Do you want a solid stable foundation for your shed? Something you could park a car on if it would fit through the door? If you know how to pour a concrete slab foundation for a shed, that’s what you’ll have.
You’ll have a permanent foundation with a smooth level floor on which to build your shed. It will also easily anchor your shed too.
All you need to do is stake out your site, remove the topsoil, build and level your forms, spread and compact 4” to 6” gravel, lay out some rebar, and you’re all set to pour your concrete slab.
I know it’s not quite that simple, but it’s also not that difficult if you follow the steps laid out below.
In this article, I’ll walk you through the steps so you know how to pour a slab foundation for a shed. I’ll discuss some pros and cons, the materials you’ll need, and give you some helpful tips along the way. Hopefully, you’ll find this article a concrete addition to your shed foundation repertoire.
- Do I Need a Foundation for My Storage Shed?
- When to Use Concrete Slab for a Shed
- How to Pour a Concrete Slab for a Shed in 12 Easy Steps
- Step 1: Planning for your Slab
- Step 2: Buy Materials
- Step 3: Prepare the Slab Construction Site
- Step 4: Mark the Location of the Base
- Step 5: Dig Out the Foundation
- Step 6: Build Strong Concrete Form
- Step 7: Spread the Gravel
- Step 8: Install Rebar Net
- Step 9: Pour Concrete
- Step 10: Level Your Concrete Slab
- Step 11: Finish and Edge the Concrete
- Step 12: Ensure Proper Concrete Curing
Do I Need a Foundation for My Storage Shed?
All storage sheds should have a foundation; whether pier, gravel or concrete. The foundation keeps the shed off the ground, reducing the transfer of moisture from the ground into the shed material and its contents to prevent mold and rot.
It provides a level, stable base on which to sit or build your shed so it lasts longer, and it can be used to anchor your shed from wind damage.
When to Use Concrete Slab for a Shed
A concrete foundation is a permanent foundation, which will withstand frost movement if properly constructed. It is ideal for flat ground but can be constructed on the sloped ground with some extra material and work.
If you need a solid, stable foundation that will support a great deal of weight, concrete is your best bet. A concrete slab foundation is also excellent for ground level access.
This foundation works for small 2ft x 4ft generator or recycling sheds up to 20ft x 40ft storage sheds, or larger. You can place plastic or metal sheds on a concrete slab and also anchor them directly to this foundation. It is an excellent foundation and anchor for a prefabricated shed, or on which to construct almost any style of shed too.
A concrete slab foundation has some limitations. It is a permanent foundation which means it is heavy and doesn’t move easily to another location, especially if it is large.
It requires a compacted gravel pad under it and a stable soil structure too, so it’s not recommended for wet soft soils unless you’re planning to do a lot more work.
- Concrete foundation makes a great shed floor
- An easy floor to sweep and wash
- Supports heavy loads
- Ground level access
- A solid, stable foundation for any shed style or material
- Can anchor your shed directly to it
- Keeps wood off the ground reducing the chance of rot
- Can settle into the ground over time
- More expensive than other foundation options
- Can crack if poorly built
- It is a permanent foundation, hard to remove
- Expensive for a shed
To Mix and pour, or order in.
The size of the concrete pad and your budget are usually good indicators whether to mix the concrete yourself or order ready mixed delivered. If you feel in need of a really good work out, and the pad is small, you may feel up to doing it yourself. If it’s a larger pad, you may want to order it in.
A 60-pound bag of concrete mix usually makes ½ a cubic foot of concrete and an 80-pound bag a bit more. It’s a lot of lifting and mixing to do, even if using a portable mixer.
By hand, in a wheelbarrow or mixing tub, it can be back-breaking labor. Rented portable mixers are usually electric or gasoline powered and will mix 2 to 18 bags of concrete mix. You’ll still have to lift it into the mixer and then move it to the concrete pad site too.
A cubic yard of concrete will do approximately an 80 square foot 4” thick pad; not counting the thicker perimeter edge. A cubic yard of concrete delivered in my area is $225.00.
At $4.50 for a 60-pound bag in my area and 54 bags in a cubic yard, the cost for the pleasure of doing it myself would be $243.00 + tax. If I rent a mixer, the cost goes up. It’s a personal decision,
If you decide to order in, phone around for the best deal. Many companies have a minimum load and some charge mileage and time on site, so if you have to unload the mixer truck by wheelbarrow it could get costly.
How to Pour a Concrete Slab for a Shed in 12 Easy Steps
A concrete slab has many positive attributes as a shed foundation, but it’s a very labor-intensive undertaking. It also is permanent so any mistakes are also permanent. For this article, I will use a 12’x14’ shed when providing examples for study.
Required Tools for this Project
Some of the tools you may have already, some you may borrow from friends, others you can rent. If borrowing or renting, return in as good condition or better when you’re finished with them. You may even have friend/helpers who bring needed tools along too.
- Tape measure: to measure distances, elevation, thicknesses and more
- Hammer: to hammer nails, drive stakes, vibrate concrete form edges
- Drill with a screwdriver bit: I use a drill to drive screws instead of nails
- Saw: to cut lumber and stakes to length
- Round bladed (digging) shovel: digging earth and spreading gravel…and mixing concrete
- Steel garden rake: leveling soil, gravel and spreading concrete
- Tamper and/or plate compactor: for compacting soil and gravel
- Concrete mixer – electric or gasoline powered: mixing concrete
- Masons line: a strong line that doesn’t stretch and sag
- String level: level forms using strings from side to side and corner to corner
- 4ft level: level forms and concrete
- Hacksaw or hand grinder: to cut rebar
- Concrete float: smoothing concrete
- Trowel or hand float, bull float and edger: smoothing the surface and rounding edges to reduce chipping
- Work gloves: protect hands from blisters and from the lime in the concrete
- Dust mask: keep concrete dust out of your lungs
- Safety goggles: protect eyes from concrete
- Rubber boots: concrete is wet and hard on leather boots, rubber boots wash easier too
A long list, but I tried to make it a complete list. Some items you can do without, or exchange for others.
Required Materials for this Project
For a concrete pad foundation, the material list is smaller than your tool list. The numbers or amounts will reflect the 12’x14’ example but can be adjusted to suit your project.
The materials you’ll need are concrete, ½” rebar, gravel, wood to form the pad perimeter, and 2x4s for stakes and kickers to hold the forms level and in place. I also put 6mil poly down on the gravel before the concrete goes down. It prevents moisture from seeping up into the concrete and also keeps the concrete from drying out too quickly after the pour.
I’m going to cover the material needs as if you opt for the full experience of mixing your concrete. If you decide to have ready mix delivered, the company will often ask the dimensions and then give a quote on the job.
When buying the concrete mix, you’ll need to decide what you need and the weight of bag you can manage. Concrete mix usually comes in 60 and 80-pound sacks but is available in 40 and 50-pound sacks in some areas.
Concrete comes in 3 main kinds:
Crack Resistant Concrete Mix – has a synthetic fiber in the mix, which helps prevent cracking and eliminates the need for the mesh. It’s rated at 4000 psi
Concrete Mix – the normal or original concrete in a bag, also rated at 4000 psi
5000 Concrete Mix – a higher strength mix rated at 5000 psi, it is good for cold weather applications
6000 Concrete Mix – an even higher strength mix which also includes synthetic fiber to make it more crack resistant, also rated at 6000 psi
For a concrete pad foundation for a shed use the original concrete mix. It will provide you with the same end product as the other two types and cost less.
The thickness of the concrete pad depends on what you plan to put into it. Most sheds and workshops are fine with a 4” thick slab. If you’re planning to store heavy items like barrels full of steel balls, choose a 6” thickness.
How Much Concrete
The dimensions of your shed and the thickness of the slab determine how much concrete you need. For our example of a 12’x14’ shed, I’ll use a 4” thick slab with a double thick perimeter.
To calculate your concrete needs multiply the length by the width of your shed, then multiply by the thickness; remembering to change inches to feet (4” ÷ 12” = 0.33 ft). There are 27 cubic feet in a cubic yard of concrete, so divide the total cubic feet by 27 to get the amount in cubic yards.
12’ x 14’ = 168sqft x 0.33 = 56 cubic feet of concrete for a 4” pad. I’ll need more if I double perimeter thickness to support a heavy structure. 12’+12’+14’+14’ = 52ft x 0.33 = 17.3 cubic feet. I’d need 56 cubic feet ÷ 27 = 2.07 cubic yards of concrete for the 4” slab. Plus 17.3 cubic feet ÷ 27 = 0.64 cubic yards if I double the perimeter wall construction.
If I’m using 60-pound bags, I’ll need 163 bags for a 4” thick pad with an 8” thick perimeter. Each bag makes ½ a cubic foot of concrete and would cover approximately 14.76” x 14.76”x4”.
If I’m using 80-pound bags, it’ll take 123 bags to do the same job. Each bag would cover approximately 21.4” x 21.4” x 4”. Always order 10% extra for miss calculation; as long as it’s kept dry and clean, it can be returned.
If I were using ready or pre-mixed concrete, I’d order 2.75 cubic yards if the company delivers in ¾ units or rounds up. A cubic yard will cover approximately 80 sqft at 4”s thick. I’d have something formed up to use any extra concrete too – you pay for it anyway.
The cost of pouring a concrete slab depends on where you live. I’m about 30 miles from the concrete plant and a mile from my closest building supply store.
The building supply store waives the delivery charge if my order is large enough. Unfortunately, if I’m renting compactors or concrete mixers, I have a 30-mile drive, each way.
To have pre-mix delivered in my area is $225.00 a cubic yard; they do ¾ increments above 2 yards. $225 x 2.75 = $618.75 with 1 ½ hours on site before extra charges.
If mixing 163 bags manually, and including 16 bags for the extra 10%, I’m in for $805.50 before tax. A 60-pound bag cost $4.50 in my area, and I’d prefer moving 60-pound bags to 80. If mixing 3 bags at a time every 10 minutes, it’ll take me close to 9 hours! Rent a mixer!
Gasoline powered 9 cubic foot mixer in my area runs $71.00 for a 4 hour rental. If I’m using up 2 hours on travel I’ll need it longer. Even though it’ll hold 18 – 60-pound bags, do 12 to 14 at a time.
I usually put a measure of water in first, then add half the bags, add more water, and add the rest of the mix. Save the final water amount to get the consistency you want – not too wet, but not too dry.
A good porridge mix is ideal. Even at 14 bags at a time, with 3 – 4 mixes an hour, it’ll still take 3 to 4 hours to mix. The more hands to help, the better your back will feel.
To ensure a solid base for your concrete pad, a compacted gravel base which is 4” – 6”s thick needs to be put down. For heavier loads or in cold areas with deep frost, you may want it 8” – 12”s thick.
If your excavation is too deep, you’ll need extra gravel to bring it up to the desired level. For our example, I’ll use 6” of compacted gravel.
The best gravel to use is a ¾” crushed stone mixed with smaller pieces and dust; often referred to as 21-A by gravel companies. It compacts very well and gives a solid level base for your concrete.
The thicker the gravel pad, the more compacting you will be doing. The gravel should be laid in 3-inch thick layers and compacted before the next layer goes down.
To calculate the amount of gravel you need, measure down from where the top of your concrete pad will be to the base of the excavated soil. Subtract the thickness of the concrete pad, and you are left with the thickness of the gravel.
Multiply the thickness of the gravel by the dimensions of your concrete pad, and you get the cubic foot measurement off required gravel. For my example, I’d use 0.5’ (6” of gravel) x 12’ x 14’ = 84 cubic feet ÷ 27 (cubic feet in a cubic yard) = 3.11 cubic yards of gravel. It’s better to order extra. I’d bump that up to 3.5 cubic yards since I’d use more when compacting.
Step 1: Planning for your Slab
Determine where you want to build your shed. The flatter the ground, the better, but you don’t want low-lying areas that may pool water or stay wet.
You have decided you want a concrete pad foundation for your shed. The final size should be the exact size of your shed. You want the framing anchored tight to the edge so the cladding can overlap the concrete by approximately 1½”.
The overlap creates a seal where the wall and slab meet preventing weather and insects from getting into your building. The outside shed dimensions, or it’s perimeter, are the measurements for your concrete pad.
If you want a ramp up to a doorway, or the pad extended on one side for extra storage space, you can build them at the same time, or add it later. Once you’ve decided on the final size, you’re ready to move on in your planning.
You need to decide now if you are going to mix and pour the concrete yourself, or deny yourself that pleasure and opt for pre-mixed concrete. Some of the decision may be made based on where your job site is. Ready-mix or premix may not be available in your area, or it’s too difficult to get it to your site.
The concrete you use for your concrete pad is another decision you need to make. Are you going to use rebar and steel mesh or use concrete with a synthetic fiber in it?
Use both, or use no steel or fiber? The greater the risk of frost movement in your area, the greater the need to reinforce your concrete pad with steel or fiber.
The thickness of your slab depends on the climate you live in, the type of shed you will place on the pad, and what you’ll store in the shed.
A 2” slab might be good for lightweight plastic structures and light loads like a bicycle, in warm climates.
A 4” slab is good for most climates and loads like snowblowers, motorcycles and even cars. While a 6” slab will stand up to cold climates and heavy loads like milling tools, stacked steel plate, or heavy machinery.
If you are building a wooden or heavier steel structure on the pad, the perimeter of the pad is often twice the thickness of the pad. The extra thickness supports the weight of the building and provides thicker concrete for the wall anchors to be set in.
L-bolt anchors are available in 6,8,10, and 12-inch lengths. They are designed to be set into the thicker concrete perimeter before it sets.
If you’re placing a shed onto the pad and planning to anchor the shed to the pad, a 4” thick perimeter is great since ½” diameter anchor bolts need to be sunk 2¼”s. The minimum depth in the concrete for the bolt to be effective is 4½ times its diameter: so 4½ x ½” = 2¼”.
This is ideal for wedge anchors that are added after the concrete hardens. You don’t want the anchor sticking through the concrete into the gravel or it will rust and permit moisture to seep up into the shed wall.
One last item in the planning is Insulation. If you’re thinking of using your shed for an office, playroom or in-law suite sometime, then you may want to add a layer of high-density rigid foam insulation. The insulation is laid on top of the gravel and the concrete poured over it.
Step 2: Buy Materials
The first material you’ll need to buy is lumber for the batter boards, stakes, forms, kickers. Also, buy a very straight 2×4 to use to screed the concrete level.
Use 2×10 to form a 4” pad or 2×12 to form a 6” pad. You need enough 2×4 to cut into stakes for every 2 – 3 feet of the perimeter, plus the same number of kickers.
Remember to get anchor bolts. You need enough anchor bolts to do 12” on either side of each corner and door, plus every 4 to 6 feet.
I also get a roll of 6 mil poly to go on the gravel to prevent moisture from seeping up and to slow the drying out process of the concrete.
Take some time to call around for quotes on gravel and concrete. If you book delivery for when you hope to be ready, you can always call and reschedule.
If you wait to book until you’re ready, it may not be available. The same goes for any equipment rentals, book ahead for the date you hope to be ready.
Step 3: Prepare the Slab Construction Site
Before you begin to dig, you want to check with the utilities to make sure you are clear of underground utility lines or right-of-way. A concrete pad is permanent, so you don’t want to have to move it. Check with your local building department about permits you need, plus any HOA rules for your neighborhood.
Clear your planned site of any brush, trees, rocks or obstructions. You want a clear workspace around your site too, so clear an extra 3 to 5 feet in all directions. Tree roots can cause concrete to crack over time, so keep that in mind if your site is close to trees.
Plan your concrete pad construction with the weather in mind. If it’s too cold concrete doesn’t set well, if it’s too wet concrete is hard to finish, and if it’s too hot, it can cure too quickly causing the surface finish can peel. You also want to make sure you have time to do the job properly.
Note: Call 811 or go to call811.com before you dig to have any underground utility lines marked.
Step 4: Mark the Location of the Base
Once the underground utilities are marked, and you have the permits in hand, you’re ready to lay out the foundation. Measure in from any setbacks or the lot line and drive in a stake to mark your first corner. Measure from that stake to find and place the rest of your corner stakes.
For larger pads set up batter boards a foot or two outside the planned site and use them to identify the corner location and square up the corners.
To square up your corners an easy method is to measure the diagonal distances from each pair of opposite corners and adjust one until both are the same. Remember to make sure your length and width stay the same.
Run mason or string line around your corner stakes and use a string level to level the string. Also, check the level diagonally between the corners.
Mark on the stakes where the string is when level, it will make it easier to reset it the string is moved. Use the marking paint to outline the pad on the ground.
You’re now ready to put your shovel into the ground.
Step 5: Dig Out the Foundation
The size of your concrete pad is the area you need to dig out. If you’re planning to put down 6” of gravel and a 4” pad, and you want the top of the pad to be at least 2” above ground level, then you need to dig 8” down.
If the top of your pad is 5” above the ground, then the shed siding can overlap 1”, and you’ll still have 4” between the ground and any wood. You need to remove the sod and 5” of soil from your build site for the top of the pad to be 5” above the ground. Remember to level the soil base too.
Check the level of your excavation as you dig by measuring down from the string line. Once you have dug out a level base, remove any loose material and tamp it down. Remember, your excavation needs to be larger than the pad so you can fit in the forms.
If your site is fairly flat and the pad not too big, you can do it yourself with a shovel. If the pad is large or the ground not flat, you may want to rent a bobcat or excavator for a day; it’s about $250/day in my area.
If someone else in your neighborhood is planning or doing a similar job, you may be able to go together and save some money.
If you don’t want to do the excavation yourself, contact a professional. They’re not cheap, but they know how to use the equipment and will be much quicker.
You’ll also avoid any damage to you or your property. If you have friends or neighbors who may need work done, you may be able to split the cost.
Step 6: Build Strong Concrete Form
Begin by cutting your 2×10 or 2×12 forms to length. For my 12’x14’ example, I’d use 4 – 2”x10”x14’ planks. I’d make sure 2 were 14’ long and cut the other two 12’-3” long. The extra 1½” on each end is for attaching to the 14’ planks. If you can’t get planks long enough, then connect two with a 4’ length of same dimension lumber, also known as a cleat.
Place a long side (14’) plank in your excavation and stake it at one end with one screw or nail. The screw or nail acts as a pivot point as you level the other end of the plank against a stake. Once the plank is level, attach it to another stake, so it is staked at both ends.
Next place a short side (12’-3”) plank in the excavation and stake it, so it overlaps the other plank. Attach it with one screw to a stake at its end, or to the other plank. At the opposite end hammer in another stake and raise the plank until it is level, then attach it to the stake.
Repeat the process until all the forms are placed and level. Check the diagonal measure to make sure the corners are square and recheck that the forms are level. Also, check that it is level from side to side; use a string line and string level to do this.
With the forms leveled, hammer in reinforcing stakes every 2 to 3 feet. Concrete is heavy and wants to flow outward, so it puts a lot of pressure on the forms.
Check the level again using a string that’s attached at each end of the plank. You can adjust a bow or a crown when attaching to the support stakes.
Screw or nail all stakes to the planks. To provide more support at each stake, hammer 2×4 kickers diagonally into the ground and attach them to the top of the vertical stakes.
Step 7: Spread the Gravel
With the forms in place, you’re ready to for your first layer of gravel. Don’t have the delivery truck dump it directly into the pad area; it needs to be compacted in 3” layers or lifts. Fill the formed area with 3” of gravel. Use a vibrating plate compactor and make 3 or 4 passes. You can rent a 14” compactor in my area for about $90 a day.
Spread the second layer, or lift, of gravel. Keep all other layers about 12” back from the forms. This will allow for a thicker concrete perimeter to support the walls of your structure and to set the anchor bolts in.
If the gravel is too dry, mist it with water from the hose to help it stick together and compact better. Make 3 or 4 passes with the plate compactor for every 3” layer of gravel.
Check that the gravel is level and measure down from the top of the form to make sure the depth of the gravel is correct and that you have enough room left for your 4” or 6” concrete slab.
Step 8: Install Rebar Net
Concrete makes a permanent slab on which to build or place your shed, but it is brittle. It needs reinforcement to give it added tensile strength and to prevent cracking.
You can use steel rebar, or steel mesh, or opt for having synthetic fiber added to the concrete before pouring. Do a price comparison and choose what best suits your plan and budget.
Before laying out the steel, lay out the rigid insulation if you’re using it, and tape the seams together. The 6 mil (or thicker) poly is rolled on top of the insulation if using it, or on top of the gravel.
Overlap and tape the seams. The poly slows the drying of the concrete by preventing the moisture from seeping quickly into the ground, making the concrete set better.
Rebar is available in ½” diameter and comes in lengths up to 20 feet. Cut the rebar with a hacksaw, reciprocating saw, or power grinder with a steel cutting disk.
Use lengths to form a 2-ft or 4-ft grid pattern. Overlap lengths by at least 6” where necessary. Use wire ties to connect joints and where the rebar cross each other.
The rebar needs to be suspended in the middle of the concrete to be effective. Rebar chairs are wired to the grid to lift it off the ground. If you need to wheelbarrow the concrete, leave the rebar on the ground and lift it up with a rake, hoe, or your hands as the pour progresses.
Sheets of 6” grid steel mesh are an alternative to the rebar grid. They need to be wired together and lifted off the ground with rebar chairs.
The rebar chairs need to be wired to the mesh too and frequently placed to keep it off the ground. Use a plank to wheel the concrete into place instead of trying to lift the mesh up after pouring concrete.
An alternative to the grid or mesh is using concrete with synthetic fibers added to it. The fibers are mixed into the concrete, increase the tensile strength, and help to prevent or limit cracking.
The fibers are throughout the concrete instead of only in a set grid pattern in the middle. The synthetic mesh is now being used in residential, industrial and commercial projects without the steel grid.
Regardless of which reinforcement you use, make sure to place two lengths of ½” rebar in the deeper 12”s around the perimeter. This will give the thicker concrete more strength as it acts like a footing and supports the walls of the shed.
Step 9: Pour Concrete
Pouring concrete is a labor-intensive fast-paced process. Concrete waits for no one! Make sure you have all the tools, materials and helpers ready.
Mark the location of doors and openings on the outside of the forms so anchor bolts can be placed more easily. I recommend you mark the bolt locations on the forms so they don’t interfere later on with the placement of studs. Check that the forms are level and corners square.
Safety Tip: The lime in concrete is very hard on skin, and concrete is abrasive, wear gloves and protect your skin. Wash concrete off clothes and skin before it dries. Those who are mixing the concrete should also wear a dust mask and eye protection as well as gloves. Inhaling concrete dust or getting it in the eye can be painful and cause long-term issues.
You need two or more large wheelbarrows and enough strong helpers to keep everything moving. Plan how the concrete will get to the formed pad, the closer the mixing is to the pad, the less muscle to move it.
Check the weather forecast, and postpone if necessary. Rain will ruin the surface and soften the ground for heavy equipment and work. If it’s too hot or windy, it can dry and harden the concrete faster than you can work it.
Before pouring the concrete, spay a mist of water over the steel grid. The water on the steel will help the concrete bond to it better.
If you’re mixing your concrete, make sure it is the consistency of porridge; it should slump some, but not run. The instructions are on the bag.
The bigger the pad, the longer it will take. Take turns mixing and wheeling the concrete. Fill the pad beginning at the corner furthest from the mixer.
As you cover a 2sqft section, lift the steel, so it’s in the middle of the concrete. Work to fill one end and then move out from there. Tap the outside of the forms to help remove any air bubbles or pockets as you fill inside them.
If you are having premixed concrete delivered, plan out the route to get the truck as close to the site as possible. Take into account the size and weight of the delivery vehicle too.
When you order the concrete, tell them the dimensions and use of the project. They’ll recommend the best concrete for the job.
Step 10: Level Your Concrete Slab
The concrete should be slightly above the forms. Use a shovel or rake to spread the concrete around and move it into low spots as needed.
Once one end is filled, use the long 2×4 screed board to begin smoothing the concrete level with the forms. Angle the screed board toward you and work it left and right as you pull it to you.
There should be ½” to 1” of concrete in front of it as you pull it toward you to fill in any low spots.
Set the anchor bolts into the concrete as it is screeded. 2”s of the threaded bolt should be above the surface. Lightly smooth the surface around the bolt.
The bolts can be straightened as the concrete hardens if they tip a bit.
Begin smoothing the concrete with a hand or bull float after a section has been screeded. Make 3 to 4 passes with the float to smooth the surface, keep the leading edge slightly raised by lowering the float handle.
You want to smooth the concrete, not plow it. Don’t float the concrete too much as it will draw water to the surface and weaken the surface.
While a section is being flattened with the screed board and another smoothed with the float, the mixing, wheelbarrow moving and filling of the pad continues.
The size of the pad determines how quickly it will be filled, screeded, and floated smooth.
When finished with tools, wheelbarrows, or the screed board, clean any concrete off them and let them dry.
Step 11: Finish and Edge the Concrete
Once the slab has all been floated smooth, wait for the water to bleed or seep out of the concrete and pool on the surface. When the water disappears, the concrete is ready to finish. If it is a hot day, you’ll have to work quickly. If it’s a cooler day, it may take an hour or two for the water to evaporate.
Edge the concrete where it meets the forms while waiting for the water to disappear. Rounding the edge reduces the chance of chipping and makes it easier to lay the bottom plate flat. It the edger sinks more than 1/8” and leaves a groove, smooth it out and wait a bit longer.
The concrete is ready for finishing when the surface water disappears, or it loses its sheen, or when you push your thumb onto the surface, and it doesn’t leave an imprint.
If it’s a large pad that you don’t have time to wait for the perfect moment, you can make 2’x2’ pads from rigid foam to kneel on. You want a smooth finish, but don’t work the surface too much, or it may flake when dried.
Finishing the concrete using a hand float to push pebbles beneath the surface and remove any bumps or marks left from the bull float or edger.
Use a steel trowel after the float for a smoother finish. Work it like the float, but you may need to press down hard to draw some soup or slurry to fill in air bubble holes.
Keep it almost flat as you work in a sweeping motion trying to leave no visible mark. For large slabs you may want to use a power trowel. It’s gasoline-powered 4 or 5 trowel bladed propeller like a machine that can be used to provide a smooth finish.
If you want a no-slip surface, skip the steel trowel stage and draw a bristle broom across the surface to give a slightly roughened or broom finish.
Once the concrete is smoothed, edged, and anchor bolts place and upright, and everything is washed up, you can take a break.
Step 12: Ensure Proper Concrete Curing
For concrete to cure to its maximum strength, it should be kept wet to cure slowly. Mist the surface with a hose 2 or 3 times a day for 5 to 7 days, or spray it with a cure and seal compound after finishing the surface.
The compound is available from most building supply stores. You could cover the concrete with plastic to slow the curing process too.
Keep the plastic off the concrete while it hardens. The plastic can discolor the concrete too.
Let the concrete harden for a day before removing the forms. The surface will be soft and will scratch or chip easily.
Wait a couple of days before building on it to let the concrete set. After 24 hours concrete is 50% cured, at seven days it’s 70%. It will take about 28 days for the concrete to be fully cured.
I hope you’ve found this article a concrete addition to your building file. A concrete slab foundation may seem like a lot of work, but it is a real asset to your shed.
Gather the tools and materials you need, follow the steps, and have lots of helpers. It makes a great foundation and an awesome floor!
Your comments and questions are appreciated. If you know someone who is considering pouring a concrete slab, please share with them if you liked the article.