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Foundations

Are your property's piles in good order?

Piles are short timber or concrete posts embedded in the ground under your house that support the rest of your house. Piles support the bearers, which in turn support the joists under the floor of your house.

Problem

If your house sits on damaged timber or concrete pile foundations, it can fall off the piles during an earthquake.

To check

  • In general
    Are the piles:
    • standing upright
    • evenly supporting the bearers
    • not weakened or exposed by ground excavation or being too close to the top of a bank?
  • Timber piles
    Are your timber piles showing signs of rot or borer damage when you jab a screwdriver or knife into the pile?
  • Concrete piles
    Are your concrete piles cracked or crumbling?

Solution

If your piles are damaged, misaligned or undermined, they need to be repaired or replaced. This isn’t an easy task, so get professional advice.

Is your house properly tied to pile foundations?

Problem

 If your house is not properly tied to its pile foundations, it can slide off the piles in an earthquake.

To check

  • Are the pile / foundation connections in good condition?
    Check the connections between the:
    • bearers and the piles
    • bearers and the joists.

They should not be loose, rusted or broken.

Solution

Make sure all your piles are connected to bearers and fix any broken ones.

There are special fixings on the market that can be fitted to existing concrete and timber piles.

Concrete piles
Insert a hole through the concrete piles near the top, roughly the same distance from the top and each side of the pile.

Thread wire through the hole and fix it to the opposite sides of the bearer.

Use galvanised wire and fasteners unless your house is located within 500 metres of a coastline or harbour – in which case, use Z-nails and skew nails made of stainless steel so they don’t rust.

Timber piles
Connect timber piles and bearer together using Z-nails, one on each side of the pile and bearer. Place skew nails into the bearer to fix it to the top face of the pile.

No pile connections
If you have no pile connections at all, consult a building professional.

Unbraced pile foundations.

Are your pile foundations properly supported by bracing?

Problem

If your house is not braced to its piles, it can topple over in an earthquake

To check

  • Are the piles braced?
    First, look under your house and check if your piles are braced.

    If you see diagonal timber braces between the pile and bearers or joists, your house may be sufficiently braced.

    Houses built since 1980 may already have bracing or anchor piles, which are deep enough to stop them toppling over.
  • Does your house have perimeter walls (these are the outside perimeter of your foundations)?
    If your house has no perimeter walls or only horizontal boards, you may benefit from additional foundation bracing.

    If your house has a concrete perimeter foundation wall, your house may already be sufficiently braced.
  • Do you have any concrete perimeter foundation walls?
    If your house has a concrete perimeter foundation wall check:
    • Is the wall is cracked or crumbling? if the wall is cracked, your pile foundations may have moved.
    • Is the timber framing still adequately connected to the wall?
    • Have any fasteners rusted away?
  • Does your house have concrete corner walls?
    If your house has concrete corner walls, consider adding bracing, especially if the walls are more than 600mm tall.

Solution

Consult a chartered professional engineer to design the right bracing solution for your house, which may be timber or sheet bracing.

Timber bracing
With timber bracing, braces at a slope of not more than 45 degrees are fixed between piles and bearers / joists or concrete corner walls. Each brace is connected to the underfloor framing that runs parallel to it, whether that is a bearer or a joist.

Sheet bracing
With sheet bracing, material such as treated plywood or fibre-cement is used to bring rigidity and strength to the structure. Leave gaps or openings for under floor ventilation.

Unreinforced concrete foundations.
Unreinforced concrete foundations

Do you have reinforced concrete slab foundations?

Problem

Concrete slab foundations cannot topple like pile foundations, but they are shallow and can be badly cracked by ground slumps and liquefaction, especially if the slab is unreinforced. A properly reinforced slab can better withstand earthquake effects.

To check

  • Ask your local council
    It’s difficult to tell if a concrete slab is unreinforced by examining it, but your local council may have building details on file.

Solution

If you know you have an unreinforced concrete slab foundation, get advice from a chartered professional engineer or a foundation specialist.

Pryda foundation strengthening kits

Keep your house on its foundations in an earthquake, using the fixtures and fittings in the Pryda foundation strengthening kit. Each comprehensive kit has enough fixings and fittings for up to 20 piles – enough to greatly improve your home’s earthquake resilience. 

Before buying, you need to know if your house piles are concrete or timber as there’s a different kit for each. Pick up a kit right away from your local Mitre10 MEGA or order from Bunnings or PlaceMakers. For more information about the kits visit Pryda's website.