Building Proper Deck Footings Part II of II

This past week, we began discussing a topic about building proper footings for decks, the type of deck typically found in the backyards of residential buildings and homes. There are a litany of constraints and guidelines related to proper footing construction.

The outline of our discussion follows.

Last Week’s Topics:

This Week’s Topics:

  • Footing Depth
  • Calculating Deck Load
  • Digging and Pouring Footings

You can go back and take a look at last week’s article at this following link.

Footing Depth

Proper footing depth proves critical for deck stability and frost protection in Washington DC’s climate. Footings must extend below the local frost line so shifting ground during winter freeze-thaw cycles does not impact the structure. The frost line depth in Washington DC averages around 30 inches. This means most footing types for building element footings should be designed to extend below this depth at grade to prevent frost upheaval, in most cases. Given the region’s seasonal temperature fluctuations, building codes often require frost-protected footings to prevent heaving and sinking issues. An even deeper footing depth would provide an even greater buffer against frost effects in Washington DC’s soil conditions, but increasing depth has a nonlinear cost increase, and ensuring footings extend adequately below grade provides key protection from the impacts of frozen ground in metro area winters. 

footing depth

Digging holes for footings will likely mean digging down through topsoil into dense subsoil or compacted fill. Precast piers can, in some cases be an alternative, and can in some cases, achieve comparable depths with minimal digging, in certain ideal conditions. Certain helical screw piles can be drilled to required depths or resistance, in some cases , as well.

Calculating Deck Load

To size footings appropriately, the deck’s load must be calculated, that is the load which will be transferred to or imposed upon the footings. The main components of the load calculation follow:

  • Dead load – Weight of the deck itself including joists, beams, decking, and other structural parts.
  • Live load – Moving or temporary weight like people, furniture, and snow.
  • Environmental load – Wind, seismic, and other forces.
  • Concentrated loads – Heavy objects like hot tubs.

The calculation starts by determining the deck’s footprint in square feet. For a simple rectangular deck, multiply length by width. Then factor in:

  • Decking – 2-3 lbs per sq ft depending on wood type
  • Joists – Around 3 lbs per linear ft for common 2×8 or 2×10 joists
  • Beams – Up to 10 lbs per linear ft for 4×4 or 4×6 beams
  • Railings, benches, steps – Can add several pounds per linear foot
  • Live load – Usually at least 40 lbs per sq ft for occupancy
  • Snow load – Can range from 20-50 lbs per sq ft in heavy snow areas
  • Concentrated loads – Per manufacturer specifications
  • Wind and seismic – Check local building code requirements

These items are summation for the estimated total deck load. This will help determine the size footings needed. Some key footing load capacities (pounds per linear ft) to use as minimum targets:

  • 12” round – 1,800 lbs
  • 16” round – 2,800 lbs
  • 20” round – 4,000 lbs
  • 16×16” square – 1,600 lbs
  • 24×24” square – 6,000 lbs

The values of calculation stated are rough ballpark estimates and the actual individual values very significantly from project to project and from aspect of or element of construction.

calculating deck load

Digging and Pouring Footings

Digging and pouring footings is simply heavy and laborious work.  Here in Washington, DC, footings must often be dug by hand.  There are multiple different services underground which are packed into the tight areas of the city including even in the back or rear areas of houses. Even on private property these typical service elements will often be buried underground.

  1. Water supply lines – Underground water mains distribute pressurized potable water to properties through service lines running from the main to the building. 
  2. Plumbing waste lines – Sewer pipes carry wastewater and sewage from buildings to municipal sewer mains or private septic systems for treatment and disposal. 
  3. Electrical services – Underground power cables bring electricity from utility poles to buildings through conduit and deliver it to electrical panels and circuits. 
  4. Gas service lines – Natural gas is piped from street mains to buildings via buried gas lines to fuel appliances and equipment like furnaces, stoves and generators.

Locating services often will mark approximate areas of pipes or cabling buried underground, but they do not guarantee the locations marked and only provide a level of confidence to within approximately 3 feet of the markings in most cases. On private property in the city  because property lots are generally very small, this often doesn’t leave much space to install footings to areas that are not within a potential range of probability or danger for being close to these services underground. Within these ranges, it’s required or at least best practice to dig by hand so that if you uncover one of these lines such as plumbing waste or water service, or electrical service, sewer or even gas, you don’t hit it with an excavator which will destroy it. The cost to repair it and the cost of the disruption to other neighbors in the city can be immense.

Outside of city areas like Washington DC, you can often use excavators or larger equipment and it will save a significant cost of labor and save a lot of time.  Equipment like mini-excavators and compact track loaders can dig underground footing excavations. These heavy machines can make easy work of excavation.

For larger projects, full-size excavators may be brought in. Their extended reach allows digging from a distance for minimal site disruption. Power augers can also be used to dig larger footer holes down to depths of several feet.  In constrained spaces where equipment can’t reach, hand digging with shovels is still required. 

Spoils are excavated soils, unsuited for reuse, the removal of spoils from job is a major effort. Dump fees at municipal landfills have skyrocketed due to reduced disposal capacity from shortened operating times and personnel during COVID-19 pandemia. Fuel surcharges tacked on by haulers have also driven up the per-load costs of disposal.

It’s often best to optimize excavations to minimize spoils. Concrete tube forms reduce digging compared to square forms. Some soil gets reused onsite as backfill or spread around the property if permitted. Overall removal costs still account for a substantial portion of project budgets or overruns in the case of differing conditions.

For deck footings, concrete mixed for at least 3,000 PSI compressive strength is standard. This achieves suitable bearing capacity for typical residential loads. Higher strengths up to 4,000 PSI may be specified for heavier commercial or multi-level decks in concentrated load paths or poor soils.

Concrete gains strength and cures  as cement hydrates and hardens through chemical reactions with water. Portland cement makes up about 15% of a typical mix. Sand and gravel comprise the aggregate which occupies around 60-75% by volume.

Mixes are occasionally customized with plasticizers and air entrainment additives to achieve a dense, durable concrete that withstands light freeze-thaw conditions during the curing process.   Concrete can be tested for a slump measurement which indicates the looseness in concrete.  Footings do not need loose concrete or a high slump concrete and in most cases a lower slump tighter concrete, as long as it is fully hydrated, is generally higher strength. To ensure adequate workability for pouring and consolidating in footing forms. A slump of 3-5 inches is typical. If concrete is too dry, water or plasticizer is added to the mix to achieve desired consistency, especially when outside of typical ambient temperature ranges.

Concrete is often placed in lifts no more than about 8-18 inches deep to allow for proper tamping and consolidating with rods and compacting vibrators. This eliminates voids and air pockets that weaken concrete. Upper lifts are often placed once lower lifts have hardened enough to avoid aggregate  segregation.

J-bolts and anchors are inserted at stipulated depths and alignments while concrete is still plastic. Templates may be used to position hardware accurately. Concrete is carefully finished at proper elevation to receive posts.  One of the many best practices for finishing concrete footings and piers for decks is to build the center of the footing a little bit higher than the outside edges of the footing and then finish with a relatively smooth trowel texture to allow rainwater to drip off the edge instead of pooling up in the middle of the footing. This then allows the majority of the water to run away instead of accumulating and freezing in the center of the footing which extends the longevity by avoiding damage from water build up and freezing thaw action in the middle of the footing top. 

digging and pouring footings

Concrete should be protected while curing, within certain ambient weather conditions to promote strength gain. Damp burlap and plastic is placed over footings to retain moisture. Typical curing generally takes at least 7 days before building on footings.

Test cylinders can be cast, for additional cost, to check concrete strength over time. Cylinders are most often crushed at 7 days and 28 days to verify minimum PSI. Cores may also be extracted from finished footings for verification, but coring is normally much more expensive.

With the proper footings constructed, a deck can be built on a solid foundation that will provide lasting support and minimize structural issues down the road. Always use a contractor who is trained, experienced, and understands the local DC regulations for specific footing requirements and all areas or aspects of related and associate construction, like our company. Proper planning, grading, and construction techniques will lead to a solid deck that you can use safely for decades of use.

Use a contractor who understands and cares about doing things right.  Always, feel free to reach out to us here at Dupont Decks and Patios. Contact us today.

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