Shed base epiphany

[Damian Hemsworth]

I'm planning on building a timber workshop this summer at the bottom of the garden.

As stated in a previous thread, this is on uneven ground and I have to take up some really rubbish existing concrete slab, which is proving really easy as its only an inch thick in places..... no wonder it was listing to one side when the old shed was on it!

Any hoo..... I intended to build a surrounding wall out of some of Wickes finest 7N dense blocks, lay another slab within this retaining wall and Bob's your Uncle!

I've since been musing and thought (after reading some other threads), Could I build the wall to the exact size of the shed (3.6m x 4.2m) and build the shed directly onto this?

Would I need to use 6x2s for the floor joists and would I need any concrete piers within the perimeter of the block wall for additional floor support?

It strikes me that by doing this would save on time, effort and more importantly money and if I install a DPM on the blocks before the sole plate goes on, it should be reasonably long lasting shouldn't it?

I appreciate any suggestions from you pros!

Kind regards
Damian

 

[freddiemercurystwin]

Provided the walls have an adequate foundation.

 

[Damian Hemsworth]

I'm aiming to dig a trench 2 foot deep by 1 foot wide, fill with 1 foot of hardcore and a further 1 foot of concrete on top an which to start building the block wall, is this sufficient?

 

[freddiemercurystwin]

Why the hardcore? You don't need or want hardcore under a trench footing.

 

[Damian Hemsworth]

Really? ok....... is 12in deep concrete a suitable foundation?

 

[freddiemercurystwin]

6" would be adequate provided you go down to firm ground.

 

[oldbutnotdead]

As for your floor joists, to use 6 x 2 you'd be wanting a central dwarf wall at 3.6m span. And you'll have to ventilate the floor void and cover the joists with some sort of flooring and fix the joists to the wall- you'll find the costs start getting quite high for timber. Doing a traditional hardcore/sand/dpm/concrete floor (build your block wall then use the top of the blocks as your level for the concrete) as long as access isn't too restricted will be physically harder work but technically less demanding and give you a much stronger and longer lasting structure.

With the rest of the shed, ideally you'd want your DPC 6" above outside ground level- maybe put another course of blocks above floor level then your sole plate with DPM underneath it- means the timber might get wet but won't ever be standing in water.

 

[Damian Hemsworth]

I understand that if I use 9x2 I will be fine for the joists won't I?

 

[oldbutnotdead]

9 x 2 at 400 centres will do the span as long as you're not going mad with heavy machine tools or stuff like that. If you're getting the timber for free or cheap from somewhere then fine, if you're paying the going rate then do some sums before you commit to the timber floor. For any decent sort of floor sheeting (18mm OSB or ply or chipboard) you're at £5/sq m or so, 9 x 2 will be costing £2.50/metre or so, you need about 36 metres so you're at £170 before any ironmongery. Plus (unless you're building above ground level) you'll need to dig a hole to allow for the depth of the joists plus air circulation space- there'll be most of a skip load of soil there almost certainly and skips aren't free.

 

[theprinceofdarkness]

I built a shed on slightly sloping ground, difference over 12' was about 6". dug a trench below topsoil level, 12" wide and 6" deep. Built a shed sized wall from the stepped foundation. Put top soil inside the walls and rolled it well. I slung in any hardcore I could find. Topped the centre area with 4" of concrete.
I lined the top of the base with a DPC and laid my floor joists FLAT, so giving a small air gap and built the shed of these. In retrospect this was wrong as the floor was always cold, Doing it again I would have put the floor joists the "right" way up and included insulation under the floor. The shed had a medium sized lathe in it (400lbs) for 32 years and for its last five years a one ton milling machine without any problems.
Frank