Making the McMansion Green? Concept #1 – The Green Roof

McMansions. Love them or hate them, the term is now embedded in our lexicon and has a knack of polarising opinions. McMansions refers to what we commonly view as oversized detached housing, for example a family-of-4 home with 4 bedrooms, 3-4 bathrooms, media room, 3 car garage, no eaves and no landscaping.



Apparently new Australian housing is the biggest in the world [per capita], and homes in New South Wales are the biggest of the big. We’re just not getting it. There are a range of reasons, including how they’re marketed, how banks value property [size does count] and ‘keeping up with the Joneses’ [aka Status Anxiety].

house sizes

Source: Building Product News

Whilst these homes might meet current building code energy efficiency standards, they’re still twice as big as our homes used to be.

But let’s park the reasons and talk some outside-the-square ways in which we might make these homes more climate resilient in our near future. What might some of our solutions be for these future renovation rescues of entire suburbs?

The first thing I’m curious about is how could we retrofit a green roof? Whilst I’m not necessarily a one-eyed green roof devotee, there is a time and a place – and this could be one of them. Green roofs on existing homes would reduce energy consumption, improve comfort, reduce heat-island effect and improve micro-climate, create habitat and treat stormwater runoff.

[Primer: a green roof is essentially a planted roof. An extensive green roof is one that isn’t intended to be trafficable, so it’s lighter weight and has a shallower soil profile. An intensive green roof is one that is trafficable and has heavier structure and deeper soil. They can be expensive but also save energy by acting as a living blanket over the roof, insulating the home underneath.]


Source: Green Garage

Where it gets tricky for a McMansion is that green roofs tend to lend themselves to shallower gradients, not to 20deg-30deg pitches. We’ll assume for now that a McMansion roof structure [typically prefabricated timber trusses] has enough load capacity to bear a fully soaked green roof.

Here’s how it could potentially be done;

  1. Remove the existing cladding – whether it be concrete tiles or metal decking. Metal decking could remain if the load isn’t too much. Replace with marine ply board;
  2. Add the requisite layers of waterproofing, drainage cell, insulation and geotextile;
  3. Add the perimeter angles to hold the soil/planting [sounds like it could be a tricky detail, but it is possible];
  4. Add the soil profile and planting. For this one there are various methods available – I didn’t have any luck sourcing Australian examples/products so the US it is. There are proprietary soil stabilisation products available for steeper slopes with in-situ planting, or there is planting in plastic trays or even mats which come ready-established.

Source: ZinCo Green Roof Technology [subsidiary in Australia]

green roof



Source: This Old House

For steep green roofs there are also some additional challenges;

  • water can run off quickly, so soil depth and plant species become important;
  • the apex of the roof might not hold any water at all, so irrigation might be necessary in drier areas;
  • the pitched roof has different solar aspects, so different planting is required on either side [just like vegetated valleys/hills]
  • access for maintenance becomes more challenging and risky.

Whilst there are a raft of other lower-cost measures that can be taken to make a McMansion greener, I think this one will be fascinating in that it doesn’t seem to have been decently explored yet. Sounds like a niche market to me.


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