Which roof covering suits your garage?

A detached, unheated garage has different needs from a workshop used in winter. Both can get damp: a wet car, melting snow and work inside all introduce moisture. Allow for ventilation and a way for surfaces to dry. Simply closing the building tightly will not remove the moisture.

Where a garage adjoins the house, consider how its roof meets the wall and the rest of the building envelope. This junction needs a different detail from the exposed edge of a carport. Establish where the heated space ends and what the design requires before choosing the covering and the layers below it.

T18 or T35 profiles may suit an unheated garage if their load capacity matches the support arrangement. T35 has deeper ribs, but that alone does not determine the permissible span. Check the thickness, steel grade and geometry of the particular sheet against the manufacturer’s data or a structural calculation.

A sheet that reaches from the eaves to the top of the slope can eliminate end laps. First check that it can be manufactured, delivered and lifted at that length. On shallow slopes, laps and flashings need particular care. Two sheets sold as T18 do not necessarily have the same minimum pitch.

No. Fleece on the underside of the sheet temporarily holds some of the condensate and releases it as conditions dry out. It needs air and an opportunity to dry between wet periods. It does not provide the thermal resistance of an insulated roof or replace all the layers required above a heated room.

Intermittent heating can bring warm, damp air into contact with cold surfaces. Before relying on fleece alone, consider how the space is ventilated and used. If it must hold a steady temperature or protect moisture-sensitive equipment, have a complete insulated roof build-up designed.

An insulated roof panel combines two facings with an insulating core, which can simplify construction above a heated garage. A PIR core can provide the required thermal resistance at a relatively small thickness. Span and loads also affect the choice of panel thickness; price alone is a poor guide.

The panel needs suitable supports, fasteners, seals and junction details. Roof and wall panels differ in their outer profile and joints. Minimum pitch depends on the system and whether the roof has end laps. Check the fire requirements for the whole assembly, particularly where the garage connects to the house.

Tile-effect sheets can help the garage roof match the house. Compare the actual colour and spacing of the tile pattern. Sheets cut to length suit a simple roof slope; smaller modules may be easier to handle where the roof shape is more complex or access is restricted.

The garage roof must meet the minimum pitch for the chosen sheet. If it would clash with an existing window, ask the designer to resolve the junction; do not reduce the pitch below the stated limit. The layers beneath tile-effect sheets must also suit the heating and ventilation of the garage.

Even a small roof can leak at the wall, eaves or verge. Wall flashings must direct water onto the covering while allowing for movement at the junction. Simply sticking metal to the render with a general-purpose sealant is insufficient. The ends of the flashing need a detail that keeps water out as well.

Size the drainage for the area that feeds it and plan where the water will go on the plot. Avoid discharging it where the ground beside the garage stays wet. If snow can slide from a higher roof, assess the load on the lower slope. A gutter is not a snow guard.

Compare complete systems: trapezoidal steel sheet with any necessary underlying layers, tile-effect sheets with accessories, and insulated panels with their junction details. Include supports, transport and labour. Extras can account for a considerable share on a small building, so sheet prices alone tell you little about the cost of a weathertight roof.

Choose systems suited to the intended use before comparing like-for-like quotes. An inexpensive uninsulated roof may work well for one garage and require costly alterations in another. Allow safe access for servicing and plan for winter conditions before the roof is built.