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Garden Structures #1- The Basics - Footings

Woodwork for the Garden
by Lawrence Winterburn
January 1, 2000

As a garden designer creates a magnificent space from a parking lot, a carpenter can create something beautiful where there was none. Occasionally a garden designer of lesser talent may produce a creation that seems outstanding. Five years in the future, the design has not aged as gracefully as expected and now requires serious work to correct. I think you know where I am going here. Garden structures are not all created equal.

For those who will hire a professional to build something for them I offer some technical insights to help discern the knowledgeable from those who are putting on a show. For those who will build it themselves, I hope to bring your technical theory up a level and allow you to build something that will age more gracefully. In a future article we will delve into some subtleties of garden structure design.

First, I will offer a word of caution. Never attempt to use any tool or machine you have not read the manual for and/or feel comfortable using. Accidents happen quickly, and you can compromise your safety by not being familiar with and following proper safety procedures. "I Can Garden", or Lawrence Winterburn, can accept no responsibility for such accidents.

Footings

As a home needs proper footings and a foundation, garden structures require a good foundation to resist forces exerted by frost, clay swell or wind. Even the most beautiful structure can end up in disaster if the footings are incapable of doing the job. Recently, during calls from insurance companies to quote repairs, I have seen gazebos, decks, and fences that have collapsed and ended up as total write-offs, due to an improper footing and/or structure. Remember the average gazebo retails for $7-$15,000, a great deal of money wasted in these cases. I gave quotes to the insurance companies to rebuild the structures as they were constructed, as requested. I informed them that since they were not structurally adequate, I could offer them no warranty whatsoever. I then showed them the quote to redesign the structures, and build them properly.

You need only drive down any street in suburbia to view many examples of poor footings. An uneven deck, a fence leaning, an arbor out of square, are all good examples of what not to do. Most homeowners have no idea what is adequate and are later shocked to discover the footings are poorly constructed. Building departments are coming along, but their specifications are often lacking and based on outdated methods; i.e., a pergola required to be supported by sonotube footings (which will be loose in the ground and could eventually cause failure). Thus the pergola must be redesigned to include bracing to prevent the structure from racking. Another design flaw, the use of sonotube footings without reinforcement, beneath a pergola or gazebo, which could also cause failure. Concrete has very little strength without reinforcement.

Information from your local building department is assembled based upon current general knowledge. It should be looked upon as the minimum standard requirements, which a first class job should surpass. The building department is there for the sake of public safety and not as the arbiter of the levels of high quality construction.

Footings are limited to being as strong as the ground they are contained in. In excavated soil, they are destined to move (sink). You will be better off when installing a deck or structure within 4 feet of the dwelling, fastening to the foundation and/or wall of the house to stabilize the structure. The ground in this area has been previously excavated and is not suitable to support a load.

Why do footings lift?

The source of the problem, concrete installed to ground level. Since the top of the hole is larger than the base, a lip for frost to catch is formed.

As the concrete is lifted by frost, soil falls into the base of the footing, which causes the footing to remain—A little higher every year.

Why a Two Part Footing Works

Our standard two part footing consists of a post bearing upon 6" of concrete and encased in concrete for the next 1.5’. The rest of the footing is topped up with 3/8" gravel.

*Illustration shows how compression over time locks the concrete in place

  1. Screenings compact or settle, allowing the surrounding soil to expand. The concrete, however, will not compact. Over time the soil will encroach over the concrete thus locking the footing in place.
  2. When pressure caused by the expansion of freezing soil or the swelling of clay-based soil as it absorbs moisture, is transferred to the screenings, they will give way, allowing the ground to rise and fall independently of the footing.
  3. Screenings will drain effectively and also allow for evaporation, keeping the footing relatively dry.
  4. Limestone screenings will inhibit growth of vegetation, which retains moisture and causes wood rot.
  5. A footing is only as solid as the ground around it. The screenings will conform to the contour of the ground and adapt to seasonal changes or those caused by extreme winds, etc. The post will be noticeably more stable than traditional concrete to grade footings.

Caution:
* Always call for utility locates prior to digging.
* Never begin building anything without first consulting with your local building department.

Lawrence Winterburn is president of the Winterburn Group Woodwork design and installations. The company supplies and installs "One of a Kind" and traditional garden structures in Ontario, Canada. His One of a kind plans and structures are sold worldwide through Gardenstructure.com.
http://www.gardenstructure.com
http://www.winterburngroup.on.ca
Email- plans@gardenstructure.com

Address: RR#3 Site A Box 11
Elmvale, Ontario, Canada

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