Shed Construction – Foundation and Floor

I am currently writing some course notes for a shed building course that I’ve been asked to create for Holmesglen Tafe. The notes are kept pretty concise, so don’t expect pages of details, (they certainly could be – I could write a book on the subject- one day I might!)

Foundation Options

There are a number of options for the base of the shed, and what you end up choosing will be based on a combination of cost, location, and intended function of the shed. These obviously are closely related to the shed floor (and in some instances are one and the same). The forms that are available are obviously very closely related to the different options for house construction.

Dirt / Earth

In some cases, there may be no specific requirement for a shed floor, and therefore the foundations of the shed are thought to also be non-existent. This will very quickly lead to problems, and is definitely not recommended. That is not to say that an earth floor to the shed isn’t fine, and in some cases it is a good solution, but the foundation for the shed itself (under the walls) is still an important component.

A shed sitting directly on the ground is not only unsecured (and therefore highly venerable to wind damage), but is likely to corrode around the base very quickly. If you choose to have an earthen floor, you still need some form of foundation, whether this be on-grade, or concrete.

On-grade foundation

This form of foundation is particularly low cost, and very easy to construct. It isn’t strictly a permanent foundation, but never-the-less is quite suitable for shed construction. It involves placing the foundations directly onto the ground (a scoria bed, concrete or brick bed or earth) and on top of which is placed a treated timber frame (for the floor), or simply the shed walls themselves (and the floor can then be concrete tiles, or earth). A layer of damp course is strongly recommended, irrespective of the final choice. A bed of scoria or similar underneath is a good idea to aid drainage, and help prevent the foundation from sinking into the soil.

In any event, given the wind strength in Melbourne, it would be worth considering sinking a concrete pier in each corner (or pouring one) to have something to secure the shed to, rather than just relying on the weight of the shed to hold it in place in severe weather conditions.

Pier foundation

This is quite a traditional construction method, and is often used for houses, and decks. It involves sinking a grid of piers (concrete or treated wood) into the ground (often concreted into position), and then a wooden frame is constructed on top.

Concrete Slab

Another very common foundation (and floor) construction is the concrete slab. Even so, there are some different ways these are used (and laid). You can box up where the concrete is going to be poured, and once complete construct the shed on top, or take an easy route and build the shed on a basic form of on-grade foundation, then pour the concrete slab into the shed itself.

Sounds a bit rough and ready, but it can be effective, and you can be sure the shed isn’t going to be going anywhere after that! It is not my favourite solution. For one, it leaves the base of the shed particularly vulnerable to corrosion. On the other hand, the shed is secured down soundly, and it is very unlikely to have water penetrating under the walls.

Floor Options

This is closely related to the choices made for the foundations, and there are many variants available. These can include:

Earthen

The floor when you don’t need a floor. One advantage – cost.

Wooden Flooring

This can be floor sheeting (either a commercially available form such as yellow tongue, or simply particle board, or similar), or tongue and groove boards, or decking. In any case, some form of underfloor barrier is definitely recommended (builder’s paper or foil, or at least a roll of plastic). There are advantages to a raised floor – services can be run under-floor (electricity, plumbing, dust extraction (if the shed is to be a workshop) etc. Having under-floor insulation obviously helps in the variable climate conditions.

A particularly good choice for workshops etc when you are going to be standing on it for long periods of time – it isn’t as hard on the legs and joints as a concrete floor. On the other hand, it is more expensive, you need a higher ceiling for the space lost underfoot, and it isn’t able to sustain the sorts of loads that a concrete floor can manage. Some woodworking (and particularly metalworking) tools can weigh in around ½ a tonne.

Wooden flooring can be placed on top of a concrete slab, which does provide a bit of the advantages of both options.

Concrete Slab Floor

There are advantages and disadvantages to all the systems, and the concrete slab is no exception. It is generally very strong and able to cope with significant loads, and particularly if you are going to be moving items around. It is relatively cheap, and by using something like a dynabolt, items can be secured rigidly to its surface. On the other hand, it is harder to stand on for long periods, so some form of rubber mat or similar would be a good idea, and if you laid it yourself, it can be tricky getting it really level. If you overwork the surface trying to smooth it, you can also find you have concrete dust being produced for a long time afterwards as well. They are also quite cold in winter, and trap heat in summer. One thing that is often missed is to seal the concrete once it has cured, as moisture can still easily pass through the slab if this is not done.

Both my current sheds (and a previous shed) have concrete floors, because as much as a nice raised wooden floor would have been ideal, cost and functionality were also considerations.

There are various grades of concrete – 20MPa, 25MPa and 28MPa. The 28MPa is relatively expensive, whereas the cost difference between the first two is only a matter of a few dollars per m3, so I tend to err on the side of 25MPa. That, combined with rebar and a 75 – 100mm thickness results in a very robust shed floor.

5 Responses

  1. I wonder if you can help me? I have just ordered a plastic keter shed for my allotment. The allotments are prone to flooding so it will be better to have the shed higher up (50cm approx). Because of the flooding, I can’t have a concrete base under the shed, partly because the other allotment holders will not be happy (understandably). I am wondering if you could suggest any solutions. The other thing I need to consider is keeping the shed in place so that it doesn’t float away! The ground at the back of my allotment is higher anyway and I had a vague idea of building a circumference wall (incl. foundations) with breeze blocks and then filling the area with soil (?) and then hammering it down as hard as I can and then covering it with pea shingle. The wall would be slightly higher than the base of the shed to solve the problem of the shed floating away, but the plastic shed floor would be directly on the pea shingle. Does this sound at all sensible/realistic? Any other ideas?

  2. Hi Rachel,
    My first thought would be, if possible – why not sink a couple of poles into the ground as stumps (from treated pine or similar – designed for in-ground situations). These could be concreted in to given them extra support, which wouldn’t cause the same grievance with the other allotment holders, as they could always be dug out at a later date.

    For the rest of the shed, you could then use concrete blocks resting on the ground and run treated pine stringers between them that the shed is then built on. Ideally, you’d sink stumps for the whole area (at least the corners and center of each wall, plus one in the middle for a small shed), and again bolt stringers on top. Or at least, some stumps and then strategically placed blocks on the ground (or on a scoria bed) to support the other sections. Those that are in the ground are specifically to stop the shed succumbing to flood / wind etc.

    The shed will then be the required distance off the ground for the floods, and if it got to the point where the shed would be at risk of washing away, it is anchored down by the stumps.

    You can also get concrete stumps that you’d dig into the ground (and optionally concrete in). It is the same concept that is used for houses that are not on a concrete slab.

    Hope that makes sense!

    Regards,
    Stuart

  3. In college, the engineering machine shop had a wood block floor. I do not know how ‘deep’ it was, but they were approximately 4″x4″ end grain that showed through. There were some large machines (a 48″ x 20’+ engine lathe among others) but they were on concrete footings, and the wood went around the concrete footings.

    Some computer type people have approached me recently about making a ‘raised floor’ for a ‘hobby’ group. Basically something on .5m grid, like are used in computer rooms. I would not recommend it for a shop floor, but rooms where you would like to have a chase underneath the floors for cables or plenum (air ducting) might be a nice but different floor. Inexpensive ‘carpet tiles’ can be laid across the raised flooring squares to hide them too.

    Just a few thoughts on floors. I am considering building a shed, and the wood block looks interesting as a replaceable floor system.

  4. The shed partially built (with a stream near by) is sinking 3 1/2 ” on one corner, when that corner was dug it filled with water, 3 bug bags of concrete were poured in the fole 42″ deep, laid an 8×8 on top of that and then the rim joice nad bolts on top of fitting, it still sank, is there a solution besides tearing it down? Its 30 degrees this am and altho below freeze line, is there a solution to “right it” ?

  5. It sounds to me like you have water flowing the ground on the subsiding corner. Substantial subterranean flows are not uncommon, although they are rarely so close to the surface. I imagine the soil is sandy and highly permeable. If this is the case then you have no option but to rebuild elsewhere unless you have a very substantial budget for very deep footings.

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