Choosing a covering for a heated or unheated garage roof

A garage may be merely a shelter for a car or a workshop used for many hours. Heating, car washing and storing wet items change the humidity conditions. Even in an unheated building, a car brings in water and snow. Vapour may condense on the cold underside of the covering, particularly where air exchange is limited.

Record the intended use before choosing the material. If you plan to heat the garage later, it is worth taking this into account when designing the roof. Adding insulation from the inside may require changing the details and checking for condensation. Not every simple enclosure can be safely converted by gluing boards between existing profiles.

On a cold garage, trapezoidal steel sheet may be an economical covering if its parameters suit the structure. T18 and T35 differ in profile height, but there is no single permissible support spacing for each of these names. The sheet thickness, steel grade, geometry and loads matter. The quote should specify the particular product, not just the category.

Check the required slope and sheet lengths. When joining along the length, different rules may apply than for a single run from eaves to the highest edge. Do not reduce the pitch on the basis of a general claim that trapezoidal steel sheet is suitable for any flat roof. Water must drain efficiently even after allowing for deflection of the structure.

First ensure ventilation appropriate to the way the garage is used. An anti-condensation coating can take up some of the water forming on the underside of the sheet. Its effectiveness depends on the possibility of subsequent evaporation. In a permanently damp and unventilated room it will not act as an inexhaustible store of water.

Such a layer does not replace insulation or a sealed barrier. It is also worth checking its behaviour at the eaves and laps in accordance with the instructions. Improper termination may encourage water to be drawn up. If the garage is to be finished on the inside, the assembly should be assessed before it is covered. Once enclosed, detecting the source of moisture is often much more difficult.

You can build a layered roof with insulation and suitable waterproofing, or use a roof sandwich panel. In the second solution, the facings and core form a ready-made element of a lightweight enclosure. Its thickness, span and fixings must be selected. A wall panel does not become a roof panel merely because it has a similar core.

PIR insulation boards provide high thermal insulation at a relatively small thickness. This does not mean that every thin panel will deliver the required thermal performance for the whole garage. Check the declared thermal transmittance of the chosen product and the thermal bridges at the joints. Also take into account fire requirements, particularly where the garage adjoins the house or has a specific pattern of use.

Tile-effect metal roofing can be a good choice if the garage is to match the house visually and has a suitable pitch. On a simple roof, it is worth comparing sheets cut to size with modules. The small format is easier to transport, but a long sheet can reduce the number of transverse laps. Both solutions require correctly spaced battens.

Do not choose a profile solely because it resembles the roof of the main building. Also check the coating, the colour in real light and the proportions of the flashings. A small garage may call for a quieter detail. If the pitch is too low for the chosen model, it is better to use a solution approved for such conditions than to try to rescue the appearance with more sealant.

In the cost estimate, include the structure, covering, insulation, flashings and drainage. Add fixings, transport and any equipment for lifting the panels. For a small garage, accessories can account for a large part of the price. It is worth comparing two complete variants, e.g. a cold roof made of trapezoidal steel sheet and an insulated roof made of sandwich panels.

Also check the planned installations and how they will be serviced. Heavy equipment or photovoltaics change the loads and require suitable fixings. Do not leave this until after acceptance. A well-chosen garage roof should match the actual use over the coming years. Then the simplicity of the solution goes hand in hand with durability, rather than with the need for rapid reconstruction.