I am in the process of ripping out an old tin shed and will be replacing it with a much larger block/brick shed (dimensions 5M long x2.5M wide x2.5M high), ground is compacted Sand Stone. the shed will have a shallow pitched roof. As this will store garden cushions etc, I was planning on building a double skin shed (inside skin from lightweight concrete blocks outer skin bricks with a 50mm cavity to stop the damp. Normal DPC membrane. Wall width including cavity 252.5mm Footing trench 1 metre deep, Footings 750 mm Wide, Footing Slab depth 252.5mm, I've worked this out that it will be cheaper to dig the footings and rather than filling the 1m trench with concrete (5:1) Ballast, Concrete. I would only fill the trench with the required 252 deep and then use blocks to get out of the ground. With this calculation I've estimated I would need 3 cubic metres of concrete. What I can't figure out is how many bags of Portland 25KG cement I'd need and how many bulk bags of ballast. Appreciate someone helping me out here. 1. to make sure I'm not going over the top with the footings etc and 2. with the calculation for the materials. thanks
Ridge, bearing in mind 3cu m of concrete is half a load of concrete from a commercial producer I would be going down the readymix route. Consistency of mix, all poured at once and speed. The only thing you need is good access but even if it means taking down a couple of fence panels and a post it's still worth it. If you insist on going down the hand mix route go to your builders' yard and ask them to do the calc. In my view you are going way over the top. I assume you're building a workshop or dry store?
Thanks for the response. I am considering readymix for all the reasons you mention. It's not going to be workshop just s shed with garden furniture cushions my wife wants kept dry. However, I believe I'm using specifications for a general building. I am considering single skin after working out how much the footings are going to cost. Any suggestions on footing dimnensions ? Thanks JB
It is very overkill for a structure to store garden cushions, it would be as strong as most houses and 5m x 2.5m is quite large for that purpose The main reason to build a cavity wall structure in the garden would be if you wanted it to be habitable in the future, in which case you should put insulation in the cavity. Otherwise you could build a single skin structure and just waterproof the outside. Similarly with the floor slab, 100 mm should be more than sufficient Why are you going down 1m and then building up ?
I think your right and maybe I'll look at single skin, block work as "as you say", its only a shed. thanks for the advice on the slab. my intention was to use the broken up concrete for the old slap that it replaces as hard-core, cover with soft sand and then put a DPC sheet down and then 150mm slab. Will reduce that down to 100 if that's overkill Reason for going down 1 metre was based on what I had read about load, if it doesn't have to be 1 Metre Deep than that's also good. Saves time/effort. How deep is recommended for this type of structure ? Cheers
When building houses on places with lots of ground heave and to get below the frost line, we start below well below the ground level and then build up but of course then the trenches are quite wide so access to the first course is easier. The width of the foundation should be a minimum of 100mm wide each side of the wall and the top a minimum of 150mm below the ground. The depth of the foundations differ due to the ground conditions, if you have stone underneath then 300-600m will be sufficient. You could go thinner with the shed floor but the savings are minimal. What we suggest these days is to put in a bag of fibres/filament into the concrete mix. What this does it replaces the need for metal mesh and also reduces the risk of the concrete cracking as it cures and its only a few pounds a bag. I don't know if you have seen this site but there is lots of good pictures and diagrams http://www.pavingexpert.com/featur03.htm
Seems OTT to me, have you looked into a log cabin or other wooden type structure? You can get double walled log cabins (at nearly double the price of single wall).
450 is wide enough for a house foundation, 1000 deep would work in clay and you have "compacted sandstone" As said above overkill. If you shallow fill then don't forget you need to backfill propery
thanks for all your comments. Have decided to build it in a single skin of 102mm Lightweight concrete blocks. Dug a few test holes and the ground is firm. looking at footing dimensions as follows: trench depth depth of 600mm, footing width 302mm wide (102mm for Block, 100mm each side), Footing depth, 150mm. that reduces the concrete down to 0.6 m3
I've got strip foundations going in for a single story extension, they are 600mm wide and 1.5m deep. 100mm block, 100mm cavity, 100mm brick and 150mm left each side as required. I've got readymix coming, to fill the trenches. Worth the money considering you get the quicker speed of laying, it's consistent, and fairly easy to tamper.
Forums
Facebook
Twitter
You Tube
Instagram
Shop