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Plans for the 400-sq.-ft. addition looked straightforward: a new dining room, a pantry, and a full bath stretched out along an existing wall of the house. Ordinarily, the wood-frame structure might have gotten concrete footings and stem walls if not a full foundation, but designer Michael Maines decided to build the structure on helical piles instead.
“She doesn’t care about additional basement space; she wants to keep costs as low as practical; and she doesn’t want to create unnecessary carbon emissions or other negative environmental impacts,” Maines said in an email about the project for his mother-in-law. “Additionally, access is tight and the septic system drain runs under the addition…. So piers just seemed to make sense.”
The 10 hydraulically driven steel piles will cost the homeowner $3000. Maines, a builder and designer as well as a contributing editor at Fine Homebuilding magazine and a GBA Expert Member, didn’t price out the concrete work, but he guesses it would have cost twice as much as the piles. Then there’s the excavation and related issues he’s also avoiding.
Maines’s decision to opt out of concrete in favor of helical piles neatly sums up their appeal on both economic and environmental grounds: They are faster, less disruptive to the building site, and often cheaper—and with none of the carbon baggage that comes with concrete.
Those advantages are helping to propel the industry into a new era, 182 years after the first helical piles (then called screw piles) were used to support a lighthouse on the coast of England. While builders like Maines may choose them for new construction, they are more often used in residential work for foundation repairs and reinforcement. On commercial and industrial projects, helical anchors and helical piles have a variety of uses.
Despite…
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25 Comments
hi, thanks for posting. i dont quite understand this statement
"When piles are not cut off at or near grade, a structure built on top of them may have a tendency to wobble. The problem can be corrected with bracing that’s welded to piles, or by leaving the tops of the piles closer to ground level."
Max, because helical piles have a small diameter, they don't have very much resistance against lateral movement. If you're placing girders directly on them it may not be an issue, though I know of a project nearby with that situation and when you walk on the floor system it shakes a bit more than it would on a regular foundation. When the floor system is built up on posts, as mine is in the project Scott photographed, there needs to be angle bracing to keep the posts from shifting. On my project we'll add diagonal 2x4s, but if you wanted to you could extend the metal piles higher and weld or clamp a metal brace instead.
I was wondering, looking at the picture, how that was addressed. Did designing that added bracing require getting an engineer involved?
That space looks a whole lot more pleasant to work in than a crawlspace.
Charlie, the project is in process--we just stood exterior walls yesterday. It's attached to an existing house so lateral resistance is not as important. But after we drop 2x4 sub-joists below the 2x10 joists to get a 13" insulation cavity, and cover that with plywood for air and critter resistance, we'll add diagonal 2x4 knee braces at each post. It's definitely a more pleasant space to work than a sealed crawlspace, aside from the fact that we put down 3/4" crushed stone and I keep forgetting my knee pads. We'll eventually wrap the space with vertical board skirting.
If I was designing a new house to be entirely on piers, I would get a professional engineer involved. Techno Metal Post also has engineers on staff who can answer a lot of questions.
To me the two photographs illustrate how many of the weaknesses of the system can be designed out. The floor of the outbuilding supported directly on the piles would be difficult to work on, leaves an awkward space underneath, and the exposed piles, although structurally adequate, look unfinished, flimsy, and temporary.
That airy underside of Micheal Maine's addition avoids all those issues. If the floor, and attachment 0f the walls to the existing house, provide adequate shear-resistance, I'd be inclined to try and not add bracing to the posts.
Malcolm, my situation was just lucky in that the existing first floor was already high above grade, but at minimum I use the IRC minimum clearance of 18" between grade and the underside of the floor system. I've crawled through 10-12" spaces and it's not fun at all. (Beams can extend below the 18" clearance.) But even 18" can leave the floor system higher than a conventional foundation.
Michael, what size metal braces do you recommend for lateral bracing, and how extensively do the piles have to be braced? thanks!
Hi Max, my only experience is with wood bracing--I'd talk with the helical pier company, and/or a licensed engineer. The Techno Metal Post website includes this image, which looks like probably 1" diameter bracing.
Thanks for the info Michael.
Hi Mike,
Do you have any information on the plumbing chase below the structure?
I'm working on a design on helical piles and have a few hairbrained ideas, but curious what you've come up with.
Thanks!
Edit: I just re-read and realized I was confused. Sounds like there was existing plumbing under the addition, not new.
Chris, we are adding a bathroom so there will be plumbing in the floor assembly. I was careful to lay out the joists so the toilet drain can go straight back to the house, and the shower and sink drains can be drilled through the 2x10 joists to tie in with the toilet drain. The 2x10s are larger than structurally necessary, partly to allow for drilling.
Yesterday we gusseted 2x4s to the bottom of the 2x10s so we have about 13" in the floor cavity. More than half of the insulation will be below the drain so I'm not worried about freezing. My insulator does a lot of floors on piers for seasonal lake homes, and he likes to make a removable polyiso foam panel below plumbing chases. We may do that here but I'm not sure yet.
On a previous project we used oversized I-joists for the floor system, which allows you to drill holes for plumbing, reduces thermal bridging and allows longer spans.
Another idea I designed for a current project before the builder decided to go with a slab system for time and budget reasons (WarmForm by Bygghouse) was to make a small crawl space, about 10' x 10', and put the rest of the structure on piers. That would have given us a place to make plumbing connections and also would have braced the entire floor system against lateral movement.
What are your ideas?
Hi Mike,
Thanks for responding! I'm planning a build on piers, which I'll hopefully start next summer.
I'm thinking web joists for the first floor & pack with dense-packed cellulose. If they're deep enough, I don't think I'll have to worry too much about the lines in the floor.
What I'm stuck on is how to get the well & septic lines into the house, and how deep to go with that detail.
I thought of just many layers of foam board with some sort of wood-walled "basement". Maybe a 4'x4' or something smaller.
I've also considered a small concrete chase, then insulating. This would help with lateral strength of the house (something I'm still not sure how to accomplish well on a building entirely on piers).
My goal is no concrete. I may have to grin-and-bear a slab for the garage. I'm not sure if I can afford all the lumber & time it'd take to make a strong enough floor to park on. TBD.
I once built an addition with a kitchen on piers. The inspector gave me the idea to box-in the plumbing in the stud bays and *not* insulate in front of them. I used the thickest foam board I could fit behind/above/below and made a small insulated cavity to the drywall. Then I caulked/sealed it all the way around. The end result was a highly air-sealed, reasonably-well insulated cavity, covered by drywall, that could stay "warm" by the conditioned space.
I built that addition for a friend many years ago in Burlington, VT. I'm fairly certain it hasn't had any issues, yet.
Chris,
I've done a couple of insulated cores. After working out the kinks (the first was too small relative to the height from floor to grade and kept freezing), on the last I poured a 4 ft x 4 ft square of concrete stem walls. with foam interior insulation continued down to the frost level. I located it under a utility room, and left most of the top open so that it relied on the heat of the house, rather than supplemental heat. That meant that if it froze, the rest of the house had too, and you had bigger problems to deal with. Once done it is vitally maintenance free and can be forgotten about.
Thanks Malcolm! I'm in Maine, Zone 6a (wet). Frost is 5-6ft. I was thinking something similar to that. You found that 4x4 worked well, it sounds? What size was too small?
Edit: I read some of your other comments on this matter. Good advice! Thanks Malcolm!
Chris,
My advice is all based on my experience here on the milder West Coast where the frost level is around a foot. There may be more efficient ways to deal with protecting the core when it has to go as deep as you would have to. Perhaps using wing insulation as they do with Frost Protected Shallow Foundations? I hope Michael or some others will chime in with their thoughts on that aspect of the design.
Chris, on a new home on piers in Harpswell that I was involved with we used thick sheets of EPS foam to make a transition box for utilities--kind of like a mini-basement. Pretty much like Malcolm describes except without the concrete. I think the carpenters ended up wrapping the foam with pressure treated plywood, which would help with UV and critter resistance.
Thanks Mike! That's kind of what I'm hoping to do. When I do basement/dug-out egress windows I usually box it in with 6x6's. I was thinking the same thing, just lined with several layers of EPS.
Pic: https://www.pinewoodconcepts.com/wp-content/uploads/2020/09/egress-window-basement.jpg
Michael, All: Very happy to read this article. For an upcoming Passive build, referencing Mr. Maine's excellent article on minimizing concrete in a slab on grade foundation vs optionally a foam-encapsulated raft slab foundation a la Warmeform. In a variant of your stem wall or perhaps an improvement to a thickened edge raft slab, I've been pondering for a while using helical piles to improve both anchoring and support while reducing total cement required. General idea was scrape topsoil, insert helicals and then tie their post caps to the ring and intermediate support beam's rebar before pouring with presumptively minimized concrete. Same idea for the EPS-encapsulated raft slab: screw in the piles before dropping the foam on top of them, then proceed as normal. Solves the problem of cross-bracing. Not sure this makes sense when I see the linked installed costs survey results. However, would welcome thoughts or comments. Thanks in advance.
We did a guest house in a floor zone a couple of years ago on piers. Built an insulated core box for water and waste. Put in some heat tape just to be safe. No problems yet, and we've had a couple of very cold winters in between.
Interesting article. Bravo to Mr Maines for looking to an alternative way to build.... I have owned a home on Long Beach Island NJ for 50+ years now. Part of the home is on helical pier reinforced spread footings and a different portion is on wood pilings driven into the ground. Both performed well during Hurricane Sandy. For the last 50 years almost all new homes on LBI have been built on wood pilings although helical piers are now sometimes used. It is hard to justify the helical piers since the solid wood piles do not create a hinge point and are cheaper. Despite the marine environment, there is no evidence wood pilings with the proper treatment levels have rotted and there are thousands of houses built on them.
Concrete is the greatest construction material ever invented-it is almost perfect....except for the imbedded carbon...I am having trouble justifying using dozens of yards of concrete in a residential home to build basements, crawl spaces, stem walls and slabs....when there are perfectly good alternatives..
For a chase, try a large plastic conduit-the black ribbed kind or corregated metal, 30 inches diameter, filled with rock wool with the pipes running up the middle..
Hi! Thanks for the article -
I was planning on going with helical piles but I am building close to the grade - only 15" from grade to finished floor height - So even if i can get it to work structurally, with my insulation, etc I still am only a couple of inches above grade - seems like I am just inviting a problem... so slab on grade seems the only option... This seems like an important aspect to note...
best
Jamie
Jamie, that is a logistics problem, and also a code violation if your location uses the IRC, which requires 18" clear except at beams.
I framed one free standing addition on piles last october here in northern vermont. the piles were all out of line due to the rocky nature of our "soil". the three girders were built on top of 6x6 posts to level out the building as it was sloping with 4' of drop over 24 feet. the girders were not square, and we framed the floor joists square on top of the girders.
One observation I came away with was to make sure the installer of the piles sets them inside the building the dimension. ours were set on the edge, and with the girders not square the floor had to grow to full cover the girders. if the girders could have been installed with the floor overhanging 6" on each edge it would have stayed to plan.
after my one experience on a sloping site I wonder if there was much of a cost savings versus concrete columns. there was a lot of extra labor and materials building up the piles to get to a place we could frame the floor.
Helical piers are not always used for cost saving measures, particularly in shallow installations as opposed to very deep applications; not to mention given how expensive they have become in some areas. I recently got a quote from the same installer I used in 2007 to install the same size helicals he installed back in 2007. The price was double. Concrete has not doubled in price by me. However helicals have their place.
Around here it is rare to see helicals end in a wood post. Instead people often use a 2 7/8 or larger diameter round pier/pile and simply extend it right up to the beam -thereby eliminating one joint/pivot point and some labor.
By the way, in residential use we talk a lot about helical piers or piles, but there are also a variety of push piles and other driven piles and, of course, wooden poles, pilings and piers installed a variety of ways. Last but not least there are interesting prefab concrete items like “Perma Columns” that may be cost effective for those who do not want to put wood in the ground
My customer spec'd the helicoils as a cost saving measure. he works for a large commercial builder in southern New England and they use them on big projects regularly. he didn't want to spend the extra money to extend the the steel posts.
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