Putting down roots: embedded poles in pole-frame houses

A pole sunk into the ground for a pole-frame house offers structural stability and resistance to bending but increases the possibility of long-term decay as well as termite attack. Here's a guide to pole-frames:

Greater stability means that less bracing between poles will be required, as long as they are placed at the proper depth.

• In a pole-frame construction the required depth for embedding poles varies between 1.2 metres (four feet) and 2.4 metres (eight feet).
• The depth depends on the type of soil and the safe working pressure it will bear, the load imposed on the pole and the size of the pole.
• Poles are aligned in pre-excavated or hand-dug holes, each with a concrete bearing pad at the bottom.
• Once the pole is accurately positioned, the hole is backfilled with concrete or similar material.

There are three major types of backfill:

• Soil taken from the holes mixed five-parts-to-one with cement, dampened and progressively compacted as the hole is filled
• Concrete backfill to within 50 centimetres (20 inches) of the surface, tapered at the top and covered with coarse gravel on crushed rock (or full concrete embedment carried above soil level and tapered)
• Gravel or crushed rock, which is progressively compacted during backfilling.

Above ground:

• Instead of sinking them into the ground, poles can be founded above the ground on brackets that are set in small concrete pads.
• Handling is easier: poles set on concrete pads are up to 1.5 metres (five feet) shorter than poles embedded in the ground.
• They're also less susceptible to termites and general deterioration, and are easier to inspect.
• The bottom of the pole needs to be checked to receive a steel "U" plate. When the "U" plate is bolted into position, it will support the pole vertically, although temporary bracing will be required to prevent movement.
• Permanent diagonal bracing may also be necessary to stiffen the structure against any horizontal shifting.

Some metals may start to corrode when put in contact with wet CCA-treated (chromated copper arsenate) lumber, especially if attached during the first couple of months after the lumber has been treated.

• Use hot-dipped galvanized steel fasteners and coat any bolted joints with multi-purpose grease or solvent-based bitumenous paint.
• Use of internal as well as conventional external supporting poles provides extra support and means roofs and ceilings can take on almost any size and form. This roughly finished pole supports a rustic ceiling.

Embedded poles provide great strength and stability.

• Bury CCA-treated poles at least 1.5 metres (five feet) deep on a concrete ­pad — to give resistance to bending — and encase them in concrete and rammed earth.

Bracing stiffens the building frame and prevents horizontal movement.

• Diagonal braces can be ornamental as well as functional.
• Make them of sawn lumber bolted to poles or metal rods fixed to a central tension collar.
• Lumber can also be used to make face-fixed or cut-in knee braces.

Pole houses are often built on hillsides or other exposed areas where winds can be fierce, so the form of the roof needs careful thought.

• Aerodynamic forces dictate that low-pitched roofs (less than 30 degrees) are generally subject to stronger suction and lift at the roof surface than roofs with a steep pitch during high winds.
• As suction decreases in proportion to the increase in pitch a hip or pyramid roof with a slope greater than 30 degrees — between 30 and 40 degrees positive (or inward) pressure occurs on the windward slope of the roof — has the best wind resistance of all but is the most complex to build because of the varied angles of the rafter cuts.

There are many benefits and risks to using pole-frames when building or renovating a house. Always make sure your structure is safe and up to code, and make sure you weigh the risks and rewards of your choice of structure.