A concrete shed base is without a doubt the strongest and most durable form of shed foundation. It is also the most costly and labour-intensive (however it should be remembered that the foundation can also be used as a concrete shed floor).
This article gives you the information you need to decide what type of concrete foundation is right for your site and how to build a concrete shed base. This article outlined the principle that shed foundations (in contrast to house foundations) are allowed to move! As concrete foundations are at the most robust end of the scale you should make a choice if the foundation is to be a 'floater' or solid.
Apart from the small section below the rest of this article is about 'at grade'/floating foundations.
Solid/frost proof foundations.
Design of permanent foundations is a skill that requires knowledge of local ground conditions, the causes of foundation movement and local methods of construction that ensure movement does not occur. Because of the wide variety of these factors throughout the world I do not intend to cover in detail this type of foundation in this article.
If it is critical that your foundation does not move I recommend that you consult a local Structural Engineer, who will have knowledge of the most appropriate type of foundation for the prevailing soil in your area. The foundation design will cost a small amount. This reminds me of a cartoon I once saw of a couple of Renaissance Italians standing in front of the leaning tower of Pisa saying 'I guess we should have paid the extra 1,000 Lira for a proper foundation design'.
A floating concrete foundation is typically four inches thick (100 mm), which despite moving slightly should last as long, if not longer than the shed built on top. The slab can be plain (un-reinforced) concrete or a light mesh of steel reinforcement can be added for extra strength and to control cracking.
There is no need to build a separate shed floor.
It can be hosed down if it gets dirty or wet.
The floor can be built closer to ground level, thus minimizing the need for a shed ramp.
A concrete shed base is very durable, damp garden machinery can be left there and there is no danger of the floor rotting as a timber floor could.
A disadvantage of concrete floors is that they can be cold on the feet if you are using them as a work shop or hobby room. If you intend to use a shed with a concrete floor for these purposes then a timber sub-floor or an insulated screed will make the floor a lot warmer.
The main items of work are:
3. Mixing concrete
4. Placing and finishing the concrete
If the area of your shed is firm and dry then remove the topsoil to a depth of 100 mm. Place a layer of stone to a depth of 50 mm. If the ground is not so good you may wish to increase the depth slightly or in local areas to get rid of soft spots. See here for tips on setting out the excavation, selection and placing of the stone layer.
The formwork for a concrete shed base will most likely be 18 mm (3/4") plywood with 50x50mm timber stakes knocked into the ground at two feet (600 mm) centres. The formwork should be set up off of the level stone/earth base, so that the top is level all the way around and that none of the posts stick up above the top of the formwork. The formwork needs to be sufficiently robust that it will be able to contain the wet concrete and act as a guide for the timber straight edge tamper when it comes to leveling the concrete surface. The area of the pour should be lined with a PVC Damp Proof Membrane (DPM). The purpose of this membrane is two fold;
- To stop loss of water from the wet concrete into the ground during pouring (this would weaken the concrete)
- To keep the contents of the shed dry by stopping water from the ground penetrating up through the finished floor.
The specification for the polythene DPM will be 1200 gauge, the brand-name 'Visqueen' is commonly used in the UK. If laps are needed in the membrane these should be a minimum of 350 mm and sealed with a suitable tape obtained from a builders merchant.
3. Mixing concrete
The main methods of getting concrete are; mixing your own or getting a ready mixed supply in.
If you are mixing your own, hire a concrete mixer for the day, it will make this heavy work a lot easier. Each cubic metre of concrete weighs about two and a half tonnes. A 10'x 12' shed base four inches thick will have over a cubic metre of concrete in it, so be prepared for some work here!
Concrete is a mixture of cement, sand (fine aggregate) and coarse aggregate. Mixes vary in the proportions dependant upon the properties required. For concrete for a shed base order 'ballast' from your builders merchant,this is pre-mixed fine and coarse aggregate. Mix the ballast with cement in the ratios recommeneded by the supplier, 1:5 by volume is typical. Add water to the mix in small amounts, keeping the mix on the dry side (a sloppy mix will be hard to handle and result in poor quality concrete.)
Alternatively contact your local ready-mix supplier and order a suitable truck mix. Ready mix concrete suppliers have a range of concrete mixes, for a shed base specify a minimum of C20. This means that it will be designed to have a crushing strength of at least 20N/mm² after 28 days. Be prepared for a bit work transporting the wet concrete to the site and consider what ramps etc you may need to get the wheelbarrow up steps etc.
4. Placing and finishing the concrete.
Place the concrete from the barrow starting at one corner and working outwards. The con crete should be placed evenly and slightly proud of the formwork. The concrete is finished using a timber straight edge (4"x2"). Starting at one end of the slab, level the concrete off moving the edge side to side as you progress. With the concrete shed base leveled off you now have a 'rough tamped' finish.
When the slab is complete, cover with polythene or damp Hessian sacking. The purpose of the covering is to keep the moisture in. Concrete does not dry, it 'cures', if the surface becomes dry during the early days (first 3 days) it will become dusty and weak.
Concrete gains strength slowly. The longer you keep the covers on the better. The concrete will be stiff the next day but the surface will mark if you walk on it. Concrete gains its design strength after 28 days (this is a standard strength test on commercial concrete pours) however after seven days it will be fine for the next stage of construction.With the concrete shed base finished you are now ready to start building the rest of the shed.
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