1 TOPIC 28: RISK ASSESSMENT © Unitec New Zealand Construction Systems 1.

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TOPIC 28: RISK ASSESSMENT

© Unitec New Zealand

Construction Systems 1

Introduction


As mentioned in the previous lecture on weathertightness, one of the factors that impact this is the level of possibility of a building in a specific location leaking. This is called a buildings ‘risk’ level, and all buildings require this risk level to be determined.

Once determined, the cladding system can then be selected, or if already selected, can be shown to be compatible to the level of risk assessed.

The Compliance Document NZBC E2/AS1 External Moisture contains the workings of the risk assessment process within.

Risk Assessment


RISK ASSESSMENT USING E2/AS1

Risk Assessment

The 4 steps to using the E2/AS1 Risk Matrix:

1. Obtain detailed drawings of the building

2. Assess each external face of the building using NZBC E2/AS1 table 1

3. Complete the building envelope risk matrix using NZBC E2/AS1 table 2

4. Determine a suitable cladding using NZBC E2/AS1 table 3

Risk Assessment1. Obtain detailed drawings of the building

The drawings must include the following:

- Site plans

- Floor plans

- Elevations

- Knowledge of junction and construction detailing



2. Assess each external face of the building using Table 1

Risk Assessment

Table 1 identifies 6 risk factors, names them from A-F and further divides each area of risk into four(4) grades low, medium, high and very high.

Risk Factors:

A: Wind ZoneB: Number of StoreysC: Roof/Wall Intersection DesignD: Eaves WidthE: EnvelopeF: Deck Design

Risk Assessment


RISK FACTOR A: WIND ZONE

Wind driven rain has more potential to penetrate a building exterior than rain in calm conditions, and this is supported in reports on building failure where buildings on more exposed sites tended to be more vulnerable and were exposed to greater levels of wall wetting than buildings in sheltered locations.When scoring each face of the building the same wind zone is applied to each face.

What is a Wind Zone?A wind zone refers to the wind forces that affect a building (as determined in the NZS 3604). The wind zone is determined from a complex calculation that considers such things as; the wind region the site is in, the terrain (built-up or open), topography (the slope of the land) and the degree to which the site is sheltered or exposed. The wind zone is usually calculated by the designer or engineer producing the building plans after a site visit to establish the above characteristics of the site.

Risk Assessment


RISK FACTOR B: NUMBER OF STOREYS

The number of storeys a building has is a direct correlation to its height. The taller the building the more exterior wall area and potentially more junctions and envelope penetrations (windows and doors, etc) it will have, raising the risk of water penetration.

Increasing the number of storeys results in an effective increase in the catchment area of a wall exposed to wind-driven rain. It means more water will be running over any vulnerable areas associated with window and door openings, penetrations and other junctions on lower levels. The upper levels will also be more exposed to wind and rain with less shelter from surrounding landscape, vegetation or other buildings. Scores assigned in the risk matrix are as shown above.

Risk Assessment


Many houses may have a partial upper storey. Depending on the type of upper storey, a particular wall may be classified as ‘one storey’, ‘two storeys in part’ or ‘two storeys’.

Risk Assessment


RISK FACTOR C: ROOF/WALL INTERSECTION

While roofs have not been identified as causing major weathertightness problems, there are often issues of water penetration related to the intersections between walls and roofs.

Low- Risk Intersection Designs:The least risky is the fully protected intersection provided by traditional eaves. The eaves direct roof water away from the wall framing and also protect the top of the wall from rain.

Risk Assessment


Medium-Risk Intersection Designs:Partly exposed intersections have no or minimal eaves to direct rain away from the wall framing, and provide limited protection for the top of the framing from wind-driven rain.

High- Risk Intersection Designs:High-risk roof/wall intersections are those where exposure of the junction allows water to run over areas that are vulnerable to penetration.

Risk Assessment


Very High-Risk Intersection Designs:This classification applies where upper walls terminate within the boundaries of the lower exterior walls, such as in multiple level roofs, clerestoreys, dormers, chimneys etc. These types of roof-to-wall intersections incorporate junctions that are exposed to water run-off and create opportunities for leaks to penetrate directly into interior wall framing below.

Design features such as enclosed balustrades or parapets includepotentially vulnerable junctions located directly above the wall framing. Water can run over these junctions, and a failure in this location poses a potential hazard for the wall framing below.

Risk Assessment


RISK FACTOR D: EAVES WIDTH Roof overhangs provide some degree of protection to the walls below – however as eaves reduce in width the level of protection reduces. Also dependant upon where eaves occur on the face of the building the extent of protection offered to walls below is varied, so the definitions take into consideration at what level the eaves are located.

Risk Assessment


RISK FACTOR E: ENVELOPE COMPLEXITY

More complex buildings generally have more junctions and penetrations and may have more than one exterior cladding –consequently they have greater potential for water penetration

Low-Risk Building Envelope complexity:Simple building envelopes have limited corner junctions, together with few penetrations and junctions. With a single type of wall cladding, junctions between claddings are minimised.

Risk Assessment


Medium-Risk Envelope Complexity:If building envelopes meet all the criteria required for low-risk envelope complexity except they have two types of cladding, these will be classified as medium-risk.

Other medium-risk building envelopes have more complex shapes, with more corner junctions. They may have curved and/or acute angled corners, but will still have limited penetrations and junctions. With a single type of wall cladding, junctions between claddings are minimised.

Risk Assessment


High-Risk Envelope complexity:High-risk building envelopes have complex shapes, with many corner junctions. They may have curved and/or acute angled corners and/or multiple (three or more) cladding types, and will tend to have the following features.

Complex floor plan, with many corner junctions, and possibly curved walls.Corners may be at acute angles. The number and complexity of vulnerable corner junctions is increased. With multiple wall claddings there are inter-cladding junctions vulnerable to water entry. Window and door units are conventional in design, without specialised windows such as box, bay or conservatory-type glazing. There are no exposed attachments such as pergolas, with fixings that penetrate wall claddings.

Risk Assessment


Very-High Risk Building Envelopes:Very high-risk envelopes are an extension of the factors covered for high-risk envelopes.Floor plans are very complex, walls may be curved and corner junctions may be at angles other than 90degrees. There may also be added risky features, such as pergolas, attachments, box windows and upper-level walls that finish within the boundaries of the floor below.

Risk Assessment


RISK FACTOR F: DECK DESIGN

Decks and balconies, particularly enclosed water proof decks, and the junctions of decks with the building envelope, have the potential for water penetration - this category defines different balcony and deck types and their building location.

Timber Slat decks:The level of risk depends on the design and location of the deck. Some are considered low risk if they do not lead to vulnerable penetrations of the external wall and are freedraining.

Risk Assessment


Enclosed Decks:E2/AS1 defines an ‘enclosed deck’ as one that has an impermeable upper surface and is closed on the underside. This means any moisture penetration endangers the framing of the deck and adjoining walls and is difficult to detect. The areas most vulnerable to moisture penetration are solid balustrades and deck-to-wall junctions.

3. Take the level of risk for each risk factor from Table 1 and assign a value from Table 2

Risk Assessment


The assessed risk scores for six key risk factors are added together in order to arrive at an overall risk score for the wall face or elevation.That score is then used to determine suitable wall claddings from Table 3 of E2/AS1.This table effectively determines the need for a drained cavity behind those wallcladdings that are included within the scope of the Acceptable Solution.

Risk Assessment


4. Take the total risk score for each face and apply it to Table 3 to determine what suitable wall cladding options are available

Risk Assessment


Each face may have different total scores and so different options may be available form Table 3.

Typically, if one cladding type is to be used, the cladding selected may be based on the highest total and all lower totals disregarded.

Risk Assessment


Worked Example:

This is a reasonably simple single-storey house in a low wind zone, with a lined basement garage. The design has a hip roof, one type of wall cladding and two decks (one at ground level and the other cantilevered above the basement garage).

In this example, a simple elevation approach has been taken which considers each elevation of the house as a whole, regardless of the number of wall faces that may be present within a single elevation.A more detailed wall face approach where each wall face of the house is assessed separately. This means complex designs will need more assessments completed in order to cover the whole design.

Both approaches may be used for assessment. The elevation approach tends to be better suited to simpler designs where there is little difference in the overall scores resulting from either approach. The more detailed wall face approach is likely to suit more complicated designs. A‘hybrid’ approach can also be taken, which incorporates elements of both systems.

Risk Assessment


Risk Assessment


Reasoning: The west elevation is Wall 8 (as Wall 6 for this worked example is assessed as part of the south elevation), and is one storey in height. Eaves are wider than 600 mm, leading to low risk scores for the roof/wall junctions and eaves width (as the eave is at the first-storey level). Envelope complexity is considered to be low. Deck design is also low risk, as the timber deck is at ground-floor level.The total risk score of 0 for this elevation falls at the bottom of the low risk band of 0 to 6. The combined total score of the factors could increase up to 6 without changing the overall risk classification.

1. Complete the Risk matrix forms:

Risk Assessment


Reasoning: The north elevation is Wall 1 and is one storey in height for most of its length. Eaves are wider than 600 mm, leading to low risk scores for the roof/wall junctions and eaves width (as the eave is at the first-storey level). Envelope complexity is considered to be low. Deck design is also low risk, as the timber deck is at ground-floor level, while the cantilevered deck at the other end does not touch this wall.The total risk score of 0 for this elevation falls at the bottom of the low risk band of 0 to 6. The combined total score of the factors could increase up to 6 without changing the overall risk classification.

Risk Assessment


Reasoning: The east elevation is Wall 2 (as Wall 4 for this worked example is assessed as part of the south elevation), and is two storeys in height for most of its length. Eaves are wider than 600 mm, leading to low risk scores for the roof/wall junctions and medium risk scores for eaves width (as the eave is at the second-storey level). The envelope complexity is considered to be low, and there is no deck.The total risk score of 3 for this wall falls within the low risk band of 0 to 6. The combined total score of the factors could increase up to 3 without changing the overall risk classification.

Risk Assessment


Reasoning: The south elevation includes Walls 3 to 7, and is two storeys in part.Eaves are wider than 600 mm, and this protects the top of the wall framing, leadingto low risk scores for the roof/wall junctions and medium risk scores for eaves width (as part is at the second-storey level). Envelope complexity is considered to be low, but the deck factor is high as there is a cantilevered deck at the first floor level.The total risk score of 6 for this elevation falls just within the low risk band of 0 to 6.An increase in score on any of the risk factors will increase the overall risk classification and change cladding options accordingly. For example, if the deck were changed to an enclosed type this would increase the deck risk factor to very high and bring this elevation within the medium risk band of 7 to 12.

Risk Assessment


SUMMARY:

All walls are assessed as low risk, which allows the greatest choice of direct-fixed claddings.For this design, a wall face approach would change the risk scores of some individual walls, but would not alter the risk band they fall within. This is likely to be similar for many simple designs, with little effect on cladding choices.However, differences between the two approaches tend to increase with more complex designs, with more likelihood of changes in cladding choices.

Risk Assessment


CONCLUSION:

From the summary sheet, decisions now need to be made as to the actual cladding (or claddings) the project will use.A conclusion is now made indicating the selection and compliance.

eg. ‘selected timber bevel-backed weatherboards on 20mm cavity battens which comply with the suitable cladding types from Table 3 E2/AS1 for low risk level on all elevations.’

Remember, the object of the Risk assessment is to provide a means of both determining the cladding (or the range of cladding options available), and a verification that the cladding selected complies with E2/AS1.

BCA’s will require such verification on building consent documentation and the usual way of achieving this is to show the matrices on the appropriate elevations along with a summary justifying what the risk scores are, the cladding(s) selected, and how it complies.

Risk Assessment


Worked examples of the risk matrix.

The following lecture for the AT students will consider the actual use of risk assessment based on a model house.

Any student wishing to see how Risk Assessment can be applied to other modelled examples can access the DBH website and will find other examples under ‘External moisture – a guide to using the risk matrix.’

>>FACULTY OF TECHNOLOGY AND BUILT ENVIRONMENT


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1 TOPIC 28: RISK ASSESSMENT © Unitec New Zealand Construction Systems 1.

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Transcript of 1 TOPIC 28: RISK ASSESSMENT © Unitec New Zealand Construction Systems 1.