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Where should a rain tank overflow go?
A soakaway can keep overflow from running across the surface near the house, but it needs to accept the roof’s inflow during heavy rain and drain without water pooling beside the foundation. A gravity connection to a storm drain may be easier to inspect, but that route isn’t necessarily allowed everywhere.
What do you weigh first when choosing between them? I’m particularly interested in how to judge overflow capacity against heavy rainfall, and what local rules or site conditions can rule out either option.
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6 replies
I’d first rule out anything that local rules prohibit or the site can’t support; there’s little value in sizing a route that isn’t viable. Among the options left, I’d compare how each handles the roof’s inflow in heavy rain, including where excess water goes if its capacity is exceeded. An inspectable connection is appealing, but only if it’s permitted and its overflow consequences are acceptable.
For a fair comparison, start with the roof area draining to the tank and a locally relevant heavy-rain assumption, then estimate the inflow and compare it with the soakaway’s available storage and ability to infiltrate - or the connection’s permitted outlet capacity. I’d also trace where surplus water goes if either route is temporarily outpaced; capacity on paper matters less if the overflow ends up beside the foundation.
If both routes look adequate on paper, how much weight would you give uncertainty about the soakaway’s infiltration in practice versus the connection’s permitted capacity and easier inspection? I’d find that reliability difference useful in making the comparison.
I’d give the infiltration uncertainty real weight, but wouldn’t automatically favor the connection just because it’s easier to inspect. If a soakaway’s performance is uncertain and there’s no safe overflow route, that’s a serious concern; if the connection’s permitted capacity is tight, inspection won’t make it cope with a downpour. I’d lean toward whichever has the more dependable margin and a safe overflow path.
What evidence would make you comfortable relying on a soakaway - such as a site infiltration assessment that reflects wet conditions, rather than just a capacity estimate on paper?
I’d want an infiltration assessment based on site conditions that reflect wet ground, not just a nominal rate or a capacity figure on paper. I’d then compare that evidence with the proposed soakaway’s storage and the expected roof inflow, and check that any excess has a safe route away from the foundation. A good infiltration result matters less if the overflow still pools by the house.
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