How to build an AG pipe and gravel trench for shed drainage
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In the absence of a proper drainage system, water could build up near the bottom of the shed, causing corrosion at the footings and making the inside of the shed wet. This moisture will eventually ruin whatever is kept inside the shed. With an AG pipe drainage system, the shed will stay dry and last longer.
Does your shed need an AG pipe and gravel trench?
Not every shed requires an AG pipe and gravel trench. Before deciding on such a solution, you need to determine whether the water issue comes from below or above the ground.
Surface water originates from above. Rainwater hits the roof, flows off, and then travels across the ground toward the shed. If this is the issue, installing a gutter system with downpipes and ensuring the ground slopes away from the walls can solve the problem more cost-effectively and with less effort than digging a trench.
Groundwater poses another problem. In the case of rainfall, it travels sideways within the soil and may end up surfacing above the level of the shed. The floor may appear to be moist, yet there is no evidence of any runoff water. This situation may also apply in cases where you observe water standing in the low end of the pad after rainfall. Another observation could be moisture in the soil for days around the shed.
This issue worsens on sloped blocks. Water travels downhill through the ground and accumulates on the uphill side of the shed. In such cases, a trench across the top side can be particularly effective in managing the water flow. We go into the site challenges of grade in our guide on building a shed on sloped ground, which is worth a read if your pad sits on a slope.
What you need: materials for an AG pipe and gravel trench
What you require is affordable and easily accessible at any hardware or landscaping shop:
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Slotted AG pipe, which can be either the flexible corrugated AG pipe or the rigid slotted PVC type, with a diameter of either 65mm or 100mm
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Drainage gravel, usually 20mm blue metal or river pebble that has been washed, and not crusher dust
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Landscape fabric, also known as geotextile, which is used to cover the gravel in order to prevent soil from blocking the pipe
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An outlet point where the water can exit safely, either through a stormwater connection, a soak well, a rubble pit, or lower ground inside your property
The required equipment includes a spade, which will be used on shorter lengths, and a trenching machine for longer lengths. Other equipment includes a wheelbarrow to haul the gravel and some string line with a line level.
How to install an AG pipe and gravel trench, step by step
Mark out and dig the trench with the right fall
Begin by marking where water tends to gather along the shed's side, typically on the uphill or lower side. Dig a trench between 300 and 600 millimetres deep and about 300 millimetres wide, adjusting the depth as needed if the water table is high. Ensure the bottom of the trench slopes consistently towards the outlet, maintaining a gradient of at least 1 in 100, which means a 10-millimetre drop for every meter of length. Use a string line with a level to verify that the slope remains even throughout the trench.
It's important to leave enough space between the trench and the shed's foundation. Digging too close can destabilise the footings and potentially weaken the shed's structure. Therefore, you should maintain a buffer of at least 300 millimetres from the slab edge.
Line the trench with landscape fabric
Line the whole trench with the landscape fabric, making sure that it fits properly at the bottom and on both sides. Have plenty of the fabric left at the top since you need more for folding it over later. This is necessary because the fabric allows water to pass through, but it does not allow the soil to enter the gravel and block the pipe. There should be enough fabric on both sides to fold over the gravel when done.
Add the gravel and lay the AG pipe
Layer a base of drainage gravel in an approximate depth of 50 to 100 mm inside the trench wrapped in the fabric. Then, lay down the AG pipe directly on top of the gravel layer, ensuring that the openings face downwards. The construction will ensure that the water gathers inside the trench, flowing into the pipe through its openings and ensuring proper drainage rather than water accumulation. Cover the pipe with another layer of gravel, burying it at about 100 mm depth. Ensure that the pipe is placed following the slope of the trench.
Connect the outlet to carry water away from the shed
Make sure the termination point for your AG pipe ends up at an appropriate point in order to deal with the accumulated water effectively. Making a choice about where this water goes is as important as digging the trench itself. You can do one of the following:
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Discharge the water into a stormwater drain that has been authorised
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Discharge it into a soak well
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Make a rubble pit but far from buildings
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Drain it on lower land belonging to you.
It should be remembered that discharging the water onto someone else’s land is not allowed.
Common mistakes to avoid with shed drainage trenches
The failure of a drainage trench can be attributed to certain specific causes, and all of them are preventable.
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Skipping the fall. The trench cannot drain the water, as it does not have any fall, and therefore ends up becoming flooded.
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Forgetting the outlet. Water gets accumulated in the gravel due to lack of an exit point, and then ends up where it shouldn’t be.
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Using plastic sheets instead of fabric. The impermeable plastic lining ends up trapping the water in the trench, while a permeable geotextile fabric allows it to pass through.
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Diggling very close to the slab. A trench dug next to the footings of the slab can end up compromising their strength.
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Using the incorrect kind of gravel. Gravel that is not washed will clog quickly, so make sure you’re using clean, washed stone that water flows through freely.
How to maintain your AG pipe drainage trench
To maintain efficiency in your trench, make sure you check the outlet if there has been any heavy rain. It's always recommended that you clean out leaves and other debris around your outlet at least several times a year. If there happens to be an excessive amount of rain and the outlet is under stress, you may have to do this even more often. You should also check whether there is any water flowing out of the outlet after a storm.
Keep an eye on any drainage sections that run close to mature trees. Over time, tree roots can invade the drain. If you notice a slower water flow, you can usually clear the clog by running a hose or a drain jet through the pipe before the silt solidifies. This proactive approach helps maintain an efficient drainage system and prevents potential water damage.
Planning a new shed? Design drainage in from the start
Installing a drainage system once the shed has been built is bound to be expensive. The best way forward is to plan for the drainage needs prior to laying down the concrete floor. With the foundation raised above ground level and the floor above ground level, you will be able to drain out any water naturally.
Even the sturdiest sheds, for example, those made out of Australian steel, can be affected when constantly exposed to water. This is why it is important to ensure a proper base foundation. Should you consider building your own shed, our range of residential sheds includes various designs for different types of terrain. Our team will be able to design drainage into the initial plan to make sure everything works as it should from day one. This is particularly important when dealing with large areas and farm sheds.
Frequently asked questions
AG pipe is a slotted agricultural pipe that is placed within the trench and has similarities to a French drain, but is not exactly the same. A French drain is simply a trench filled with gravel that passes the water through the rocks. By adding a perforated pipe at the bottom of the trench, as with an AG pipe, you get an improved system that can carry a lot more water than a standard pipe. Most shed drains employ this pipe today, so the two terms often get used to mean the same thing.
A drainage trench for a shed must be deep enough to place it beneath the surface of the ground you wish to keep dry, and this often requires digging depths of around 300 to 600mm. It must be deep enough for the bottom of the trench to be below the surface of the shed floor or slab to collect the water before it gets to either surface. When there is an elevated water table or moist soil beneath the surface, dig it deeper.
For an AG pipe and gravel trench, a minimum fall of 1 in 100 is required, meaning that the trench falls 10mm for each metre run from the point of origin to the point of outfall. Consistency matters more than steepness. If the trench has any rise or fall in between, it will tend to hold water in those areas.
When laying AG pipe for drainage in the shed and gathering groundwater, make sure that the holes face downward. The water will gather at the bottom of the gravel and enter the pipe via the lower holes before being drained out, rather than accumulating within the pipe. The holes should always face upward only when the pipe carries surface stormwater.
An AG pipe drain can be installed in clay soil and is one of the instances when a trench earns its keep. Clay soil keeps water and drains it slowly, which means that water will stay around your shed for a much longer time than it will stay in sandy soil. The presence of gravel inside the trench creates a direct passage of water into the drain, which gets trapped in clay soil and must be covered in landscape fabric because the fine grains get stuck into the drain easily.
A properly constructed AG pipe and gravel drainage trench should last between 20 and 30 years or even longer. The biggest problem in the early failure of a drain is the clogging, where soil and sediment penetrate through the gravel and obstruct the pipe, which is precisely what landscape fabric was designed to prevent. A trench made using clean gravel, appropriate fabric, and adequate fall and drainage points will continue functioning for many years with minimal maintenance.
