When you decide to insulate your flat roof, the contractor will ask: "Shall we do conventional or inverted?" If you don't know the difference, you risk choosing a system that doesn't suit your needs or climate.
The difference is not in the materials used, but in the order in which they are laid. This order completely changes the behaviour, lifespan and cost of your flat roof.
In conventional insulation, the logic is simple: First we lay the "warm" layer (insulation) and on top we place the "waterproof" layer (waterproofing) to protect it.
From bottom to top: 1. Concrete slab. 2. Vapour barrier. 3. Thermal insulation (typically EPS, XPS or Polyurethane). 4. Lightweight concrete (falls) to drain water. 5. Waterproofing on top (Bitumen membrane or liquid-applied polyurethane).
If the bitumen membrane punctures somewhere, you see it immediately and repair it easily (a simple patch). It is a lighter construction.
The waterproofing (bitumen membrane) is completely bare and exposed to sun (UV radiation) and frost. This thermal shock ages it quickly, so in 10-15 years it will need maintenance or replacement.
Here, the rules are turned upside down (hence "inverted"). The waterproofing goes below the insulation!
From bottom to top: 1. Concrete slab & Lightweight concrete (falls). 2. Waterproofing (Bitumen membrane). 3. Insulation (STRICTLY XPS only - Extruded Polystyrene). 4. Geotextile (soil filter). 5. Ballast (Paving slabs or gravel).
The bitumen membrane is protected below the insulation and gravel. The sun never sees it, it never freezes and nobody walks on it. Practically, it never deteriorates! The concrete slab stays at a stable temperature year-round, so it doesn't expand and contract (doesn't crack).
Only XPS may be used. If you use white EPS or mineral wool, because rainwater passes through their joints to reach the membrane, they will absorb water and be destroyed. XPS is 100% waterproof. If the membrane punctures, finding the leak means lifting the gravel and XPS (quite difficult).
We have a 100m² flat roof and must decide.
We lay 10cm EPS, create falls and bond an excellent chip-finished bitumen membrane on top. The system is light. In winter the house is warm. But after 12 summers of 45°C heatwaves, the membrane "cracks" at the joints. We must call a worker to re-coat it with aluminium paint or make patches.
We create the falls, lay the bitumen membrane at the bottom, place 10cm XPS and cover everything with white, round river gravel. Rainwater passes through the gravel and XPS joints, finds the membrane and runs to the drain. The membrane permanently stays at 20-25°C, protected from UV. 30 years pass and the roof is as though it were built yesterday.
The Final Conclusion: What to choose? If your building is old and cannot bear extra weight, conventional insulation is the only option (it is feather-light). But if the building can bear the weight of gravel or slabs (checked by a structural engineer), Inverted Insulation is the ultimate investment. It costs a little more upfront, but saves you from maintenance for an entire lifetime!
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