Villa case studies

These case studies show how to choose and apply a compliance path for a villa renovation.

They give examples of the compliance path options that may be used demonstrate performance with relevant clauses of the Building Code.

They follow the six steps set out in the page on selecting compliance paths.

The case studies cover:

  • Case study 1: An extension is to be added to a villa that has an original slate roof. The owners would like to clad the extension roof with slate to match the existing roof.
  • Case study 2: The extension in example 1 is to also include cedar shingle wall cladding, without plywood backing, to match the existing wall cladding.
  • Case study 3: As part of a villa extension, a new double-glazed aluminium window is to be installed in an existing uninsulated timber-framed wall.
  • Case study 4: A proposed renovation is to include a first floor addition.
  • Case study 5: Due to its risk matrix score, the proposed extension of a villa is required to have a drained cavity behind rusticated weatherboard cladding. This will create a detailing problem, as the cladding must match and align with existing cladding, which is direct-fixed.

Villas – compliance case study 1

An extension is to be added to a villa that has an original slate roof. The owners would like to clad the extension roof with slate to match the existing roof.

Step 1. Identify the aspects of the proposed design that fall outside the scope of Acceptable Solutions The proposed roofing material is outside the scope of the Acceptable Solutions.
Step 2. Identify the Building Code clauses for which performance must be demonstrated by the design and supporting documentation.

The Building Code clauses to be addressed by this Alternative Solution are:

• B2 Durability

• E2 External moisture

Step 3. Identify the performance criteria that apply.

The performance criteria that apply include:

• B2.3.1 Building elements must have a not less than 15 year durability

• E2.3.1 Roofs must shed precipitated water

• E2.3.2 Roofs must prevent penetration of water

Step 4. Select the most relevant compliance path(s).

The most relevant compliance path options could be:

• Compliance path 3 – comparison with in-service history

• Compliance path 4 – expert opinion

Step 5. Determine what type of information is required to demonstrate compliance. To demonstrate compliance, what is needed is evidence of the performance of the existing roof in terms of durability, shedding water and preventing water ingress.
Step 6. Provide the evidence.

To provide evidence of in-service history and satisfactory performance for both clauses (B2 and E2), photographs of the existing roofing (from above and within the roof space) could be used.

To provide additional support for clause E2, an inspection report provided by a reputable building surveyor would provide evidence that the performance of the existing roofing has been satisfactory for the life of the house (95 years) and that there has been no deterioration to the building framing due to water ingress.

The drawings and specification submitted to the BCA should also include fixing and flashing details that, as a minimum, match the existing roof installation.

The only change to the original would be the installation of roofing underlay and ceiling insulation, both of which are lacking in the original construction. This would improve the performance of the new addition in both weathertightness and energy efficiency.

 

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Villas – compliance case study 2 

The proposed extension described in example 1 is to include cedar shingle wall cladding to match the existing wall cladding. The initial proposal was to install the shingles over a plywood backing, but this meant that the alignment of the cladding would be lost at the junction of the old and new, so it is proposed that the shingles are installed without the plywood backing.

Step 1. Identify the aspects of the proposed design that fall outside the scope of Acceptable Solutions The proposed cladding material is outside the scope of the Acceptable Solution E2/AS1.
Step 2. Identify the Building Code clauses for which performance must be demonstrated by the design and supporting documentation.

The Building Code clauses to be addressed by this Alternative Solution are:

• B2 Durability

• E2 External moisture

Step 3. Identify the performance criteria that apply.

The performance criteria that apply include:

• B2.3.1 Building elements must have a not less than 15 year durability

• E2.3.1 Roofs must shed precipitated water

• E2.3.2 Roofs must prevent penetration of water

Step 4. Select the most relevant compliance path(s).

The most relevant compliance path options could be:

• Compliance path 3 – comparison with in-service history

• Compliance path 4 – expert opinion

Step 5. Determine what type of information is required to demonstrate compliance. To demonstrate compliance, what is needed is evidence of the performance of the existing roof in terms of durability, shedding water and preventing water ingress.
Step 6. Provide the evidence.

Photographs of the existing shingles without a plywood backing would provide evidence of the in-service history and satisfactory performance.

Removal of a small part of the wall to demonstrate the lack of damage to the wall behind the cladding would reinforce the evidence of the satisfactory performance, and a report by a building surveyor on the state of the framing behind the cladding would provide the expert opinion.

 

 

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Villas – compliance case study 3 

A proposed renovation is to include a new extension and to insert a new double-glazed aluminium window (R-value = 0.26) into a wall where there is currently no window. The wall where the new window is to be installed is timber-framed with rusticated weatherboard external cladding, plasterboard internal lining and uninsulated, giving an R-value of approximately 0.5.

Step 1. Identify the aspects of the proposed design that fall outside the scope of Acceptable Solutions The extension is able to meet the performance requirements of the relevant NZBC clauses but the installation of the new window will reduce the thermal resistance of the existing wall.
Step 2. Identify the Building Code clauses for which performance must be demonstrated by the design and supporting documentation.

The Building Code clauses to be addressed by this Alternative Solution are:

• H1 Energy efficiency

Step 3. Identify the performance criteria that apply.

The performance criteria that apply include:

• H1.3.1 The building envelope must provide adequate thermal resistance and limit uncontrollable air flow

Step 4. Select the most relevant compliance path(s).

The most relevant compliance path options could be compliance path 9 – the use of an Acceptable Solution.

Step 5. Determine what type of information is required to demonstrate compliance. To demonstrate compliance, evidence is needed of adequate thermal resistance (schedule method from NZS 4218) and control of air flow (window manufacturer’s information).
Step 6. Provide the evidence.

The compliance path option is an Acceptable Solution, so it must be accepted by the BCA. Where the existing wall has insulation added to it, the resulting R-value of the wall is likely to be better than the performance of the wall prior to the alteration work.

 

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Villas – compliance case study 4

A proposed villa renovation is to include a first floor addition.

Step 1. Identify the aspects of the proposed design that fall outside the scope of Acceptable Solutions The proposed work requires an assessment of the existing bracing to ensure that the lower level structure is sufficiently rigid to support the upper level addition.
Step 2. Identify the Building Code clauses for which performance must be demonstrated by the design and supporting documentation.

The Building Code clauses to be addressed by this Alternative Solution are:

• B1 Structure

Step 3. Identify the performance criteria that apply.

The performance criteria that apply include:

• B1.3.1 Buildings and building elements must remain stable and not collapse

Step 4. Select the most relevant compliance path(s).

The most relevant compliance path options could be compliance path 2 – comparison to other documents.

Step 5. Determine what type of information is required to demonstrate compliance. To demonstrate compliance, it is necessary to show that the bracing levels for the existing structure are adequate to support a new upper structure.
Step 6. Provide the evidence.

There are no detailed drawings that provide information about the bracing of the existing floor. However, the building had been lined with plasterboard fixed directly to wall framing.

In this case, the building could be assessed in accordance with NZS 3604:1984, which can give generic bracing ratings for the plasterboard sheet lining. A generic bracing rating can be calculated to establish that the existing structure is adequately braced to be able to support an upper level. 

 

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Villas – compliance case study 5

Due to its risk matrix score, the proposed extension of a villa is required to have a drained cavity behind rusticated weatherboard cladding. This will create a detailing problem, as the cladding must match and align with existing cladding, which is direct-fixed.

Step 1. Identify the aspects of the proposed design that fall outside the scope of Acceptable Solutions The proposed fixing of the cladding is outside the scope of the Acceptable Solutions.
Step 2. Identify the Building Code clauses for which performance must be demonstrated by the design and supporting documentation.

The Building Code clauses to be addressed by this Alternative Solution are:

• E2 External moisture

Step 3. Identify the performance criteria that apply.

The performance criteria that apply include:

• E2.3.2 Exterior walls must prevent penetration of water

Step 4. Select the most relevant compliance path(s).

The most relevant compliance path option could be:

• Compliance path 5 – comparison with a previously accepted Alternative Solution.

• Compliance path 3 – comparison with in-service history.

Step 5. Determine what type of information is required to demonstrate compliance. To demonstrate compliance, it is necessary to show that the cladding in this situation will not permit water to go behind it despite the risk matrix requirements.
Step 6. Provide the evidence.

In this situation, a similar property very near to the one in question recently had a building consent issued for direct fixing of cladding. As the house for the proposed work is similar in all respects including location, materials used, and age, the fact that no cavity was accepted by the same BCA on a nearby property could be used to support the argument for consent in this case. 

Alternatively, evidence from both the building itself and the nearby building could be used to provide evidence of in-service history, as the properties are very similar and in close proximity to one another, so are subject to the same weather conditions.