How deep concrete for shed base? Backyard Shed Slab

Wondering how deep for a concrete shed base?

Generally, when asking how deep concrete base for shed should be, a safe answer for most sheds should have 100mm or 4″ of concrete. In some cases, you can go with less concrete, such as 75mm or 3″ for small sheds, sheds with no actual structure, such as prefabricated lightweight sheds from hardware stores and or online sources. However, for more serious sheds, where a wood frame construction is being provided and are more of a permanent shed structure for your home, 100mm (4″) of concrete is ideal.

For a shed concrete type, the typical concrete bags from hardware/home improvement stores are sufficient.

This can be a 27 MPa concrete cured at 28 days for example. That is plenty of strength for most typical garden sheds, backyard storage sheds, or manufactured shed products.

Because sheds don’t carry much weight, we are not worried about catastrophic, potentially very expensive structural concrete damage, like that which can happen with an improper home foundation wall or slab on grade. Rather, we just want to prevent premature failure a few years down the road, such as cracking or lifting problems.

Concrete shed base sub-grade preparation

With small concrete sheds, it is generally okay to place the concrete on compacted earth.

This depends on the location and quality of the soils. Also, if there are any significant drainage issues, you will want to address those. Any erosion of the subsurface soils under your concrete shed could cause structural instability.

PHOTO ABOVE: Concrete Calgary Pad Installation Service – Deluxe Concrete Services – this pad is structurally designed with rebar and sub-grade prepared properly. Note the rebar chairs provide adequate concrete cover for up to 150mm (6″) concrete slab. This is more than what is typically required for a small home garden shed.

To ensure the structural soundness of your backyard shed slab for years to come, you need to make sure your slab base is prepared properly. This is also the case if you have drainage issues.

With existing soils you need to ensure that the base is very firm, dry, and compacted soils.

Crushed gravel sub-base

Another approach for the average homeowner is to excavate a small footprint for the shed by hand, approximately 1′ beyond the edge/border of the shed and approximately 1′ down.

You can place a filter fabric at the bottom if you want, but it is not necessary.

Then, if you can get some crushed gravel, place it in lifts into the bottom of the excavated area. A lift is a term that construction companies, professional Calgary concrete companies, and excavation companies use to indicate the amount of build-up to be placed for compaction of materials like crushed gravel.

A lift of about 3-4″ is going to be fine here. So, at the foot of depth, this is about 3 or 4 lifts.

Compact your sub-grade lifts

Compacting sub-grade lifts for a small shed can be done with a hand tamper. The hand tamper is forcefully brought down onto the crushed gravel mix you placed in the base of the excavated shed footprint.

To help ensure the sub-grade gravel / crushed gravel is adequately compacted you will want to water the gravel slightly.

Do not saturate the crushed gravel mix, but make sure you get enough water so that your hand tamping will significantly create a visibly smooth or hard surface for the crushed gravel.

Once satisfied with the crushed gravel compaction, move on to the next lift and repeat the process.

Having the subgrade base excavation slightly beyond the shed concrete pad footprint will ensure that the compaction does not reduce the pad’s overall surface area. This way, there will be plenty of room for the actual shed footprint.

Reinforcement & reinforcement cover

Depending on your shed’s structural requirements, there are various options for reinforcement. This really depends on the size and seriousness of your shed. For instance, for a small shed, such as a remanufactured unit, if you even have a slab at all, you can use WWM(welded wire mesh). This product is lightweight, easy to handle, affordable, and can be overlapped to provide more reinforcement over just one sheet.

Structural reinforcement like that can help ensure the slab does not crack unnecessarily under heavier loads that might be stored in the shed.

Alternatively, you can go with actual rebar.

4″ or 100mm is not that deep of a concrete shed base however it is structurally suitable.

As with any concrete slab installation, you will want to put adequate reinforcement. You could use 15m rebar in the slab, placed at 600mm O.C. or 400mm O.C. for example.

Then, you would want to run rebar around the perimeter of the shed concrete slab.

Make sure the rebar is tied off and supported from the base.

The distance from the rebar to the concrete is known as the cover. Ideally, for a shed, you want the rebar in the middle of the shed slab thickness so that there is less chance that reinforcement will ever be exposed to open air and rust.

Concrete cover

Maintaining concrete cover is essential to ensure your reinforcement does its job. Double-check the concrete cover for your shed slab prior to pouring the concrete.

Ensure proper spacing around all rebar or welded wire mesh and that nothing pokes out of the concrete when you pour. Properly tying the reinforcement off to formwork or

Slab thickening

For super serious sheds with more structural requirements, you could thicken the concrete around the edge of the shed concrete slab.

This might be necessary, let’s say, if you have a small ramp or concrete slope up to your concrete slab.

Conclusion – How Deep Concrete Base for Shed Should be:

Concrete pads for garden sheds, tool sheds, and backyard sheds can be 3″ or 4″ (75mm to 100mm) in depth/thickness and poured using typical concrete strength from local hardware store bagged concrete products.

This is more than enough strength for a shed to ensure it is structurally sound and stable for years to come and that the concrete pad does not unnecessarily deteriorate or crack prematurely.

You also want some light reinforcement.

Ultimately, a garden shed slab is not very heavy, so you could pour it on compacted earth in some circumstances. However, if you want to safeguard the concrete against cracking, shifting, or sub-grade erosion from drainage issues, a crushed gravel base is best.

Then, if you want to tie your garden shed slab to adjacent concrete sidewalks and prevent surface elevation differences, you may also consider a crushed gravel compacted base.

Good luck and have fun!