Wood-Deck Post Footings: Poured Concrete vs Adjustable Steel

When I built the wood deck on the back of one of my rental houses, I set 24 footings on a 900 mm grid. Twelve months later, after the first wet season and a dry summer, one corner of the deck had dropped 12 mm and the whole surface had a slight lean toward the garden. Because I had used adjustable steel posts on top of small concrete pads, fixing it took me about 15 minutes with a spanner. If I had poured solid concrete piers to the finished height, that same fix would have meant breaking out concrete with a hammer drill. That one morning taught me how I want to build deck footings from now on.

The three options, and what each really costs

• Precast concrete pier block, the tsukaishi: about 1,800 to 2,500 yen each, sits on the ground or a gravel bed, no adjustment after placement, weighs around 11 kg.
• Poured concrete pier: dig a 300 by 300 by 400 mm hole, about 0.036 cubic meters, roughly 3 bags of 25 kg premix at 400 yen plus a form tube and rebar, so about 1,500 yen in material per footing but a lot more labor and zero future adjustment.
• Adjustable steel post, the kousei-zuka: 600 to 1,200 yen each, threaded body with roughly 40 mm of up-and-down adjustment, but it needs a solid bearing surface underneath, not bare soil.

Why I now use a hybrid

My standard build is a small poured pad, 300 by 300 by 100 mm, on 50 mm of tamped gravel, with an adjustable steel post set on top and the deck girder, the oobiki, carried on the post. The pad spreads the load so the post cannot punch into the ground. The post gives me a permanent way to correct settlement. For a 24-footing deck the pads used about 8 bags of premix, roughly 3,200 yen, plus 24 steel posts at about 900 yen each, so around 25,000 yen total in footing material for a deck that has stayed level for years with occasional tweaks.

Read the soil before you choose. The reason my deck settled is that the garden was old fill, not undisturbed ground. Where I dug the pad holes I could see 150 mm of loose dark soil over firmer clay. On firm clay or decomposed granite that has never been dug, a poured pier to height is rock solid and I would use it. On fill, on soft topsoil, or anywhere near a downspout that keeps the ground damp, assume 10 to 15 mm of settlement in the first two years and build the adjustability in. In a cold region where the ground freezes, the pad or pier base has to sit below the local frost depth, often 300 to 500 mm, or frost heave will lift it every winter.

When solid concrete is still the right call

If the deck is more than about 600 mm off the ground, or it carries a roof, or the soil is firm and well-drained, a poured pier to height is stiffer and I would use it. Adjustable posts have a practical height limit around 700 mm before they get wobbly, and the threaded section can seize with rust after 15 years or so in wet ground, though a smear of grease at assembly buys most of that back.

Step sequence for the hybrid footing

• Set up string lines on a 900 mm grid and mark each footing center with a peg.
• Dig each pad hole about 200 mm deep and 350 mm square, then add and tamp 50 mm of crushed gravel.
• Set a simple timber form, 300 mm square, roughly level, checking the whole grid with a rotary laser.
• Mix and pour the pads, screed flat, and let them cure at least 3 days before loading.
• Stand an adjustable post on each pad, run the laser around again, and thread each post up or down until every post top sits on the same plane.
• Lock the post nuts, fix the posts down, and set the girders on top.
• Re-check level once more after the first month, then once a year each spring.

The rework cost each option buys you. When my corner dropped, the adjustable-post fix was one person, one spanner, 15 minutes, zero material. A contractor quote to re-level a deck on solid piers, by jacking the frame and shimming or re-pouring, was 60,000 to 90,000 yen. Over a 15-year hold on a rental I would expect to re-level at least twice as the ground keeps moving, so the adjustable posts have effectively paid for themselves several times over, and I am not coordinating a contractor visit around a tenant.

One safety note

Call the utility locating service before you dig; a garden is exactly where shallow water and gas lines run. Wet concrete is caustic, so wear gloves and eye protection when mixing and pouring, and lift precast blocks with your legs, not your back.

What I would tell another landlord

A rental deck gets less gentle use than a homeowner deck and you are not there to watch it settle. Build in the ability to re-level it in minutes. The adjustable posts add maybe 20,000 yen to a whole deck and have already saved me one contractor call-out, which alone would have been more than that. That is how I have done every footing since the year that corner dropped.

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