Pier foundation for a sauna, gazebo or timber-frame house

What a pier foundation is and where it fits

A pier foundation is a set of separate reinforced concrete piers under the corners, wall junctions and support points, tied together at the top by a cast-in-place ring beam. It uses less concrete than a strip, and no pit is needed: only a hole is dug or drilled for each pier. That makes it right for light buildings on dense ground: a timber-frame sauna, an outdoor kitchen, a gazebo, a garden store, a timber country house, a terrace or veranda by the house.

We do not build this foundation under a two-storey masonry house. In an earthquake separate piers each work on their own, and even with a ring beam they carry heavy walls worse than a strip, a slab or a pile foundation designed to calculation.

Pier foundation or screw piles

For the same buildings people often choose between concrete piers and screw piles. Piers are more reliable by the sea, where steel rusts in wet ground, and on plots with pebbles and rock, where a pile helix will not go through. Screw piles are faster and need no waiting for concrete; they suit soft ground and slopes a mixer cannot reach. Sometimes we combine both on one plot: piers under the heavy stove of a sauna, screw piles under the light terrace next to it.

Pier depth and reinforcement

There is almost no frost in Batumi to push a foundation out of the ground, so the depth of a pier is set by the dense layer under the fill and topsoil. On slopes and in highland Adjara, where snow lies in winter and the ground freezes, the engineer checks the depth separately. Inside each pier we place a reinforcement cage and leave starter bars that go into the ring beam.

A plastic tube or a heavy cardboard formwork tube of the right diameter often serves as the formwork: it is set in the drilled hole, the cage is lowered inside and the concrete poured. That gives a straight pier, and the concrete does not mix with the soil of the hole sides.

The ring beam on top

Without a ring beam the piers work as separate supports, and the building settles unevenly at the corners. So on top of the piers we build a cast-in-place ring beam, reinforced and tied to the starter bars. For a timber house the frame is sometimes made of steel channel or glulam on anchors, but by the sea we more often use concrete. Between the underside of the ring beam and the ground we leave a gap or close it with a plinth with vents, so the space under the building is ventilated.

Water and waterproofing

The sides of the piers and the ring beam are waterproofed, and timber is separated from concrete by a damp-proof course: otherwise the bottom plate or joist soaks up moisture from the concrete and rots. Roof water from the building is led away from the piers by a gutter, not into the ground at the supports. If water and drainage will run under the building, the pipes pass through the ring beam in sleeves rather than drilling the concrete after the pour.

How much a pier foundation costs

A pier foundation with a cast-in-place ring beam for a light building costs from 205 ₾ per m² of footprint. A single reinforced 300×300 mm concrete pier is from 240 ₾, a screw pile with head plate from 300 ₾. For comparison, a strip foundation is from 660 ₾ per m³ of concrete. Earthworks, from 39 ₾ per m³, and waterproofing, from 78 ₾ per m², are priced separately. The final cost is set after we inspect the ground on the plot and agree the estimate.