Roofs: clay tile, insulated metal, green or concrete slab

The roof is the design decision most people take from a photograph. Clay tile because it looks right, metal sheeting because it is cheap, a green roof because it sounds responsible, a concrete slab because «we can put another floor on later».

None of those four reasons is a bad one. The trouble is that none of them comes first. What comes first is how much it weighs, and that number has been published, in an official table, since 2010. Hardly anyone opens it.

The figure nobody puts on the table: weight

NSR-10 classifies the roof as a horizontal non-structural element and publishes minimum dead loads for it, material by material. These are not anybody’s estimates: it is a table in the code, and it is what a structural designer feeds into the model.

There is the whole article in one line: clay tile weighs ten times what metal sheeting weighs. Eighty kilos per square metre against eight. On a house with 120 m² of roof, that is over eight tonnes of difference travelling down the walls into the foundation.

And it is not a difference you settle later. Swapping metal for clay tile on a house already designed is not changing a finish: it is changing the structure. The other way round works better, though it has consequences too, since a structure sized for tile and loaded with sheeting ends up oversized and paid for.

And the weight that is not weight: live load

On top of dead load comes live load, which depends on use. Here is the number that almost never joins the conversation about green roofs:

Hail, which depends on altitude

Here altitude walks straight into the structure, in a way few people anticipate. The code carries a specific hail load, and its threshold is geographic:

The duty no quotation ever includes

The same chapter carries a requirement that is not a matter of judgement and that hardly any Colombian house meets:

And responsibility is allocated in writing: the plumbing design must include both the drainage and the auxiliary system, it falls to the builder who signs the permit to approve it and hand the structural designer the volume of water that can accumulate, and the structural designer must design the roof to carry that weight without failing.

The four systems, one by one

Clay tile

The heaviest, and the one that ages best to the eye. It calls for a roof structure sized to carry it, a frank pitch and a properly resolved edge: verge tiles and ridge are where nearly every problem begins. In its favour it has something no table measures: a broken tile is replaced one at a time, with nothing dismantled.

Insulated metal decking

«Termoacústica» is a trade name in Colombia, not a code designation: it describes a metal sheet with insulation bonded or injected behind it. The code table weighs the sheet — 8 kgf/m² — and the insulation separately, by the millimetre of thickness. It is by a wide margin the lightest solution, and the one allowing longer spans with less structure.

Its weak point is not the material but the junction: laps and fixing penetrations are the route water takes, and the washers degrade well before the sheet does. It is a roof of cheap, frequent maintenance, not of no maintenance at all.

Green roof

The one demanding the earliest decision, for the structural reasons above, and the one that tolerates improvisation worst: the waterproofing sits underneath the growing medium, which is to say underneath whatever would have to be removed in order to repair it. A leak in a green roof is not a spot repair.

Concrete slab

The only one of the four that does not appear in the roof dead load table, for a conceptual reason: it is not a non-structural element. A slab is structure, and its weight comes from the structural design rather than from a table of finishes. What does get added on top of it, and does appear in the table, is the screed, the waterproofing and the insulation.

It is the option that leaves the door open to growing upwards, and that is its real advantage. But «the slab is already there and it holds» is not argument enough: what adding a floor over existing construction really demands is in adding a second floor.

Which one insulates: the question is badly put

«Which one insulates more?» has no single answer, because insulating does not mean the same thing in the four climates of the country. In a cold climate the aim is to keep heat in; in hot and dry, to use thermal mass and hold the cool of the night; in hot and humid, the job is to push warm air out at the ridge, not to seal it in better. The four official classes and what each one asks for are in a bioclimatic house by altitude.

The comparison, with its label attached

What it costs: the honest part

I do not publish roof prices per square metre, for the same reason I publish none for anything else: there is no Colombian source with a methodology, and the ones in circulation contradict each other. The full reasoning is in what a square metre costs.

What can be said, and what helps in reading a quotation, is which line items move with the system:

• —The supporting structure moves with the weight, and it is the big item. Comparing two roof quotations without looking at the structure each one assumes is comparing different things.
• —Waterproofing appears in only two of the four. With tile and sheeting, watertightness is resolved by pitch and lap; with green roofs and slabs there is a membrane, and it is a line item of its own.
• —Drainage is an item, not a detail, and on a flat roof it is the critical one: it includes the auxiliary system the code requires.
• —Maintenance is never zero. It is cheaper and more frequent on sheeting, dearer and more spaced out on tile, and on a green roof it is a standing obligation that has to be budgeted as such.
• —The penalty for getting it wrong is asymmetric. Changing system once the structure is sized is only cheap in one direction: towards the lighter option.

How the decision gets made

1. 1Start from the climate and the altitude, not from the image. Altitude decides whether hail load applies, and climate decides whether the job is keeping heat in or driving it out.
2. 2Fix the system before the structural design, not after. It is the decision that costs most to change late.
3. 3If you want a green roof, decide it on day one. Five hundred kilos per square metre do not settle onto a structure meant for something else.
4. 4If you want to keep a second floor possible, the slab is the answer, but ask for the structural design to allow for it now: providing for the floor is far cheaper than strengthening later.
5. 5Insist the plumbing design includes the auxiliary evacuation system. It is a code duty and it is almost never drawn.
6. 6Budget the maintenance of the system you chose, with a frequency and someone responsible. The roof is the element most often abandoned and the one that charges most for abandonment.

The roof is the only part of a building working twenty-four hours a day against the sun and the rain at once. Choosing it from a photograph turns out well more often than it should — and when it turns out badly, changing the tile does not fix it.