The Canterbury earthquakes have prompted many homeowners to look at what they can do to improve the strength of the foundations of their homes. Depending on the age, structure and condition of a house, work required may involve:

  • replacing missing or corroded fixings and connections
  • adding bracing between piles and bearers/joists
  • adding sheet bracing
  • remediating inappropriate excavations

Check fixings/connections between piles and bearers and between bearers and joists. A house not tied down can slide off its piles in an earthquake. Replace corroded fixings. With concrete piles, a galvanised wire should go through the hole in the pile and be stapled to the bearer with galvanised staples. Timber piles and bearers should be fixed together with hot-dip galvanised Z nails and skew nails.

 

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Add bracing to piles where there is none. Fix timber braces at a slope no more than 45° – flatter slopes are better. Connect each brace to framing parallel with it, e.g. pile to bearer or pile to joist. Sheet bracing (treated plywood or fibre-cement) can also be used so long as it does not reduce subfloor ventilation.

 

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Where you find unbraced wall framing/jack studs under a floor, adding sheet bracing can add resilience. Ensure ventilation is provided for where necessary. Alternatively, if the exterior cladding is in poor condition, replace with sheet cladding that will also perform as bracing. All edges of the sheet must be fixed to solid framing. This may mean some additional timbers need to be installed along the base of the piles. (If the existing cladding is sheet material in good condition, it probably provides some subfloor bracing already.)

 

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In some cases, previous homeowners may have excavated ground from under a house, or close to the edge of a house, perhaps to create more storage space. Where excavations have been substantial and undermines house piles, consult a chartered professional engineer.