Internal or 'within roof area' gutters, also known as box gutters, can cause significant inconvenience, damage and repair cost if water from the gutter gets into the building structure below.
Failure of internal gutters is commonly the result of:
- poor design – lack of fall, insufficient outlets, no overflows, insufficient capacity, insufficient allowance for movement, joint failure
- incorrect material selection leading to a lack of durability
- poor installation
- lack of maintenance (or difficulty in accessing for maintenance).
Wherever possible, internal gutters should be avoided during design.
To meet the performance requirements of Building Code clause B2, a gutter must remain durable with normal maintenance for not less than 15 years. However, the gutter will need to continue to perform for the serviceable life of the roof cladding system, and this is likely to be more than 15 years. Replacing the gutter without replacing the roofing is considered difficult.
E2/AS1 sets the design rainfall intensities and also makes other requirements for internal gutters.
E2/AS1 specifies no end laps for membrane-lined internal gutters and that metal gutter laps are welded.
To make internal gutters as safe and durable:
- design for a 10-minute rainfall intensity for the location with a recommended minimum design intensity of 200 mm/hour (rainfall intensity is for a storm with a 10% probability of occurring annually)
- ensure there is sufficient freeboard to prevent overflowing due to wave action in windy conditions – overflowing can occur in windy conditions when the water level in the gutter reaches a point 50 mm below the top of the gutter
- make the gutter wider rather than narrower (over the 300 mm minimum required) to allow for foot traffic during maintenance – wider gutters are easier to keep clean or repair than narrower ones
- make allowance for any discharge flow from the testing of any firefighting hydrant extending above the roof level
- detail a base that is strong enough for the gutter to be safely used as a foot traffic route, particularly if there are steeply pitched roofs draining into it
- detail snow guards or snow boards in snow-prone areas so that the gutter will remain free to drain the melted water
- do not discharge downpipes and/or spreaders directly into the gutter
- locate downpipes at not more than 12 m intervals, which gives a maximum gutter run of 6 m if regularly spaced
- provide a weir outlet and discharge into a rainhead (preferably external) that has an overflow with the bottom below the sole of the gutter
- ensure that there are no seams closer than 1 m to an outlet
- specify dome-type leaf guards to reduce the risk of downpipe blockage.
The only mention of a required minimum slope for internal gutters in the compliance documents is in E2/AS1 covering butyl rubber-lined gutters, where it requires a fall of 1:100. A fall of 1:60 provides even better drainage. Minimum gutter slopes should be applied to all internal gutters, so that:
- a fall to the outlet is always maintained so that all water is removed
- some inaccuracy in construction will not negate the fall
- some sagging in the gutter over time can be accommodated without compromising the drainage.
It is good practice to:
- provide multiple outlets (two minimum) to each gutter run
- locate gutter outlets to maintain drainage – as an example, in one project, gutter outlets were located above the downpipes (which were encased within columns to protect them), and as a result of building deflection in the gutter between the columns, the outlets ended up at the highest point in the gutter, resulting in ineffective drainage
- locate overflow outlets in an obvious position (such as a location clearly visible from a doorway) to give the earliest possible warning of a problem – overflows must discharge clear of the building and have a cross-sectional area equal to that of the downpipes serving the gutter
- not make sharp changes in downpipe direction as downpipes with sharp bends inhibit drainage – downpipe sizing needs to be increased to the cross-sectional area of the gutter.
There are specific requirements for membrane or metal gutters.
Once the gutter is installed, it should be flood tested to ensure there are no leaks. Concealed internal downpipes must be pressure tested to ensure joints are adequately sealed before they are built in.
Building owners need to regularly check gutters and downpipes for any build-up of debris.
Updated: 9 September 2014