Stormwater Infiltration - a Community Responsibility

Associated Environmental Services Ltd


Inflow and Infiltration is a result of groundwater/stormwater flowing or seeping into fixtures such as;

General Repairs

All sewer fixtures should be impervious to surface water. Imagine that there has been heavy rain in your area, could surface water pond in your yard and escape through cracks or breaks into gully traps or other sewer fixtures? If so, you will be contributing to the overloading problems of the sewer system during periods of heavy rain.


Gully trap surrounds can be bought pre-cast and installed by a do-it-yourselfer for minimal cost. Gully trap, grease trap and vent surrounds can be built by the do-it-yourselfer, but permanent impermeable materials must be used and they must be built in accordance with the Building Code, i.e. surrounds must be 25mm above a paved surface or 100mm above unpaved surfaces. You will need to place any surround into a bed of cement mortar (a mixture of sand and cement) and ensure that all openings likely to permit stormwater to enter are sealed. Small amounts of premixed mortar are available from most hardware stores. PLEASE NOTE - we do not recommend that a surround be installed around a gully trap or sewer fixture if it will result in large amounts of stormwater ponding in your yard. In this case, the fault/s can only be satisfactory corrected by regrading the offending path or yard away from the gully trap or sewer fixture and then installing a surround, or by draining the yard to the stormwater system.



- various shapes are available, purchase one that is most suitable to the shape of your gully trap

- various heights are available, purchase a surround that will most easily provide you with a

finished height of 25mm above a paved surface or 100mm above an unpaved surface.



(a) Rough the paved surface where the surround is to be placed by chipping the surface (using hammer & cold chisel) to provide a key for the mortar to bind to.

(b) Clean the surface using a wire brush, flush away dust material with water.

(c) Mix the mortar by adding water til it reaches a plastic consistency. Add small amounts of

water at a time and mix in well before adding more. Take care not to make the mix too wet.

(d) Place mortar onto the prepared surface in the area that the surround is to be placed on thereby providing a bed about 15mm thick.

(e) Soak the surround in water (including the ends) to assist bonding.

(f) Allow surface water to drain from surround and apply cement mortar to end sections of


(g) Place surround onto bed of mortar and press ends up against foundation wall.

(h) Remove excess mortar and smooth off - fill in any gaps with mortar


(a) Remove loose and soft soil from around the gully trap to form a firm base to receive the mortar that will provide the bed for the gully surround.

(b) Mix the mortar as outlined in 1(c) above.

(c) Place mortar onto prepared ground as outlined in 1(d) above. The bed of mortar should

be of sufficient thickness to ensure that when the precast surround is placed on it, the height

above the immediate ground level is a minimum of 100mm.

(d) Complete job as outlined in 1(e) through to 1(h) above.


(a) Some properties, usually those with a private drainage system installed within the last 10 years or so will be fitted with gully traps made from PVC material. Moulded PVC gully trap surrounds can be purchased from drainage and plumbing outlets and in many cases installation instructions are included. Follow the manufacturers instructions but remember, often for waste pipes to fit through the new surround it will require bends to be cut off the waste pipes and then the old bends may have to be refitted or replacement bends installed.

(b) Install surround as per manufacturer’s instructions.

(c) After fitting new surround ensure all gaps, i.e. around where waste pipes protrude through the surround, are filled with a suitable filler such as silicon.





Due to the variation in both shape and location of these gully traps and sewer fixtures the following can only give a broad outline of building and installation procedures.

(a) Prepare the ground as outlined in 1 or 2(a) above.

(b) Surrounds may be built with cement mortar, concrete, brick, pavers etc., but what ever

the material it must be made of a permanent and impermeable material.

(c) If the surround is to be made out of cement mortar or concrete and is going to require to

be built to a finished height at or above 100mm, it might be appropriate to construct a mould

into which the mixture can be poured. If you use a mould, after filling it with concrete, tap the

mould lightly with a hammer to prevent holes forming in the mixture. Do not remove the

mould for several days to ensure that setting and hardening has taken place.

(d) Bricks, paving blocks and other impervious materials can be used to construct a suitable

surround. Whichever of these materials are used, ensure a base of concrete or cement

mortar is provided as a starting bed as outlined in 2(c) above and that the materials are

bonded to each other and the supporting bed by mortar.

(e) Ensure all gaps are filled in with mortar


(a) Clean surface of surround with wire brush, clearing away loose mortar.

(b) Mix new mortar by adding water til it reaches a plastic consistency. Add small amounts of water at a time and mix in well before adding more. Take care not to make the mix too wet.

(c) Press mortar into any breaks, cracks, holes or damage in surround making it whole and complete. Ensure that any gap between the surround and the foundations of the building is filled. Surface water should not be able to infiltrate into the sewer fixture once the mortar has set.

(d) Place a barricade around surround and leave to set for two to three days.

Direct inflow is a result of stormwater being directed into the sewer system through fixtures such as;

Redirecting Stormwater

All stormwater fixtures must be connected to the stormwater system or discharged to an approved outfall. In many cases moving or redirecting a fixture to a correct outfall will require the services of a tradesperson. However, competent do-it-yourselfers are often able to undertake this type of work. Please remember, you may not discharge stormwater onto the ground if it will create a potential nuisance or health hazard to either your own or your neighbours property. If you intend to lay underground stormwater drains or undertake any underground repairs, you should check with your Local Authority. They can provide you with plans of your existing reticulation and, if required, a Building Consent to carry out the works.


In many occasions downpipes found to be discharging into a gully trap can be re-directed to a stormwater drain. This will mean the downpipe has to be disconnected at a suitable point above the gully trap and then run either round or under the house and terminate at the selected stormwater drain. Always ensure the downpipe is installed with maximum fall (slope) thereby helping to clear material from the pipe and preventing blockage. Always ensure the correct jointing compound is used for the type of material the downpipe is made of and it is securely fixed and or supported to prevent it coming loose or sagging

In some instances, there may be another downpipe on your property that is discharging to an approved outfall. You may be able to redirect the offending downpipe to discharge into the other downpipe or regrade the spouting to fall back to the acceptable downpipe. In the event of there being no suitable stormwater drain it may be necessary to install stormwater drainage (which requires a Building Consent). In some cases, where the amount of stormwater is small (less than 10m2 of roof area), you may discharge the stormwater straight onto the ground or into a soakage pit. You can get advice from Christchurch City Council Building Information Officers, registered plumbers and drainlayers or from AES Ltd.


Field drains, sumps, stormwater pipes directing or permitting stormwater to flow into the sewer system are not permitted. We recommend with these type of faults you seek the advice of a registered drainlayer before attempting to undertake any remedial works. If, after the removal of the offending stormwater fixture from the sewer system, it leaves a gap or hole, the access point on the sewer system will need to be repaired in order to prevent excess stormwater from entering.



Why is Associated Environmental Services carrying out this inspection on behalf of Council?

AES Ltd was the successful tenderer out of a group of contractors that tendered to do this work. The benefits to Council and ratepayers by contracting this work out are many. You gain the advantage of having an experienced Company carrying out this survey in a professional manner at the least cost to Council. As contractors to the Council we act as agents for them and as such are empowered by the Local Government Act 1974 to enter private property for the purpose of ascertaining whether drainage is being misused.

What is a gully trap?

A gully trap is a small drain usually outside your kitchen or bathroom. It is normally situated in the ground beside your house foundations and may have a cover or grate over it to keep out leaves and other debris. A gully trap can have up to four waste pipes draining into it and should have a surround around to prevent stormwater run off from entering it.

Why do I have to repair the fault/s?

The drainage fixtures are on your property and/or serve your house, therefore they belong to you. However, Council, as the owner and provider of the drainage system, has a responsibility to ensure that there are no private drains connected to their sewer or stormwater system being misused. As a property owner you must maintain and repair your private drainage as you would for any other fixture on your property.

Why wasn’t this fault noticed during the first Plumbing and Drainage inspection?

The drains on your property were inspected when the house was constructed and at that time they would have passed inspection. Subsequently and often over a period of many years, changes to the property take place. These changes are quite often carried out by the do-it-yourselfer and can also be illegal if they have not sought qualified advice. Unfortunately, even if a previous owner has erected illegal plumbing, it falls on the current owner to rectify it. A common fault that can arise years after the original inspection is a low gully trap surround. Through the years, property owners generally undertake to landscape their properties by constructing paths or building up gardens. With the resulting change in ground level, gully trap surrounds that were originally at the correct height can end up too low.

The amount of stormwater that could get in through my fault/s is minimal, why should I bother fixing the fault?

There are many properties in your area that have similar faults to yours. The amount of stormwater getting into the sewer system from all these properties combined is immense. If everyone repairs the faults they have on their properties, the amount of excess stormwater in the sewer system will be significantly reduced. Therefore, Council will not have to pay to convey, control and treat this excess stormwater and this will reduce the demand on rates revenue. The result will be a cleaner environment due to less pollution caused through sewer overload.

What if can’t afford to get the repairs made or I am unable to organise for them to be made straight away?

Generally, most of the faults can be repaired by a do-it-yourselfer, you may want to ask a friend or neighbour to give you a hand. If you require the services of a tradesperson, we recommend that you obtain more than one quote to be able to compare them and be sure about exactly what the tradesperson will supply for that quote. If the repairs are going to cost more than $400.00, Council loans are available and an application form can be mailed to you on request.

If you are unable to organise the repairs straight away for whatever reason, please contact us by mail or by phone. We are allowed to grant reasonable requests for extensions of time and can postpone any reinspection until an agreed date.

If you have any other queries about this survey, please contact AES Ltd on 0800 166 225 during office hours.