If you’re planning a deck one of the first considerations is the type of foundation. Every deck needs to sit on a foundation of some sort. One of the best solutions is concrete piers for decks that are attached to structures or elevated free-standing ones. If you’re not sure how to build cement deck footings, we’re here to help!
Building concrete deck footings that reach below frost level will provide solid stable support for the deck and prevent damage to your house. Clear the ground area, mark post hole locations, dig the holes, insert sonotubes, fill with concrete, insert steel rebar, and affix post connectors. Once that’s all done, you’re ready to build.
In this guide, we’ll discuss what concrete piers are and how to build them. We’ll explain how to determine the amount of concrete for each pier and what the best type of concrete is for deck footings. Our aim is to provide you with the information required to build concrete deck piers.
- What Are Concrete Piers?
- How to Build Deck Footings [Concrete Piers]
- Deck Footings and Pier Specifications
- Deck Footings Code and Building Permits
- Prepare the Work Area
- Dig Pier Holes
- Install Sonotubes
- Install Rebar
- Pour the Concrete
- Insert Deck Supports or Anchor Bolts
- Cover Tubes With Damp Fabric
- How Much Concrete Do You Need per Deck Post?
- Best Concrete for Deck Footings
What Are Concrete Piers?
Concrete deck piers can be poured onsite or precast. They are used to provide solid, stable support for a multitude of structures built on ground that provides poor support, or where frost is a concern. Their base typically sits on solid, stable ground strata to support the loads it is designed to carry.
Precast concrete piers come in different shapes and sizes and are manufactured offsite. They are delivered and placed in an excavated hole or trench which is then backfilled and compacted. Piers poured on-site often use a cylindrical cardboard tube (sono-tube) or plastic conical-based pillar form.
Cardboard cylindrical tubes are usually inserted into dug or drilled holes that reach the necessary depth and are filled with concrete. The smooth sides of the tube help reduce friction so frost lift forces won’t be a concern like they would be if just poured directly into the hole.
Conical-based pillar forms are either set into a dug hole or trench. The wider base helps distribute the loads to a broader surface and also resists lift forces as do the smooth sides of the cylinder. Both types typically are reinforced with steel.
How to Build Deck Footings [Concrete Piers]
There are different ways to support a deck, but concrete deck piers provide a strong, solid, stable base upon which to start. Preparing the work area, the size, depth, and layout of the footings, and building code compliance are much the same regardless of the type of footing. So, while we address those topics in this section, our focus is on the steps for building concrete pillars for decks.
Deck Footings and Pier Specifications
Decks attached to structures or free-standing decks more than 20” above grade or greater than 200 sqft all require immovable footings. The number, size, and depth of the footings depend on the size and shape of the deck and all anticipated loads. Plus, the load-bearing strength of the ground strata the footings will sit upon and the depth of the frost level also determine the footing depth and size.
The size of the footing depends on the size of the deck and the loads it needs to support. One inch of diameter per foot of joist span is a general rule of thumb, so a span of 10’ would need a diameter of 10”. However, always check the design specs with an engineer or your local building department.
The tributary area of the deck, the anticipated live and dead loads, and the load-bearing strength of the ground typically determine the sonotube diameter or the side dimension of a square footing or pier. Table R507.3.1 of the 2021 IRC is very helpful when determining footing size.
The depth of footings usually depends on the depth of undisturbed soil, its load-bearing capacity, and how deep the frost line is in the build area. Footings must be 12” or more below the level of undisturbed soil and the bottom of the footing at least 6” below the frost line.
The depth can vary from less than one foot to six feet or more depending on geographic location due to the frost line depths. Footing depth can be much less, though, if they are set on solid bedrock.
The size and shape of the deck determine the footing layout. Use the ledger board for reference and a measuring tape and stakes or batter boards to locate the outer corners of your deck. Attach a string line or mark the perimeter with spray paint.
The span and spacing of joists and combined loads typically determine the location of the support beam. A footing and beam calculator is helpful when determining the spacing and location of piers.
If the beam is at the outer perimeter, then the footings will be there. Some decks will have intermediate beams, and others may cantilever joists beyond the beam. Plus, the beams may cantilever up to 1/4 of their span beyond their nearest support, so the layout is important. Mark the location of the center of the concrete piers with a stake or spraypainted X.
Deck Footings Code and Building Permits
When planning a deck, contact your local utilities regarding the location of underground services on your property. There is usually a free utility service locator number to call or access online. They will send someone out to mark the locations of gas, water, sewer, electrical, and communications cables on your property.
The service is free but can take up to two weeks or more depending on the volume of calls. So, you may want to do this before you get too far into the planning or layout stage. The location of services may require plan alterations or a complete redesign.
Once the utilities are located, finalize your design drawings and submit them for a building permit. All decks attached to buildings or large elevated free-stand decks require a building permit. The cost of the permit commonly is based on the square footage multiplied by a fixed unit cost.
If you are doing all the work yourself, you may be able to argue a lower cost. But the more your challenge, the more they may challenge during inspections, so tread softly.
Prepare the Work Area
The area the deck will cover needs to be cleared and prepared prior to starting. If you’ve already laid out the concrete pier locations and deck shape, clear everything inside the perimeter without disturbing the pier locations.
Remove existing gardens, trees, shrubs, or other surface materials occupying the location. Scrape off or dig up any grass or other organic ground cover and rake the exposed dirt smooth. Make sure the ground still slopes away from the building too.
Dig Pier Holes
Depending on the depth required and the type of soil, you can use a round-nosed shovel to dig the pier holes. Dig the holes slightly larger than the diameter of the pier. Work around the center to maintain a plumb hole that remains centered. Depending on the depth of the hole, use a plastic coffee can or hoe to scoop out loose dirt.
Post Hole Digger
Post-hole diggers are manually operated contraptions available from most hardware stores. They look like two shovels connected near the base of the handles with a hinge-like coupling. The handles allow the blades to be driven in and then pinched together to lift out the dirt.
Typically, the operator moves around the hole driving the shovels in and lifting out the dirt, working to maintain the center and ensuring the hole is plumb.
Earth augers have a 6” to 12” diameter helical bladed shaft that is twisted into the ground displacing the earth upward and out of the hole. Originally hand-operated by one or two strong individuals using a wooden cross-shaft, today most are driven by gas-powered engines.
Many still require two operators though to handle the torque, especially if it hits a rock! The auger may have to be lifted from the hole to clear it, and you may also need to remove loose dirt from the bottom manually. Most building supply stores offer earth augers for rent and sale.
Hydraulic augers come in various configurations that attach to tractors, Bobcats, backhoes, and other machinery. They also use a helical-bladed shaft drill and make short work of drilling holes for deck pilings in all soil types, including rocky and heavy clay soils. This type of auger is often used if there are numerous holes to drill or for wide diameters up to 24” or more.
There is also a hand-operated hydraulic auger that is ideal for one-person use. It has a gas-powered engine on one end of a teeter-totter-like bar and a helical-bladed shaft drill on the other.
The pivot point is usually attached to an axle with two wheels that make it easier to move around. The operator aligns the auger and applies downward pressure as the blades pull the auger into the ground. The torque is countered by the frame and engine weight.
Once the desired depth below the frost line is reached, under-ream the lower portion of the hole to make it wider. This will create a bell-like bulge at the base to improve stability and reduce upward frost movement.
Depending on the depth of the hole, use a plastic coffee can or hoe to scoop out loose dirt. Depending on location, you may need to fill the bottom 6” of the hole with gravel to improve stability, support, and drainage.
Measure the depth of the hole from the top of the widened bottom to 4” to 6” above the finished grade height for the required sonotube length. Cut the sono tube to length using a hand, skill, or reciprocating saw.
Slide or twist it down into the hole until it only projects above ground the required distance. The center of the tube should align with the location of the center of the post. Build a wooden frame and screw the sonotube to brace it and keep it at the necessary height and location.
Rebar is often used to improve the lateral strength of concrete columns, so installing #4 rebar into the sonotube before or while pouring concrete is common practice. The diameter of the tube usually determines the number and size of rebar required. In most situations, one length is all that is necessary but there are pre-made 4-bar vertical rebar frames or skeletons available in different diameters and lengths for the purpose too.
Insert and center the steel prior to installing the sonotube and use wire through the cardboard sides to hold it in place. Alternatively, just set it into the hole inside the sonotube and adjust its location as the concrete is poured. Depending on your local inspection schedule, footings often need to be inspected prior to concrete being poured, so remember to set up an inspection.
Pour the Concrete
Concrete can be ordered ready-mixed and trucked to the site, or it can be mixed onsite by hand or in a mixer. The number and diameter determine the amount of concrete required. If you need more than 2 or 3 cubic yards and can get the cement truck close to the site, consider ordering it.
Pour or shovel the concrete into the sonotube until it is level with the top of the cardboard. If the rebar isn’t fixed in the tube, add it and also use it to remove any air pockets by moving it up and down through the concrete before placing it in position. Top up the concrete if necessary and do a final alignment check, smooth the surface, and leave the concrete set.
Concrete can be mixed manually in a concrete boat, wheelbarrow, or bucket. It’s best to buy bags of premixed concrete and just add water. The volume or weight of the bag determines the amount of finished concrete it contains – a 66-pound bag will typically result in half a cubic foot of concrete.
The amount of water added does affect the strength, so only use enough to make it like thick oatmeal. It needs to be liquid enough to flow into the tube but not soup-like so it won’t stay on the shovel blade.
The powder and water should be thoroughly mixed together. Mixing concrete by hand is a labor-intensive task, so if you need more than 30 bags, consider renting a power mixer.
Rent a Mixer
Electric or gas-powered concrete mixers are available in different volume sizes or in PTO-driven formats. The bags of premixed concrete are poured into the mixer, water is added, and the mixer turned on.
Once the concrete is fully mixed and the desired consistency, the mixer can be tipped and the concrete poured into wheelbarrows or other modes to get it to the sonotubes. Always wash out the mixer barrel when it will sit idle for more than 20 minutes.
Ordering ready-mix concrete is easier than hand mixing or using a rental mixer, but also more expensive. However, if you need more than two cubic yards of concrete, then it may be worth it.
You may want to phone around for quotes as some companies charge more and also have additional low volume, time-on-site, and travel charges. Access to the worksite should also be considered since the concrete still needs to be moved from the truck to where the concrete footers are located.
Insert Deck Supports or Anchor Bolts
Once the concrete has enough surface tension to prevent anchor bolts or deck supports from sinking, insert them into the concrete, align them, and let the concrete harden. Alternatively, some pros attach the anchor bolts or supports to blocks and insert the hardware into the concrete as soon as it is poured and smoothed.
The blocks hold the fasteners in place as the concrete hardens. Another method is to wait for the concrete to harden and then drill holes and use epoxy to secure the bolts or supports in place.
Cover Tubes With Damp Fabric
Covering fresh concrete with a damp fabric or keeping it damp with a misting of water 5 to 10 times a day for a week helps to slow the rate of evaporation. This is known as moist curing and will help reduce shrinkage cracks and make the concrete up to 50% stronger.
A damp cloth is often easier than remembering to spray the water. However, the fabric needs to be kept moist, especially in hot summer weather.
How Much Concrete Do You Need per Deck Post?
The radius and depth of the concrete footer or pier are typically used to calculate the amount of concrete required to fill it. There are online tables and calculators available to determine the cubic footage, or you can calculate it yourself.
The volume of a cylinder is calculated by multiplying pi (3.14) by the radius squared and then multiplying the product by the height or depth of the hole to or from the top of the form.
For example, a 12” diameter sonotube that is 48” long has a volume of 3.14×0.5×0.5×4 = 3.14 cubic feet. All inch measures are converted to foot measures to provide a result in cubic feet.
Remember, if the base of the concrete footer hole has been under-reamed to form a wider bell-shaped base, it will require additional concrete. The dimensions of the under-reamed section can be roughly calculated to determine the amount of concrete needed and added to the amount for the sonotube section.
A rough guestimate for the 12” diameter pier could be 4.5 cubic feet for the combined tube and under-reamed sections. So, this concrete footer might require the mixing of nine 66-pound bags of concrete mix.
If all deck piers are similar in dimensions, simply multiply the number of concrete piers by the amount of concrete needed to fill one. If you only need 3 concrete deck footers similar to our example, then you’ll need 27 cubic feet of concrete.
A cubic yard of concrete is the same as 27 cubic feet, so don’t order ready-mixed concrete. You’ll need 54 66-pound bags of premixed concrete, which can be mixed by hand in a couple of hours, or more quickly with a rented mixer. You may also want to have some extra bags on hand just in case the guestimate is off.
Best Concrete for Deck Footings
There are a number of different concretes that work well for deck footings. Concrete tends to require 28 days to cure to its maximum strength, so if planning to begin using the footings ASAP, select a concrete that reaches a working strength sooner. The names vary with manufacturer, but most contractors use 3500 to 4000 PSI strength concrete which achieves a working strength of 1500 PSI after 3 days of curing.
If working strength is needed sooner, consider using the more expensive 5000 or 6500 PSI strength premixed concrete. They are stronger, more durable, heavy-duty concretes that are usable sooner.
The 5000 PSI achieves 1500 PSI strength and the 6500 PSI reaches 3000 PSI in 24 hours. 3500 to 5000 PSI concrete mix meets ASTM C 387, while the 6500 PSI meets ASTM C 926 R-3.
If you’re planning to put a hot tub or spa on the deck, use the 5000 or 6500 PSI mix, otherwise go with the 3500 or 4000 PSI mix.