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Whether you buy a pre-fab kit or build from an original plan, your backyard storage shed needs a solid, square foundation. The good news is you don’t need advanced skills, special tools, or lots of money to build one, just the patience to plan properly and do the job right.
This article describes two different types of foundations for a small to medium-sized shed: an on-grade foundation that sits on top of the ground, and a permanent foundation that uses concrete footings or piers that extend below the frost line.
Either one will work. Here’s how to build them and make the right choice.
Concrete piers and beams:
On-grade Foundation: Concrete pavers:
On-grade Foundation: Timber frame
On-grade Foundation: Concrete piers and timbers
Permanent Foundation: Concrete piers and beams:
Permanent Foundation: Concrete slab:
Find out whether you need a permit and a site inspection to build a shed on your property. Local building codes may require them even for a small shed. If you build without first obtaining permit, you may have to move your shed or take it down completely.
Before you begin work, call local authorities to identify utility lines near your building site.
Plan to devote one full weekend (or one-third your total project time) to getting your building site ready and constructing the foundation, especially if you’re using concrete piers. They take time to dig, level, and square, and for the cement to set.
If you’re working from a shed kit or plan, the manufacturer will recommend a foundation type based on the size of the structure you’re building. Heed the manufacturer’s suggestions.
Over time, an on-grade foundation can shift out of square or level especially in areas with significant or prolonged freezes. It’s possible to jack up the building and shim or adjust the blocks to level the foundation, but it may be difficult to make the shed as square as it was when you built it.
If at any point you’re unsure of what you’re doing, call a qualified professional. The money you spend will be returned in the benefits of a quality job.
1.1 Clear your building site of all shrubs, trees, rocks, and stumps. Protect nearby landscaping, pavement, and other structures from damage during construction of your shed.
1.2 Outline the shed perimeter with stakes and string.
1.3 Use a line-level to level the string.
1.4 Measure the distance between the stakes, making sure the diagonal measurements are equal.
1.5 To reduce weeds and improve drainage, excavate sod and soil inside the staked perimeter to a depth of 4".
1.6 Cover the area with landscape fabric.
1.7 Top with 4" of pea gravel. This can be compacted with a mechanical compactor.
This simple, economical foundation also becomes the floor of your shed. Best for: Level, sturdy soil; lighter-weight structures without a floor kit.
2.1 Add a uniform 1" layer of dry sand/cement mix (one part cement to eight parts sand) on top of the pea gravel at your prepared site. Rake level.
2.2 Lay down concrete pavers within your staked perimeter, starting from one corner. Leave a 1" gap between the pavers.
2.3 Level and adjust each paver as necessary using a piece of scrap wood and a rubber mallet.
This foundation is a frame of pressure-treated timbers resting directly on the gravel of your prepared building site. It gives you a choice of floor materials. Best for: Level, sturdy soil; lighter-weight structures without a floor kit.
3.1 Cut 4" x 4" timbers to length, matching the perimeter of your shed.
3.2 Join with half-lap corner joints or stack two timbers high. Fasten the timbers together with landscaping spikes or lag screws.
3.3 Square the timbers and level with shims.
3.4 Fill the area within the timber frame with gravel, concrete pavers, or other floor material, or cover with 2" x 6" pressure-treating deck boards.
This foundation is a series of pressure-treated timbers supported by concrete blocks set directly on top of the ground. It’s the simplest type of foundation to build. Best for: Sturdy soil, smaller structures, slightly sloping grades, warmer climates with little frost heave.
4.1 Set pre-cast concrete blocks in place around the shed perimeter so their outside edges align with the string. For a small or medium-sized shed, you’ll need at least eight blocks plus one for the centre, or three rows or three blocks each, to support your shed floor.
4.2 Lay one pressure-treated 4" x 4" across each row of blocks.
4.3 Level the blocks. You have to level the top of the block itself and also level the blocks to each other. If a block is too high, remove dirt from underneath it; if you need to add back material, use more pea gravel.
4.4 Fine-tune with a 4' or longer level. Take your time: an out-of-level foundation will create problems later. Tip: An asphalt shingle makes for an excellent shim that holds up well to moisture and temperature changes.
TIP: Level a foundation on a sloping grade using a combination of concrete blocks, pavers, and pea gravel or insert a pressure-treated 4" x 4" to elevate the structure.
Concrete piers provide a permanent footing that resists the effects of frost heave and soft ground. The piers extend below the level where groundwater usually freezes in the winter, called the average frost-line depth. The timbers attach to the top of the piers. Best for: Most soil conditions, small or medium-sized structures, areas subject to frost heave.
5.1 Determine the exact location of the concrete piers so the outside edges of the 4" x 4" timbers are even with your perimeter string. Like the on-grade concrete block foundation, you’ll need at least eight piers plus one for the centre.
5.2 Use a posthole auger to dig holes 8" to 10". Your local building office can tell you the average frost-line depth for your specific climate and soil composition, telling you how deep you need to dig.
5.3 Tamp the bottom and lay a bed of 0-¾ crushed stone 6" deep at the bottom of every hole for drainage.
5.4 Insert a form tube into each hole and cut it so it protrudes about 6" above soil level.
5.5 Plumb the inside of the form so it’s straight.
5.6 Backfill each hole with dirt.
5.7 Mix cement and fill each tube completely. Before the cement hardens, place the footing on the surface or insert a J-bolt into the centre, leaving 1" of thread above the surface.
5.8 Use a line level to make the footing or the top of bolts the same height.
5.9 After the concrete has set, if you used a J-bolt, attach post-base hardware with the side plates oriented so the 4" x 4" beam can rest on it.
5.10 Repeat these steps with each pier to complete the foundation.
15.1 Measure the diagonal to ensure the foundation is square.
NOTE: Set the piers and bolts in the four corners first. It’s easier to make sure your foundation will be level and square. Don't worry if the tops of the footings are not precisely level with each other. You can use shims to adjust the level of the beams as needed.
A slab foundation is a flat, concrete pad poured directly onto the ground. Best for: Larger structures on level sites. Susceptible to shifting or cracking when the ground freezes and thaws. If you’re considering a slab foundation, talk to a concrete contractor about building techniques and materials that are designed to withstand freeze/thaw cycles.
6.1 Prepare your building site as in Step 1, except excavate sod and soil inside the staked perimeter to a depth of about 15" but do not cover with fabric or pea gravel.
6.2 Build a strong, steady form using plywood and 2" × 3" wood beams nailed diagonally to the outside of the form and resting on the ground to provide support.
6.3 Nail a foundation sill of 2" × 4" lumber directly to the frame. The sill should normally be at the same level as the slab.
6.4 Lay a bed of well-compacted sand at the bottom of the form.
6.5 Cover with a sheet of plastic.
6.6 Build a steel rebar perimeter and mesh to reinforce the concrete. Lay the rebar and mesh on either bricks or rocks so it settles into the centre of the concrete slab. Once the concrete has been poured and has set, the rods will be permanently imbedded in it.
6.7 Pour at least 10" of concrete over an area of about one to two feet around the inside perimeter of the form, reducing gradually to 4" to 5" at the centre of the slab.
6.8 Drag a straight 2" × 4" (screed board) across the top of the forms to level the concrete.
6.9 Use a bull float to smooth the slab and bleed the water out of the concrete. Float a trowel for a smooth finish.
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