Concrete sectional garage roofs

The wall is a series of precast concrete panels, slotted between concrete posts and bolted or grouted into place. The roof sits on top: a pitched or shallow-pitched arrangement of concrete or timber rafters carrying corrugated sheets, or in some cases a flat roof of panels or boards finished in felt. The whole kit was designed to go up quickly in an afternoon with minimal skill, and it was never meant to carry much weight.

That detail matters when you choose a new covering. The roof is light, the panel walls do not tie the building into a rigid box the way brickwork does, and the rafters are often slender. Anything heavy, such as a clay or concrete tile, is out of the question without engineering advice, and the walls may be marginal for a heavy new deck as well.

Look at the outside before anything else. Panels that have cracked or leaned, or posts that have spalled and shown rusty steel, are a concern in their own right and affect whether the roof is worth recovering at all.

For decades the standard cheap roof for these garages was a run of corrugated asbestos cement sheets. HSE's rule of thumb for premises built or refurbished before 2000 is to assume asbestos is there. Garage sheets from that period are commonly asbestos cement, so a sectional garage of unknown age is handled on that footing until a sample says otherwise.

Sound asbestos cement is not an emergency and need not be stripped the day you find out. It does need care. HSE sorts the work by condition: sheets lifted off whole fall in one category, and sheets reduced to fragments fall in a stricter one that must be notified. Dropping or smashing them is therefore the wrong way to clear a roof. Our page on how asbestos waste is disposed of explains the consignment-note side.

There are four sensible routes, and the right one turns on the roof's shape and how much of the structure deserves keeping.

• Fibre-cement sheets. A modern, non-asbestos corrugated sheet that looks like the original and fixes to the same rafters. It is the nearest like-for-like swap.
• Box-profile steel. Light, long-lasting and quick to fit, with long lengths that reduce laps. Fitted with an anti-condensation lining it behaves well in a garage.
• A flat deck covered in felt, EPDM or GRP. Only worth it where the existing roof is already flat or can be framed up to a proper fall. The recommended minimum fall quoted from BS 6229:2018 is 1:80, with 1:40 as the design target.
• Re-sheeting over the old roof. Sometimes discussed as a cheaper path; see our page on overcladding an asbestos garage roof for when it makes sense and when it stores up trouble.

Concrete panels with a thin roof above them make a cold building. Warm, damp air from a wet car or a drying rack condenses on the underside of any sheet that cools below dew point, and the drips can pass for a leak on a sound roof. Steel suffers most, which is why we cover it in a separate page, why a metal garage roof drips.

Fibre-cement is gentler on condensation than bare steel, and a lining or a layer of felt under the sheets helps either way. Ventilation helps most: a small gap at the eaves and a vent at the gable let moisture out before it settles. Garage conversions and workshops, where heat and moisture are higher, need a different approach, and the Building Regulations enter the picture. Check with your building control body, or Swindon Borough Council's building control team, before anything is lined or insulated.

Sectional garage roofs are among the quicker garage jobs, because the structure is simple. Even so, the plan depends on the covering. Sheets can go on in dry, calm weather because wind catches a large corrugated sheet and turns it into a sail. Felt and EPDM have their own temperature and moisture limits. We cover how the season changes things in our page on replacing a garage roof in winter.

On the day, expect old sheets and fixings to be lifted off carefully, rafters checked and any rotten timber replaced, then the new covering laid with correct laps and fixings and the edges flashed or closed. Where the old sheets are asbestos cement, the removal follows HSE rules for asbestos cement, and the waste goes off site with its consignment note. Please move the car, the bikes and the freezer out first, because dust and debris will fall.

We inspect the sheets, rafters, panel joints, posts and guttering, photographing each part as the survey goes on. A written total follows. Roof area, the result of any asbestos test, the state of the rafters, your choice of covering and skip access are what shift it. Check that any figure you are given states whether VAT and scaffolding are included.

Sound rafters may only need new sheets, while failed timber needs more, and we recommend whichever is lighter if it cures the fault. The workmanship carries a written guarantee of ten years, and the rubbish goes with us when we leave. WhatsApp pictures of the outside, the roof and the inside, or ring 01793 626096. More options are collected on our garage roof replacement page.