Customer Service: (253) 677-0290
Customer Service: (253) 677-0290
By Daniel Rotaru, Owner and CEO at Decks Restore. WA Contractor License # CC DECKSRL797P2. Tacoma, WA.
A few years ago nobody asked me about this. Now it comes up on maybe a third of my estimates, usually phrased the same way: “My neighbor got piles instead of concrete. Should we do that?”
The answer depends on two things that have nothing to do with the deck. What your dirt is, and where a concrete truck can park. I have used both systems on lots four blocks apart and been right both times.
Here is how I actually decide, and what I think homeowners should be asking before anybody quotes them.
A poured footing is a hole in the ground filled with concrete, with a metal post base cast into the top. The concrete spreads the load of the post out across the soil at the bottom of the hole. How much weight it can carry depends on how big the pad is and what the soil under it can take.
A helical pile is a steel shaft with one or more helix plates welded to it, screwed into the ground with a hydraulic torque head until it reaches a target resistance. A bracket goes on top and the post lands on that. It carries load in a different way, mostly through the plates bearing against deeper soil rather than a wide pad near the surface.
That difference in mechanism is the whole story. A footing needs decent soil right where you dug. A pile can reach past bad soil to something better.
Ours is not uniform ground. Glacial till, clay, silt, sand lenses, and in a lot of older neighborhoods, fill that somebody placed decades ago and never documented.
Wet clay is the specific problem. Saturated clay loses bearing capacity, which means the same footing pad that would be fine in August might not be fine in February. It also holds water in an open hole, and it collapses the sides of that hole while you are standing there looking at it.
Before you take anyone’s word about your soil, you can look it up yourself. The USDA’s Web Soil Survey lets you draw a box around your parcel and pull a report with the soil series, drainage class, and engineering limitations. It is free, it takes about ten minutes, and it tells you whether the words “poorly drained” show up next to your address.
One caveat, and it matters. That survey is mapped at a regional scale. It tells you what is generally there. It does not tell you that the previous owner backfilled six feet of construction debris in the corner where you want your posts. Only a test hole tells you that, and on any job with real load I dig one.
Concrete is still the default for good reason.
On a flat, reasonably drained lot where a truck can reach, it is cheaper per point than anything else. There is a prescriptive path in the residential code, meaning that for a normal deck you do not need an engineer to sign off on a standard footing. Every inspector in Pierce County has looked at ten thousand of them. There is no proprietary hardware and no single supplier.
If your lot is flat, your soil drains, and your driveway gets a truck within chute distance of the holes, concrete is probably the right answer and anybody selling you something more expensive should explain why.
Piles earn their price in conditions where concrete struggles.
The part I find genuinely valuable is the torque. A calibrated head reads resistance as the pile goes in, and that resistance corresponds to a load capacity. When the installer stops, you have a number for that specific pile in that specific spot. With a poured footing you have an assumption about soil bearing, and you find out whether the assumption was right several years later.
The practical advantages stack up on hard lots. No excavation, so no spoil pile and nothing to haul away. No cure time. They install in the rain and in the cold, which matters if you want a build to start in January. The machine fits through a standard gate. And on a lot with mature trees, screwing a shaft down does far less root damage than digging a two foot hole.
They also solve the fill problem. If there is four feet of unknown fill over good native soil, a footing bearing in that fill is a bad idea. A pile goes past it.
Per point, piles cost more. That comparison is real and I am not going to pretend otherwise.
But per point is the wrong unit. Count the whole job. On a difficult lot, concrete brings a pump truck, spoil disposal, days lost to holes that fill with water, and sometimes larger pads than you planned because the soil came back weaker than assumed. Add all of that up and the gap narrows, and on the worst lots it closes entirely.
The rough shape of it: on an easy site, concrete wins on cost and it is not close. On a hard site, they land in the same neighborhood and piles often win on schedule. The harder the access and the wetter the ground, the more the math moves.
I have taken out enough bad work to have opinions here.
Concrete footings fail in a few predictable ways. Undersized pads for the load, which is common under hot tubs and outdoor kitchens that got added later. Concrete poured into standing water, which never reaches its design strength. Footings bearing in fill that nobody checked. And posts sitting directly on concrete without a standoff base, which wicks moisture up into the end grain and rots the post from the bottom.
Piles fail differently. The main one is installation to a depth instead of to a torque, which throws away the entire advantage of the system. Wrong bracket for the load path is another. And in soil near salt water, corrosion assumptions matter, because a steel shaft in aggressive ground has a service life that depends on coating and section thickness. I wrote about how hard our environment is on metal in Tacoma’s salt air and what it does to deck hardware, and the same logic applies below grade.
Three questions, whichever system is being proposed.
Did you dig a test hole, and what did you find? If nobody has opened the ground, the foundation quote is a guess.
What soil bearing capacity is the design assuming? A contractor doing concrete should be able to answer this without hesitating.
For piles: will I get an installation log showing the torque and depth for every pile, and what is the evaluation report number for the product you are using? The log is the deliverable that proves you got what you paid for. If an installer will not commit to giving you one, that tells you something.
Flat lot, drains well, truck access, ordinary deck height: poured footings.
Slope, unknown fill, saturated clay, tight access, or a winter start date: piles.
Heavy concentrated loads: it depends on the numbers, but I lean toward piles when I want verified capacity under a point load rather than a calculated one. That comes up most often with hot tubs, which I got into in detail in the load math on adding a hot tub to an existing deck.
Existing deck, and you are not sure what is under it: find out before you spend anything. Our seven point inspection checklist covers what to look for, though the foundation is the part you usually cannot assess yourself.
Are helical piles allowed by code in Tacoma? Yes, as an engineered foundation system. They are typically approved through a product evaluation report and the installer’s design, rather than through the prescriptive footing tables. Permit requirements are covered in more depth in permits and regulations for deck construction in Tacoma.
Do helical piles work in clay? That is one of the conditions they handle well, because the plates can be sized and placed to develop capacity in cohesive soil. Saturated clay is harder on an open excavation than it is on a pile.
Can I put piles under a deck that is already built? Sometimes. Access underneath is the limiting factor, and the existing frame has to be temporarily supported. It is a common repair on decks whose original footings were undersized.
How deep do they go? Until they hit the target torque, which varies by site. Two piles on the same lot can end up at noticeably different depths, and that is the system working as intended rather than a problem.
Is concrete stronger? Neither is stronger in the abstract. A correctly sized footing on good soil and a correctly installed pile both carry the load they were designed for. The failures come from bad soil assumptions, not from the material.
Decks Restore is a deck building company in Federal Way and Tacoma, and we use both systems depending on what the site calls for. We work across the North End, Proctor, Old Town, Stadium District, Browns Point, and out through Puyallup, Gig Harbor, and Federal Way.
If you are planning a new deck and you want a straight answer about your foundation before you commit to a design, request a free quote. I will come out, look at the access, and tell you which way I would go and why.
Decks Restore Tacoma office: 748 Market St #173, Tacoma, WA 98402 Federal Way office: 33530 1st Way S Suite 102, Federal Way, WA 98003 Phone: (253) 677-0290 Hours: Monday to Friday 9:00 AM to 5:00 PM, Saturday 9:00 AM to 2:00 PM, closed Sunday WA Contractor License # CC DECKSRL797P2 Serving Tacoma, Federal Way, Gig Harbor, Puyallup, Kent, Auburn, Covington, Bonney Lake, and the surrounding South Sound.
More on our process and recent local projects on our page for deck builders in Tacoma.