Pier Foundations Protect Trees
Summary: Pier foundations resemble wood decks or a common table. The actual foundation is a series of vertical columns, or piles, that transfer the building load to the soil. Beams are built on top of the vertical columns that hold up the walls of the house or structure. These specialized foundations work great around trees or in other locations where it is difficult to dig a traditional foundation.
DEAR TIM: I want to build a deluxe garden shed, but I have serious concerns about where to place the shed on my slightly sloping lot. The best location for the shed is under a giant maple tree. A traditional foundation system I feel would cut into far too many roots. Placing a concrete slab on the sloping ground is also somewhat of a challenge. What foundation system can I use that will cause the least stress to the tree and provide plenty of oxygen and moisture to the tree? How is it installed? Barbara H., Loveland, OH
DEAR BARBARA: Gosh! Maybe you should just come over to my house to see what I am doing. Believe it or not, I am in the midst of building a garden shed for my wife Kathy and I am facing the same set of challenges. Placing our shed under the maple tree in our back yard was important for many reasons. In fact, I had to obtain a zoning variance to build my shed because some of the tree's roots were in the way of my foundation.
I decided to use a traditional pier foundation system that is very common in many parts of the nation. A pier foundation is very different from a traditional foundation. Whereas a traditional foundation has a continuous-spread concrete footer that transfers the foundation wall load to the soil, a pier foundation has small square or circular pads of concrete located at strategic locations around the outer perimeter of the structure. Some larger structures need additional support pads inside the outer boundaries of the structure, but I doubt your small garden shed will require this.
Large diameter posts made from treated wood or steel rise up from these concrete pads and connect to the underside of the structure. In essence a structure with a pier foundation mimics a standard table with legs. By raising the structure up off the ground using this foundation system, air can still get to the soil and any water that runs across the soil can still get to all parts of the tree's root system.
If your shed is square or rectangular in shape, you may only have to dig four 16-inch diameter holes in the ground to support the entire structure. These holes would be located at the four corners of the structure. The posts that come up from the ground connect to horizontal beams that support the floor joists and exterior walls. If your soil strength is sufficient, the concrete that is poured at the bottom of the holes will easily support your shed for years to come.
I would also recommend that all lumber for the entire floor system of the shed be treated lumber. Many people do not realize you can purchase treated lumber plywood. When you use this plywood for the shed's floor, you don't have to worry about condensation and mold on the underside of the flooring that would quickly rot out regular wood flooring. If you bring in wet gardening tools, damp soil or if water blows into a door, it will be of no concern as it will not cause wood rot in the treated-flooring system.
If you have seen a traditional wood deck that sits four or more feet off the ground, you have seen a pier foundation system. A typical deck has two or three posts away from the house and these posts are connected to a beam. One end of the floor joists of the deck rest on top of the beam or are hung from the beam with special framing hardware. Often the other end of the floor joists are hung from a board that is attached to the house. Had the carpenters used additional posts and a beam near the house, the deck would sport a full-fledged pier foundation system.
Be sure you use the right nails, bolts and hardware with the treated lumber. The newer treated lumber requires hardware that has an extra-thick zinc coating to resist corrosion. Residential treated lumber has a different chemical composition that can cause rapid corrosion of traditional metal fasteners.
|
|
Comments:
Pete Giesbrecht 15 Feb 2008, 21:31
Is it possible to build a remote cabin in the range of 20 feet x 24 feet in
a remote location with a 10/12 pitch roof on piers? Or are there simpler
foundations that would do the job? The property is on a slope that has a
fall of 400 feet in half a mile. Can you help? Getting large equipment into
the site not possible so all work is by hand. Pete Giesbrecht Calgary
Alberta.
AsktheBuilder 16 Feb 2008, 10:24
Pete,
It is absolutely possible. I can't help as I am busy up building my own place in New Hampshire.
Sean O'Grady 21 Feb 2008, 12:50
Hi is their any specialist way of laying a foundation for a detatched
dormer bungalow close to a full grown oak tree that would appease uk
planning regs.
Mike 25 Feb 2008, 17:00
One problem that I see with the above photo is that all the weight of the
building is being supported by the nails between the beams and the piers.
This might not be an issue for a small shed, but for anything larger or
something with a heavier dead and live load, I would be more inclined
towards going with a design that did not put the load in shear with the
nails. Using a larger diameter pier and notching it on the sides so that
the beams are supported by the notch would give you quite a bit more
structural stability.
All one has to do is look back at the old plantation houses along the Mississippi River between New Orleans and Baton Rouge to see that a pier type construction can last a long time. In fact, this type of construction has lasted longer down there than many of the concrete slab on grade designs that were built after WWII in the New Orleans area. The pier design is not quite the same though since the old ones were likely either old growth cypress blocks or masonry and the beams of the building sat on top of these. One of the main advantages for that area was that as the ground slowly subsided over the years, it was a simple matter of jacking up the building a bit and placing a few shims or new layers of bricks underneath. When you're building in land that is basically semi-dried-out swamp land, this works quite well. As it turns out, the old guys seemed to know a bit more about how to build that the new (i.e. circa 1950s) guys...
AsktheBuilder 29 Feb 2008, 16:52
Sean,
I have no answer. You need to talk with your planning officials and see what relief they might offer.
AsktheBuilder 02 Mar 2008, 07:23
Mike,
Thanks for your comment! Dude, those beams are resting on notches in the vertical piers or posts. It is hard to see it in the first photo, so I have added a closeup shot. In the top photo you can clearly see the notch in the middle pier on the left side.
Mike 02 Mar 2008, 10:59
I stand corrected then. The original photo looked like you had used 4x4s
for the piers and had just nailed to them. The new photo should provide
enough detail for those who are reading your article but don't realize that
notches should be cut to ensure the the load was resting on the pier
securely and not just the nails. Does the most forward right pier support
two beams at the corner or is it just supporting one beam? From the photo,
it looks like it's supporting just one. If that is the case, it would not
be as structurally sound as if you had the corner supported there.
AsktheBuilder 02 Mar 2008, 11:16
Mike,
No worries! Your comment was appreciated as it brought up a pet peeve of mine. Look at the new photo I added - the close up of that exact corner post. You can see that it is notched so there is bearing for both beams. This is why I prefer 6x6s as they give you the needed space to work with.
Calvin 26 Mar 2008, 17:13
Hi
I've been researching the issue of foundation problems. My home was built in 1973. I purchased the house in 1998. I notice that the house etension has settled some since then. An structural engineer has determine that I will need roughtly 10 resistance piers to stabilize the extension. I've been trying to determine if there is some sort of cost per pier standard. I've gotten one estimate thus far and I think it is high. But I don't know much about these thing, so I was attempting to get some type of price standard, if any exist.
AsktheBuilder 27 Mar 2008, 07:31
Calvin,
A price standard does exist. It is the Law of Supply and Demand. You do not yet have enough data. Go get two or three more bids. Ask the engineer *which* contractors he knows of that do the best work. Ask him which contractor he would hire to fix a structural issue at his own office. View all comments |
