Any measures adopted to improve the flood resistance of ceilings need to recognise the much lower probability of the floodwaters reaching the ceiling due to the extra elevation over the floor.
ADVANTAGES DISADVANTAGES SPECIAL FLOOD PROVISIONS
Fibre Cement (Section 5.6)
• Minimal water damage
• Unlikely to collapse if flooded
• Water resistant fibre cement ceilings are unlikely to need removal for repair
• Not commonly used for ceilings
• More difficult to remove and replace than plasterboard
• Where the area above the ceiling is confined (eg intermediate floors, cathedral ceilings), use non-absorbent insulation (eg polystyrene, foil) to reduce the risk of decay to timber joists and underside of floors
• Insert small air vents in the ceiling to relieve pressure from trapped air in the room and ventilate enclosed areas to reduce risk of timber decay
Plasterboard (Section 5.6)
• Less expensive than alternatives
• Easy to remove and reinstall or undertake patch repairs
• Likely to sag due to increased weight from absorbed water and loss of strength
• Can collapse if there is a loss of strength and water trapped above
• May be damaged by trapped air pressure in floods that almost reach the ceiling
• Insert small air vents in the ceiling to relieve pressure from trapped air in the room and ventilate enclosed areas to reduce risk of timber decay
Increasing vulnerabilityIncreasing vulnerability
2
ROOFS
Any measures adopted to improve the flood resistance of roofs need to consider the reduced probability of the roof flooding due to the extra height above the floor.
ADVANTAGES DISADVANTAGES PROVISIONS FOR PROTECTING STRUCTURAL PERFORMANCE
Traditional Pitched Roof
• Good access for cleaning and repairs
• Generally good ventilation
• Able to support a range of light and heavy roofing materials
• Non-tiled roofs or roofs with sarking may need additional ventilation
• Roof Truss – careful detailing required to help avoid potential weakening of timber truss connections upon immersion
• Terracotta or cement roof tiles absorb moisture
– increased weight on roof frame should be taken into account
• Sheet metal roofing can add strength because of its structural properties and its ability to span
Low Pitch (Near Flat) Roof
• Low height and lighter supporting structure
• lower costs generally
• Greater need for thermal insulation
• Roofing or lining may need to be removed for cleaning and repair
• Difficult to ventilate effectively
• Consider using insulation that does not absorb or retain moisture
Increasing vulnerabilityIncreasing vulnerability
REDUCING VULNERABILITY OF BUILDINGS TO FLOOD DAMAGE
23 SECTION 2 CONTROLLING RISK EXPOSURE THROUGH FLOOD AWARE DESIGN
2.5 KEY RECOMMENDATIONS
To effectively limit flood damages, key recommendations have been prioritised into three categories to assist consent and certifying authorities set appropriate housing control policy (Table 2.4.2). These priorities are based on information in Table 2.4.1 and the risk implications of the various recommendations. For example, systems / components close to the ground such as ground flooring and the lower storey wall structure have been assigned a high priority.Table 2.4.2 Summary of key recommendations for flood aware residential housing in high risk (flood) areas
Priority 1 = measures needed to achieve effective flood aware design in the possible to unlikely flood probability range (e.g. 1 in 100 to 1 in 500 AEP).
Priority 2 = measures which are worthwhile but may not be considered essential Priority 3 = measures which only provide benefits in very low probability events
Priority Measure
Building Type
1 • In areas of higher risk from deep flooding, adopt 2 storey housing with double brick or masonry walls for lower storey for strength and ease of repair and to reduce damage costs by availability of higher upper storey
• Consider use of multi level buildings, which usually comprise of flood resistant concrete/masonry structural elements. Such buildings have lower floors which are used for commercial or common purposes. This allows elevation of the residential premises above areas exposed to a more frequent threat from flooding
• In areas where the ground level is higher but the risk from inundation is still high, adopt flood aware housing for single storey buildings with measures detailed in this table
Foundations
1 • Ensure that adequate regard is given to the properties of the soil types under potential flood inundation, drainage and the impact from flow velocities
• Support foundations on the same stratum
• Protect exposed areas, including embankments Ground Floor
1 • Raise floor to provide protection from local overland flooding and ponding
• With slab on ground in areas of high silt deposition, use deeper slab rebate to hold more silt without the build up of silt bridging the wall cavity
Wall Systems
1 A. Cavity brick (double brick) or masonry walls for the lower storey of 2 storey homes in areas of deep inundation
• Provide for ingress of water to balance hydrostatic forces inside and outside the walls via vents and flaps (which are compatible with the energy conservation requirements)
• Also include openings into the cavity brick walls to facilitate removal of silt from the cavity
Components located at a higher level, such as ceilings and roofing have been assigned a lower priority due to the lower probability of being flooded and thus the resultant lower damage risk.
The final decision on the application of these prioritised recommendations by the consent and certifying authorities needs to be based on merit, which can be determined through the floodplain risk management study and plan preparation process. Through this process the full acceptability of flood aware residential housing recommendations can be finally assessed by balancing technical merit against socio – economic and household financial impacts.
2
Priority Measure
Wall Systems 1 for
lower storey and 3 for
upper storey
C. Interior Wall Linings
• Horizontal jointing to reduce replacement costs
• With linings used on external perimeter walls, raise lower sheet to provide narrow gap behind skirting board to aid post-flood ventilation and cleaning
1 for lower storey and 3 for
upper storey
D. Insulation
Use insulation such as polystyrene panels, which is :
• Waterproof and non absorbent
• Drains and dries quickly
• Resistant to retaining silt
• Maintains its shape after loading
• Anchored to withstand buoyancy forces Ground Floor Doors
1 • Doors fitted with a pet flap, which open both ways to facilitate the effective movement of water both into and out of the house
Intermediate Flooring
2 • Waterproof / resistant timber for flooring and joists
• Allow for loss of strength, if engineered timber beams are used
• Ensure ventilation which is needed for efficient drying and reducing chance of timber decay
Ceiling Linings
2 • Insert small air vents to relieve pressure of trapped air
• Ensure ventilation of enclosed areas to reduce the risk of timber decay Stairways
2 Straight and wide stairs with treads and risers of comfortable proportions to facilitate relocation of contents from ground to upper floors
Fasteners
3 • Given that flooding is a relatively rare occurrence above the 1 in 100 AEP flood level, most bolts, nails and screws do not warrant corrosion free alternatives
• In more corrosive environments or critical areas (i.e. where any loss of strength cannot be tolerated e.g.
balcony, which supports a live load), consideration should be given to using galvanised or stainless steel alternatives for fasteners
Roofing
3 • Traditional pitched roof with painted sheet metal roofing (e.g. colourbond) to ensure the strength of the roof
• Roof truss based designed to avoid weakening of the timber truss connections due to immersion