Small outbuildings do not legally require foundations. However, a good base will help to improve the performance, life expectancy and appearance of your new timber building. Since a shed, workshop or summerhouse is a costly purchase and may sit on your property for decades, it's worth some effort to make sure that it lives up to its potential.
This article is the second in a series giving step-by-step instructions for preparing a base for a timber outbuilding. Ideally, a shed base needs to
For a more detailed discussion of these concerns, please see 'Shed Bases - Four Things to Think About.' For a few tips on planning for your shed if you will not be able to put significant effort into a base or foundation, please refer to 'No Shed Base? Quick Solutions.'
Many people who plan to install a shed and do not have an existing paved area, patio or wooden deck to set it on and will probably choose to create a base from gravel and pavers. This is a fairly easy project to do yourself using materials from a paving supplier, builder's supply or DIY superstore.
First, determine the size, shape and nature of the bottom of your shed or outbuilding. Then, add 20 cm to the length and the width. That's the size of your gravel base. Why is it slightly larger than the bottom of the shed?
To allow water to drain into the gravel under the shed, rather than getting the shed base wet. The water that collects around the gravel will eventually drain down into the soil below. The gravel should be 15 to 25 cm deep. You can calculate the volume by multiplying: length x width x depth = volume Easy. Add a little extra, just in case you dig a little too deep in places.
Now, take a look at the area where you plan to set it. Is it level? What type of soil is already there? If the soil doesn't drain well, then you should add a greater depth of gravel.
Obviously you're going to have to dig out this area before you add the gravel, but there are a couple of other things you should think about finding, first. It's a good idea to buy a piece of geotextile that's big enough to fit under the gravel both on the bottom and at the sides.
That will let water drain through, while keeping your gravel free of dirt that could clog it up and prevent it from providing good drainage. To determine the right size for the piece of geotextile cloth, just take the size of your gravel area and add two times the depth to the length and width. You will also need to figure out your tie-down strategy.
Depending on how they work, the tie-downs will be added either before or after the geotextile, and they should be placed so that it will be easy to attach them to the timber building, either at the sides, at the base, or through the floor. You can use spikes or corkscrews driven into the ground below the gravel, or you can use tie-downs with broad bases that will use the weight of the gravel and pavers to help hold the timber building down.
Obviously, that will work best if the gravel is fairly deep and/or the pavers are heavy. Finally, you'll need some course sand or fine gravel for directly under and between the pavers. It should be fine enough to create a level surface and to fit between the pavers and course enough to allow drainage.Alternatively, you can use a fine gravel for the whole thing.
If the area isn't level, then think about adding a mini-wall on the high side and creating a path for water to drain around the edges of the shed base.
Once you've dug up the base area, laid down the geotextile and added the gravel, you'll need to make sure that the gravel is level. You'll need a long, straight piece of wood and a long spirit level. Long levels are expensive, but they can often be borrowed rather than purchased. Finally, add your pavers to the top.
Make sure you leave spaces between the paving tiles, and fill the spaces in between them with fine gravel. You can use concrete or clay paving tiles, bricks, or anything else that can give a reasonably flat, strong surface.
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