Bases for Garden Buildings - Basic

Garden Building Base Advice. 

One of the most important things to get right for your new garden building is the base. This is especially important for the larger building due to the weight. This is critical when dealing with log cabins and some of the larger summerhouses or wooden garages.

The main points to make sure are correct with all bases, no matter the size of building going onto it are:

  1. It must be flat, level and square and not be susceptible to subsidence at a later date.
    The entire area of the building must be supported across its floor, in the case of log cabins this refers to the foundation beam running the perimeter. If a floor is being installed this must also be correctly supported. 
  2. It should be at least the buildings footprint and where possible exactly to it, this will mean that any rain fall will drain away in to the grass/gravel surrounding the base rather than hitting a solid base and splashing up against the side of the building. Our show site at Woodmeadow Garden Centre in Northampton is an excellent example of this, the main show area is concrete or slabs and all our buildings need very regular treatment and cleaning because rainfall splashes up against the bottom. The other advantage of a shed base to the exact footprint is that no water will be allowed to sit in any dips below the building which may cause the floor to rot over time.

There are three main types of bases that can be used; these are listed below with some example products to give you an idea of the size of the building that they can accommodate:

Timber Bearers Base: this is an easier less permanent solution and normally cheaper than the slab or concrete alternative. Similar to decking frame base we would use timber lengths secured to piers to create a sturdy level platform on which to assemble your building. Make sure you use pressure treated timber which is guaranteed against rot for longevity.

Paving Slabs Base: This is a simple and popular method and good if you want to be able to move the building in the future. The base can be removed with minimal disruption. If the groundwork is done properly this can support some of the larger buildings.

Concrete Base: This is always the best method especially for log cabins and the larger summerhouses. This method is always recommended if you are having a larger log cabin where weight is a consideration.

Depending on what building you are having or its eventual use, it is also a good idea to incorporate a damp proof membrane into the base. This is usually applied in the sub base.

Timber Bearers Base: This is the simplest and cheapest method but is only really suitable for the smaller sheds and summerhouses

This method is simple. First remove any overhanging branches, hedges, bushes etc that will affect your new shed. The area must be firm, level and square and where possible to the foot print of the building. Mark out your area using pegs and string and then clear the vegetation, dig down to the required depth (usually about 3”-6” depending on the bearers being used) and level the area using a spirit level, make sure it is well compacted if you have disturbed a lot of soil. Once the area is level you may want to consider laying a weed suppression blanket. Lay your bearers as required onto the level ground checking them with a spirit level across both planes; you can also consider the use of bricks or slabs to lay the bearers on if there is a risk of movement over time. We will always give you advice as to the bearer configuration but for our own Taylors manufactured sheds we have bearers at either end and then space them about a foot apart to ensure the entire floor is supported through its length. The best bearers to use will be pressure treated of 6”x2”, 4”x2” or 3”x2” dimensions, you can of course use any size you fancy, numerous people find old sleepers to be good but these are quite hard to level.

Paving Slabs Base: Paving slab bases work very well for most buildings but it can be tricky to get right. There are a couple of ways to do it. One way is where the entire footprint is covered in slabs, this would be the case with a log cabin. The second way is to space them out at right angles to the floor joists, and spaced evenly, normally about every 2 ft, this way less slabs are used.

A special consideration for this is to make sure there is a row centrally when a two piece floor section is used, if you are unsure whether your new shed is this way configured please ask us, we’re always pleased to help.

Mark out the area and clear the top layer of vegetation. Make sure the area marked is totally square and as level as possible. A good way to check the area is perfectly square is to measure across the diagonal. Each measurement should be identical.

As with most things there are alternatives. The alternatives are a dry sand/cement or a hardcore and wet cement. It depends greatly on your preferences. For this article we’ll stick with the dry mix.

Compact the ground as best as you can and make sure it is as level as possible, lay a layer of dry sand/cement normally a couple of inches is fine. Compact this layer and make sure this is as level as possible, back to front and side to side. Start in a corner and lay your first slab – this must be level in all planes, failure to get this correct will throw the whole base off. If a corner is too high, remove a little sand / cement, if too low add a little and use a rubber mallet or a piece of wood to bed it in. Lay the other slabs with as much care and using the same system taking measurements from the first slab. It is best to do these a row at a time. The slabs must be perfectly level across the area to ensure the floor of your shed is supported throughout its length.

Concrete Base: This is by far the best method and will give superb support, this really is the only method for the larger shed and summerhouses especially if they are odd shapes such as corner summerhouses or octagonal summerhouses. This is essential on the larger cabins such as our range of residential log cabins. The only real downside with concrete is if it goes wrong it is extremely hard to remove.

As a rough guide we tend to recommend bases of a depth of 3” for normal sheds or summerhouses such as a 10’x14’ shed. For log cabins or wooden garages a depth of 4” on a layer of compacted, finely broken hardcore is preferable. Ideally half the base should be above ground.

Start by clearing the area as above. Peg the area using piece of wood set to the outside dimension, set them at each corner and then at about 2’ apart on the sides. Attached to these is “shuttering” ideally 2” thick and set to the depth of the finished base. A couple of pointer here; make sure the corners are totally square and that it is totally level across the top of the shuttering both side to side and front to back, also make sure the pegs you are using do not protrude above the shuttering – it will make it extremely hard to level.

Depending on the size of base, you could go for the easy option or the slightly more labour intensive; if it is a large base say for one of our Bespoke Garages (the largest we’ve done so far is 45’x30’) you would be best to order in a cement lorry with the correct load, they can poor it in while you level it. For smaller bases it may be more economical to mix it yourself.

If you’re going to mix it yourself it’s always best to be as close as possible to the base so you can shovel it in or alternatively use a wheel barrow for distances. Make sure you use a consistent measuring container for the amounts needed, a 3 gallon bucket is ideal. If the base is quite large it may be an idea to hire a cement mixer. An ideal proportion for the mix is 1 bucket of cement and 5 buckets of “all in 20mm”. This is a ballast made from gravel (20mm means the average size of the gravel) and is available at any builders merchant. “All in 20mm” is normally sold in bags of 40kg. 1.25 bags are needed on average for 1 cubic foot of concrete. As a basic rule calculate the volume required and then add a third for compaction and then add a bit as the worst thing is to be under. It is quite hard to work out the exact amount required due to the undulations of the ground so always over estimate. For an 8’x6’ shed we would recommend about 20 bags of “all in 20mm ballast” the ratio of cement to this is 1:5 so you would need 4 bags of cement.

Mix your concrete using the ratio above and gradually add water, don’t make it too wet as this can weaken the mixture, it needs to be wet enough to workable and fluid, keep mixing until the concrete is uniform in colour.

Start laying the concrete with a layer across the bottom, tamp this down and make sure it’s pushed into the corners, tap the shuttering to ensure a solid edge. Keep laying more and more layers. Starting from one end, use a beam that will cover the entire width across the top of the shuttering. Use this to level the concrete to the top of the shuttering, this is done with a sawing motion, carry on across the base area leaving the concrete flush with the shuttering. Once completed, a finish can be applied to smooth the surface using a wooden or plastic float; a special machine can also be hired to do this. Don’t try to smooth it until the concrete has gone off a little.

You will need to allow about 3-4 days for the shed base to set completely, remove the shuttering and finish the edges with a layer of pea shingle or grass as required.

If you are doing this job in the winter you may need to consider protecting the wet base from frost as this can damage fresh concrete significantly. Cover the area using plastic to give it some protection.

Another small point to consider with concrete bases is if it is very large it is an idea to introduce expansion points within it, normally only needed when above about 12’x12’.

I hope the above is useful to you, if you are at all unsure get in contact with us and we’ll be pleased to help or you may want to consider hiring a professional to do the job. There are lots of resources across the net that may also be of use to you.

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