Healthy Homes Series #3 – Drainage, Eaves and Gutters – and Roofs too!

Liz Bassett Sustainability

-Mikala Dind

In this six-part series, Narara Ecovillage member Mikala Dind shares her knowledge about what makes houses healthy or unhealthy. While this article is about existing homes, the information will definitely assist anyone seeking to build a home.

One of the most common reasons for mould problems in a home is water finding its way into the building.  In fact, keeping water out of buildings is one of the most fundamental aims of building/landscape design.  If these principles are skipped or corners are cut, then the home will become a ticking time bomb, with potentially cascading problems and increasingly expensive repairs.


As you know, water will find the path of least resistance, so if you don’t redirect it away from the home, it will find its way in, either directly or indirectly.  

Sloped sites

Sloped sites need both surface and subsurface drains in ground that is higher than the home in order to move surface and subsurface water around it.  These sites have the benefit of gravity at their disposal. 

Something that’s occasionally overlooked is including a drain at the front of the garage where the driveway runs downhill towards the house.  I get concerned when builders rely solely on angling the driveway to direct the water away from the garage.  Again, it may work 90% of the time, but it’s that 10% that causes trouble –  when water gets into the garage it can damage items stored there, and dampen the slab, increasing humidity in adjacent rooms.

Flat sites

Flat sites also need drainage.  However, they need to create their own “fall”, using either drains or earthworks to make the land go downhill from the home, so water is drawn away from the house.  

Termites building their way past the cement slab to enter a home

Keep slab edges and the base of walls exposed at ground level

A house built on a cement slab needs 75mm of the slab exposed, both to reduce the likelihood of water intrusion, and to make any termite attacks easy to see (as in the photo above). 

Often gardens are built right up against the home, which can lead to water ingress into the home, increased humidity in rooms behind that wall, and hide termite incursions.  Keep gardens, mulch and compost well away from walls. Another consequence is that airborne mould spores from the garden (eg decaying mulch) can enter the home through windows.

Garden built up against a wall, causing flooding inside the home

During one mould audit it started to rain (usually something that makes my job more difficult).  However, my client, who rented the property, was thrilled.  Why?  So she could show me how the downstairs laundry (that we had already inspected) gets its own little river coming out from under the laundry cabinet when it rains.  The real estate agent had not believed her claim of the flooding, but there it was for all to see about 30 minutes after the rain began.  

The reason?  The owners had built a garden up against one of the home’s walls (see photo), covering the weep holes in the brickwork.  The result was that water now trickled into the double brick wall cavity, rather than weeping out of it.

Eaves, Gutters and Roofs


Somewhere along the line, eaves started being uncool.  That’s a shame because they protect your home.  They keep your windows (particularly wooden framed ones) dry, thus reducing the likelihood of water ingress (eg through wood rot).  They also keep the ground abutting your home drier, reducing the likelihood of bricks and mortar becoming damp and increasing the humidity in the home.


Gutters and downpipes may not be aesthetically pleasing, but they perform a vital function – to remove water off the roof and into storm water pipes before it can do any damage.  Not enough downpipes, or guttering with a higher outer lip, can almost guarantee that rain will find its way into the roof cavity, potentially damaging the structural integrity of the roof and encouraging mould growth.

Flat roofs

Another fad with even more drastic consequences is the flat roof and/or hidden guttering.  Please run a mile from either of these architectural catastrophes.  Roofs need to have a decent pitch/fall (15% or greater) so that rain is forced by gravity towards the gutters, and wind cannot easily push rain uphill and into the roof cavity (eg via the ridge cap).  I did an audit once where the roof over the outside entertainment area (connected to the building just above the back door) was actually pitched towards the building, creating really bad humidity and water damage issues.

I always recommend that my clients spend the money to over-engineer their drains, gutters, and downpipes so that they reduce the risk of water getting into their home (either via the roof or the floor).  It only takes one event of water ingressing into the home and much damage ,(both seen and unseen,) can be done.

You may be wondering why I keep harping on about humidity … well, you’ll need to wait until the next instalment as I will discuss the relationship between humidity, dust and mould.