined with floor level Insulation will assist in keeping your legs warm up to your knees [have you ever worked in an old portacabin??]

One observation is that for future maintenance you remove the weeds between the concrete slab and the fence place some weed suppression membrane and stone chips ? will assist in keeping the above air holes clear?

Hi Ken,

That's brilliant advice thank you, I reckon the tanalised timber can be flush on the concrete and it would be fine if I had to replace just that every few years. Yes I have been in a Portacabin, my boss worked in one and even though it was rather ****e, we would come up with any excuse to talk to him as it was always warmer in there than outside :)

Weed suppression and stone chips definitely for around the edges, I want o leave enough space to at least walk around the edges and keep it a bit away from the neighbors wall.


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