The garden shed foundation is the most important and the first step in building a secure structure. The foundation of most Gambrel, Gable, and SaltBox sheds may be as simple as laying out a few concrete blocks but then again just laying down concrete is not going to prevent water from getting it, it also does not tie down your timber structure well enough to hold off against a storm.
How To Start?
Our first tip for almost all newbie garden shed builders is to use timber pegs measuring around 2”x2” which have been sharpened at the end. Add a nail into each peg’s center and setup a perimeter around your intended location. The shape should be rectangular and you should double check it by measuring the area diagonally. If the dimensions are equal across each corner then it’s a square.
You can use either optical levels or sight rails to control the level of your excavation. For such a small scale excavation it is best to keep things simple by using a spirit level along with 4×2 timbers. The initial strip can be done with a spade up to a depth of around 50mm.
Ideally, you will want the finished depth to be 75mm in most cases, and a 4×2 peg can be used to measure the ground level. Cut a small piece of timber of this length and continue your excavation and keep checking with the 4×2 and spirit level every now and then to make sure you do not go too deep.
Most people make the mistake of using low-grade stone to lay down the garden shed foundation. The material we recommend using is a ‘well graded material’ based foundation. This will have a good mixture of small stones measuring 40mm and some finer ones. You can also use pulverized concrete or crushed stone. However, be careful not to use gravel for the base or one large aggregate as these may not lock together.
Adding The Crushed Stone Layer
Prior to compacting all the stones for the layer make sure to level everything first. A vibrating plate can be used to compact the stone or you can use a simple hand tamper. A hand tamper has a 20lbs weight and measures around 4”x4” which you will need to raise and then drop many times in order to compact the stone. You will also have to continue checking the levels every now and then during the process.
Adding The Pressure Treated Wooden Foundation
The timbers you lay down should be perpendicular to the floor joists of your shed floor. You should use 4×4 inch pressure treated timbers which will allow for ventilation under the shed. We recommend pressure treated timber because it lasts long, plus adding stone prolongs the life of the timber you use. The spacing between the timber bearers you lay down will differ so you’ll have to refer to the blueprints you’re using for precise measurements. The floor joists should 2×4.
The foundation is a very important part of building a garden shed. The emphasis should be on laying down a solid garden shed foundation and one which is not easily damaged by water.
Learn more about shed plans and building foundation from Ryan Henderson the author of My Shed Plans.