Under the tiles there is a structure that carries the weight of the roof, the snow and the wind. Hardly anyone sees it, but it decides whether the work needs an engineer. This guide shows the three types, the warning signs, the loads in the standard and who may sign the design.
Updated on 4 November 2026. General information. It is not a structural assessment: only an engineer, on site, can say whether the structure is safe.
The essentials
- There are three families of structure: timber (the traditional one), reinforced concrete slab and steel (APICER and CTCV, 2018).
- A sagging roof is a warning sign. It may be just a crooked batten, or structural failure and a push on the facades.
- Changing the weight, the spread of loads or the supports calls for a design and a check of the structure underneath (APICER and CTCV).
- Snow and wind depend on the place. The engineer calculates them.
- Designs for the structures of buildings are for civil engineers or civil technical engineers with valid registration (Lei n.º 31/2009, amended by Lei n.º 40/2015).
The three types and how to recognise them
- Timber: trusses (asnas), purlins (madres), rafters (varas) and battens (ripas). On short spans the truss may be missing: there is a ridge beam and the rafters.
- Concrete slab: a continuous, sloping surface that follows the pitch of the roof.
- Steel: steel sections, sometimes in «light steel frame».
APICER and CTCV describe the traditional roof as a timber structure in layers, each less strong and more closely spaced near the tiles. The names are in the roof glossary. On a slab, the tile also sits on a batten. Steel often follows the timber model.
The way to tell them apart is ours: exposed wood is timber; smooth concrete following the slope is a slab; a folded, bolted section is steel.
Timber: signs in the structure
Pests and rot have their own guide: timber, woodworm and termites.
- Sagging. APICER and CTCV link a deformed roof to structural failure, to a horizontal push on the facades and to displaced tiles. It may come only from the battens or rafters, but they always advise a structural assessment.
- Push on the walls. A well-braced wall plate (frechal) reduces horizontal thrust. Cracks at the top of the walls or a leaning wall are signs by our own deduction: call an engineer.
- Joints. The joint between the rafter leg and the tie beam of the truss must carry the vertical and the horizontal force, and must not sit too near the end of the tie beam, or it may fail in shear. Metal plates at the nodes only count if they are calculated, and so do temporary props.
- How the main structure fails: mostly by fire, rot from serious leaks, or collapse of what is underneath.
Concrete slab: what to look at
LNEC says that in reinforced concrete, corrosion of the reinforcement is the most frequent degradation mechanism, often tied to thin cover. It comes from carbonation of the concrete or from chlorides. Rust takes up more volume than steel and causes cracking and then spalling of the concrete.
Seen from below, our reading is: brown stains, concrete coming loose, bars showing and cracks along them. Damp stains call for the cause of the leak. APICER prefers insulation on the outer face of a sloping slab (see attic condensation).
Steel: what to look at
LNEC names corrosion as the main form of degradation of metals. Plain steel usually gets zinc, zinc and aluminium or paint, and corrosion depends on humidity and on agents such as chlorides and sulphur dioxide. Look for rust and peeling paint.
For panels, Kingspan (09/2024) says the sub-structure is not part of the delivery: the supports must meet the panel’s needs, and the designer or supplier assesses its structural and fire behaviour. Paroc (03/2022) asks that, before fitting, you confirm the frame is ready, with wide enough supports and acceptable tolerances, and that the steel thickness suits the screws. See the sandwich panel guide.
Which loads the engineer counts
The values come from IST lecture notes (2011) on NP EN 1991 and the national annexes. We did not read the standards.
- Roof reachable only for maintenance (category H): 0.4 kN/m² on the horizontal projection and a point load of 1.0 kN, which are not added to snow or wind.
- Snow. Three zones, Z1, Z2 and Z3. The ground load is sk = Cz [1 + (H/500)²] kN/m², with H the altitude in metres and Cz equal to 0.30 (Z1), 0.20 (Z2) and 0.10 (Z3), for a 50-year return period. On the roof it is multiplied by shape, exposure (0.8 to 1.2) and thermal coefficients.
- Wind. Zone A (almost the whole country): reference speed of 27 m/s. Zone B (Azores, Madeira, a 5 km coastal strip and mainland above 600 m): 30 m/s. Also for 50 years.
The zone of your municipality is on the map in the national annex, which we did not see.
Changing the roof covering: when it needs an engineer
APICER and CTCV say a new structure must be compatible with the building’s in weight, in the spread of loads and in the supports. Any significant change in these points requires a full design and a check of the impact on the structure underneath.
In our reading, swapping tile for a light panel (or the reverse) changes the weight and the fixings: it is a case for an engineer. A lighter roof is not, by that alone, safer: see earthquakes and roofs. See the works permit and the weight of tiles in tiles and climate.
Who may sign
Lei n.º 31/2009, as amended by Lei n.º 40/2015 (in force since 6 June 2015), says:
- Engineering designs are for engineers or technical engineers recognised by the Ordem dos Engenheiros and the Ordem dos Engenheiros Técnicos (article 10, paragraph 3).
- In annex III, table 2 lists civil engineers and civil technical engineers for «foundations and structures of buildings».
- For more complex works, the law asks for specialist status or years of experience. Category II includes structures of buildings under 15 m high, from foundation to roof, and structures with spans up to 8 m.
- The author signs a statement of responsibility (article 21) and whoever designs or directs the works must hold civil liability insurance (article 24).
A contractor or roofer is not on that list. Ask for the registration number and check it with the Ordem (our advice).
What to ask the engineer
- The type of structure and its condition, with photographs.
- The registration with the Ordem, the statement of responsibility and the insurance.
- The loads used: altitude, snow zone and wind zone.
- The weight of the new covering and of the old.
- The joints, the supports and the calculated propping.
- For steel: protection against corrosion.
- Everything in writing. See how to choose a contractor.
See also: On snow, hail and ice, see snow, hail and ice on the roof.
What we could not confirm
- The texts of the standards NP EN 1991-1-3 and 1-4 and of the national annexes, sold by IPQ. We did not see the zone map, the zone of each municipality, or any new versions since 2011.
- The category (I to IV, Portaria n.º 701-H/2008) of the design of a house roof: we did not read the portaria.
- The current text of Lei n.º 31/2009. We read the version of 14 June 2018 and Lei n.º 40/2015 as a copy; the Diário da República was not consulted. Lei n.º 3/2026 only through the OET note, which concerns article 5.
- An official limit for acceptable deformation.
- The share of buildings by type of structure in the 2021 Census.
- The visible signs of slab and steel: they are our own deduction.
Sources (read on 4 November 2026)
- APICER and CTCV, Guia para Projeto e Aplicação de Telhas Cerâmicas, 2018.
- P. Mendes (IST), Quantificação de ações em edifícios de acordo com o Eurocódigo 1, April 2011.
- Lei n.º 40/2015, of 1 June, copy of the Diário da República. Lei n.º 31/2009, consolidated version of 2018.
- Ordem dos Engenheiros Técnicos, note on Lei n.º 3/2026, 6 January 2026.
- LNEC, Durabilidade dos materiais de construção (no visible date).
- Kingspan, KS1000 RW, design and installation guide, 09/2024. Paroc, installation guide, 03/2022.