Bracing and Alterations: Why Removing One Wall Can Affect the Whole House

Why a "Just a Wall" Job Can Be More Involved Than It Looks

Taking out an internal wall, or cutting a bigger opening into one, is one of the most common renovation jobs in New Zealand homes. But some of those walls are quietly doing structural work, and removing or altering the wrong one can leave a house with less resistance to wind and earthquakes than the Building Code requires. Here is what bracing is, and why it pays to check before you start cutting.

What Wall Bracing Actually Does

Light timber-framed homes built to NZS 3604:2011 rely on specific walls, called bracing elements, to resist lateral loads from wind and earthquakes. These walls are lined with materials such as plywood, fibre cement or plasterboard, and each lining system is rated in bracing units (BUs). The standard sets out how much total bracing a house needs, and requires it to be spread across "bracing lines" running in both directions through the building, spaced no more than 6 metres apart under clause 5.4.6 (extendable to 7.5 metres using dragon ties).

Why Removing or Altering a Wall Isn't Just Cosmetic

According to BRANZ, removing an internal wall will likely reduce a building's bracing capacity, whatever the age of the house and whether or not its original bracing was ever formally calculated. The wall's contribution has to be assessed and replaced elsewhere. The same applies to cutting a new door or window into a wall that is already providing bracing: it can leave the remaining section of that wall short of what it was rated for.

"It's Just an Internal Wall" Doesn't Always Mean No Consent

Building owners sometimes assume internal wall work is automatically exempt from a building consent. MBIE's Schedule 1 exemption for internal walls and doorways in an existing building only applies if the wall is not:

• Load-bearing
• A bracing element
• A firewall (a barrier designed to limit the spread of fire, heat and structural collapse)
• Part of a specified system, such as an automatic or electromagnetic door
• Built from masonry units such as bricks or blocks

If the wall is a bracing element, that exemption does not apply, and the building consent authority will usually want evidence that the house still complies with the Building Code once the work is done.

What to Check Before Any Wall Comes Out

• Confirm the wall's construction rather than assuming. Linings and framing can be identified during careful deconstruction; BRANZ research (SR305) provides BU ratings for common older wall types that weren't covered by modern product testing.
• Map the existing bracing lines in both directions through the house, not just the wall you're removing, so you know what the building was relying on.
• Plan the replacement bracing early. BRANZ's preferred order is to put it back on the same bracing line as the removed wall first, then elsewhere on that same line if needed, and only then on a line within 2 metres of the original.

If the Wall Turns Out to Be Structural

Where a wall is load-bearing or a bracing element, at least an equivalent amount of bracing needs to be installed somewhere in the building, distributed in line with NZS 3604:2011's layout rules, even in cases where no consent is required. For anything beyond a straightforward like-for-like replacement, it's worth having an engineer check the bracing line calculations and sign off the new layout before work starts, rather than after the wall is already gone.

Thinking about taking out a wall, widening a doorway or opening up a floor plan? Get in touch with Fil Engineers and we can check what the wall is doing structurally before you start.

Sources

• BRANZ – Calculating bracing when renovating
• BRANZ – Wall bracing
• LBP.govt.nz – Know your stuff: Exempt Building Work, Part 2

This article is general information only and is not a substitute for site-specific engineering advice. Every building is different — talk to an engineer about your own situation before removing or altering a structural wall.