small time developments limited ian mccomb
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Storm in a tea cup!


Flooding is not a great word to be associated with any home.  In this article, I look at a couple of great solutions to this potential problem you could face on your site.

The fairly common issue of downstream flooding, can be a serious one when proposing infill developments, even with Tiny Houses.  This can happen if the Council stormwater system does not have spare capacity which is commonly the case. How?  Well, this can mean that if is any sort of ponding, even small increases in hard  or impervious surface can lead to the “cup” filling up and flooding homes that were not flooded before the development.

How do we prevent this? Luckily, the maths is fundamentally pretty simple, 1mm of rain on 1m2 of impervious surface generates 1L of water or for this discussion, stormwater runoff.  Whilst the extra runoff can be held up (detained) so the flow is less and the stormwater pipes have the capacity, this is less beneficial in most cases than keeping the water on site to assist underground water storage for streams and vegetation.

In simple terms solid man-made structure such as roads, concrete and rooves are impervious and lead to high amounts (but not total) runoff.  Flat, vegetated surfaces lead to much lower (but not zero) runoff.  So the basic plan is to minimise creating hard surfaces and maximise flat vegetated surfaces in the development which capture stormwater.  The two main forms of technologies that I favour as a direct replacement of impervious driveways and rooves are porous paving and green rooves.

Porous Paving

There are many forms of porous paving and several have been around for many years. However, I’m investigating the use the relatively new Grasscrete product from Stormwater360 as it can be laid in any layout and steel reinforced to handle light traffic loads in the longer term.

Green Rooves

Green Rooves have also been around for ages with sod and earth-sheltered homes.  Recent research by Landcare and the University Of Auckland has quantified that over 80% of the potential annual runoff from a roof can be captured by a green roof and then evaporated back into the atmosphere.  This avoids flooding, provides cooling and the other benefits of living plants as habitat, aesthetics and air pollution reduction.


Building a small house with green roof could reduce the runoff from a steep site and offset the impact of other hard surfaces.  As there is flood risk potential on one hand and multiple benefits on the other, planning for stormwater management is a key part of the early site design.  Luckily I think it’s fun!

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