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STRUCTURAL WOOD ASSESSMENT
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Deck footing & pier construction guide: this article explains how to prepare deck footings or piers using cardboard tubes, reinforcing steel, post connectors, and concrete. We describe using a SonoTube concrete form to pour the concrete piers or footings that will carry deck posts. This article series describes critical safe-construction details for decks and porches, including the preparation of footings, piers, and pier-to-post connections for decks, porches, & exterior stairs.
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Footings and piers are designed to support the weight of a deck through all types of weather and stress.
The following steps demonstrate the use of waxed cardboard tubes, popularly called SonoTubes, for making concrete piers. These tubes may be required by your building code, especially if the local soils are sandy or loose.
But they are convenient and practical in all circumstances.
They create smooth-sided piers that resist uplift caused by frost; make it easier to estimate how much concrete you will need; and hold moisture in the curing concrete longer, resulting in stronger piers.
This is a good time to double- check your building code and permit. Under some codes, you may be required to have the footings inspected before making the piers, or you may need to add several inches of gravel to the bottom of each footing before adding concrete. Be sure to have sufficient rebar cut to length, as well as anchor bolts or post bases on hand, as they must be added to the concrete while it is still wet.
Table of Sources of Concrete Forming Tubes
1. Do You Need to Pour Separate Deck Pier Footings?
Sometimes, in fact usually in most soils, the footings and piers can be poured simultaneously, in which case you can skip this step. Or by good construction practice we want the bottom of the hole excavated for the form tube to be handled as follows:
If your footings need to be larger than the bottom of the concrete pier and if the footings will need to be inspected by your building official you should begin here.
The hole for the footings should be below the frost line, as large as required by code (typically 16 inches in diameter and 8 inches deep for 8-inch-wide piers), and flared. Shovel or pour concrete into the hole until it reaches the top of the flared section. Take care not to disturb dirt on the sides of the hole.
Some inspectors permit a simple solid 4-inch or 6-inch concrete block placed at the bottom of the hole to serve as a footing that will carry the deck pier.
In the three-piered deck photo above you will notice that the piers are close to the breaking edge of a hill. Hillside piers need to have adequate depth and if necessary need to be pinned to bedrock to assure that the whole structure doesn't creep or fall down the hill. In this deck installation you can't see the sloping rock that we encountered when digging for these piers.
Encountering shallow bedrock may mean that your piers won't meet the depth requirement specified by your building department and local codes. The easy solution that your building inspector is likely to approve is the proper pinning of the pier to the rock.
Watch out: if your excavation of the footing and pier hole hits solid rock that is not close to dead flat, you will want to anchor the footing and pier to the rock using re-bar. Using a power drill and masonry bit, drill the top of the solid rock to accept 4-6" of re-bar to assure that there will be no pier slippage along the sloping rock surface.
2. Cut the Concrete Form Tubes to Proper Length
Cardboard form tubes are sold in lengths up to 12 feet. In most cases, you will need to cut them to the length you require. (The length will vary depending on whether you are seating the tube into a wet footing or pouring the footing and pier at the same time. Tubes can be cut with a handsaw, but the job will go quicker if you use a reciprocating saw.
Make square cuts and cut the tubes long enough so they all extend the same distance—at least 2 inches—above the ground. Measure and cut tubes for each hole individually, as hole depths are likely to inches in diameter and 8 inches deep for 8-inch-wide piers), and flared. Shovel or pour concrete into the hole until it reaches the top of the flared section. Take care not to disturb dirt on the sides of the hole.
Cut the Tubes
Cardboard form tubes are sold in lengths up to 12 feet. In most cases, you will need to cut them to the length you require. The length will vary depending on whether you are seating the tube into a wet footing or pouring the footing and pier at the same time. Tubes can be cut with a handsaw, but the job will go quicker if you use a reciprocating saw. Make square cuts and cut the tubes long enough so they all extend the same distance—at least 2 inches—above the ground. Measure and cut tubes for each hole individually, as hole depths are likely to vary by a few inches.
3. Set the Form Tubes in Place
Set the tubes in their respective holes. If you have already poured footings and the concrete is still wet, push the tubes an inch or two into the footings. Use a level to ensure that the tubes are plumb in the holes and level at the top. Begin backfilling by shoveling dirt into the hole around the outside of the tube. When 6 to 8 inches of dirt have been placed in the hole, tamp the dirt with a piece of wood. The dirt needs to be firm, but do not pound so hard that you damage the tube. Continue with this routine until the hole is filled, checking the tube’s position periodically with a level.
If you are pouring footings and piers simultaneously, you need to lift the tubes so that the bottom of each is even with the top of the flared footing section; the tubes can be suspended with braces, as shown in the lower picture. Once a tube is level and plumb, drive two screws through each brace into the side of the tube. Do not backfill around suspended tubes.
If the batterboards are still in place and you would like to be sure that the tubes are centered and aligned properly, put the string lines back in place. Drop a plumb bob from the line at each of the planned hole locations, and adjust each tube so that the plumb bob is centered.
Install the Re-Bar & Post Anchors in the Concrete Form Tubes
Watch out: before you pour concrete into the sonno-tubes your local building inspector will probably require reinforcing re-bar in your deck piers and you may also want or even be required to set post anchor bolts into the pier tops as you pour the concrete.
Your local building code official may want to inspect your piers before the concrete is poured. The official will want to confirm that piers were set to the required depth and that the re-bar was installed.
Some officials may accept photographs documenting the pier depth and reinforcement.
Setting Deck Posts in Concrete ?
Decks are most often built with posts set on top of solid concrete piers, a technique that allows you to use shorter posts, keeps the wood out of the ground, provides for easy repair in the event a post is damaged, and offers a greater margin of error.
There are occasions, however, when it makes sense to set the post itself in the ground in concrete. Done correctly, this approach produces a more rigid frame, which may be necessary for a freestanding or an elevated deck, and required where earthquakes are common.
The post can be set in a hole that is partially filled with concrete, then topped off with soil or gravel or, for even greater stability, the hole can be filled to the top with concrete. For best results, use a treated post and set the uncut end in the ground. The earth itself can serve as the form for concrete, or you can set the post in a large cardboard tube. The posts must be carefully plumbed, aligned, and braced before you pour any concrete.
Continue reading at DECK PIER CONCRETE MIX where we discuss how to figure how much concrete you need as well as procedures for mixing concrete and pouring it into the forms.
Deck or Porch Piers & Post Installation Details & SNAFUs to Avoid
The concrete forming tube with rebar inserted shows reinforcement details at a poured concrete pier at our forensic lab in New York. The contractor excavated to bedrock (he could not get below the frost line) where he then drilled into the rock to insert wired re-bar to pin the pier to the rock surface. We took this photograph to provide to the building code inspector who could not be present at the time the piers were poured.
The photos early on this page show above-grade masonry piers. Structural connectors were used to connect post to pier, post to girder, and joists to a ledger board that was bolted to the building's rim joist.
Our deck photo at left shows a structure that we were reluctant to inspect from below. The deck was relying principally on gravity to keep those girders atop the tall steel posts. On closer look, the post bottoms were also not secure.
According to Best Practices Guide to Residential Construction:
Also see STAIRS, RAILINGS, LANDINGS, RAMPS - INSPECTIONS, CODES and see Building Safety Hazards Guide for a list of other building safety topics for home owners and home inspectors.
Above are some photographs of errors: unsafe short cuts taken to "get the deck up" when proper lengths of post were not available or where piers were re-cast on top of older poured concrete piers that were tipping or collapsing. It is surprising how often we see an extra block or scrap installed to make up for a deck post that is too short. This is an un-stable structure at risk of dangerous collapse (above left).
At above right the original concrete piers were probably not dug deeply enough nor properly constructed and were tipping and sliding down hill. A repair contractor placed a new pier on top of the tipping, sliding, collapsing pier - a dangerous repair. The cross bracing may slow the fall of this deck when it collapses.
Continue reading at DECK PIER CONCRETE MIX (how to figure how much concrete you need as well as procedures for mixing concrete and pouring it into the forms.) then this article series will continue at DECK POST CONSTRUCTION
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Our complete guide to building decks, porches, & exterior stairs can be found in Related Topics above. Key articles include:
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