Garage roof replacement in Bath

Plenty of older garages in and around Bath are roofed with large corrugated sheets laid over timber or concrete purlins. From the ground, cement sheeting that contains asbestos looks much like the cement sheeting that does not. Colour, moss and weathering tell you nothing reliable. The only sound way to know is to have a sample analysed by a laboratory, and until that result is back we treat any grey corrugated garage roof of a certain age as though it might contain asbestos. That costs nothing and keeps everyone safe.

Asbestos-cement is a stiff, brittle board made of cement with asbestos fibres mixed through it. While the sheet is whole and undisturbed, the fibres stay locked inside. The danger comes when the sheet is broken, drilled, scrubbed or blasted and the fibres are released as dust. That is why the common advice is the same everywhere: do not pressure wash it, do not scrape the moss off it, do not drill it and do not smash it up for the skip. The page on cracked asbestos roof sheets explains what to do if one has already split.

Removal as non-licensed work

Under HSE guidance, taking asbestos-cement sheets off a roof counts as non-licensed work so long as they come away carefully and are not broken up on the way down. It becomes notifiable non-licensed work where the sheets will be substantially broken up, creating significant quantities of dust and debris. In plain terms, the method decides the category. Whole sheets, unfixed with care and lowered rather than dropped, sit at the gentler end. A roof that has already crumbled, or one that cannot be removed without smashing it, sits at the other. The HSE sets this out on its non-licensed work page, and every garage job we plan starts from that distinction.

Because the method matters so much, we look at fixings and sheet condition during the survey. Bolts that have rusted solid, sheets that have been painted and sealed over the years, and sheets that have sagged between purlins all affect how much care the lift needs. We write the approach into your fixed price so there is no surprise on the day.

Testing, taking away and disposal

If you do not already hold an asbestos report for the garage, a sample comes first. Our page on whether you need an asbestos survey before garage roof work covers when a sample is enough and when a fuller survey makes sense. Once the sheets are off, they are double wrapped and taken to a site that accepts them. They never go in a general skip, and they never go to the household tip in the boot of a car. Our guide to how asbestos sheeting is disposed of goes through the wrapping, the labelling and the paperwork.

Some owners ask whether the roof can be painted or sealed instead, and whether it can be left alone and covered. Both are fair questions. A coating can slow the shedding of surface fibres on a sound sheet, and an overlay can keep the old sheets in place under a new covering. Neither is the same as removal, and neither suits a roof that is cracked, sagging or leaking at the laps. We cover the sealing question on encapsulating an asbestos roof, and the overlay route on the page about overlaying an asbestos garage roof.

Our own position is simple. We tell you what the survey and the sample show, we explain the options that are open to you, and we price the one you choose. Where the sheets are in poor condition, we tell you up front and show you the photographs. Where an overlay would be acceptable and sensible, we say that too.

With the sheets identified and a method agreed, attention moves to what they were resting on. A roof can only be as good as its frame, so the next chapter looks at the purlins and the timber or steel beneath the old covering.

A garage roof is light, wide and often exposed at the edge of a drive or a garden with nothing to shelter it. Wind passing over a roof creates lift, in the same way that air over a wing does, and the lift is strongest at the corners and edges. If the covering is held by too few fixings, or by fixings that have rusted, the first gale starts to peel it back. A sheet that rattles on a windy night is already telling you something about its bolts.

Old asbestos-cement sheets are often fixed with hook bolts or crook bolts that grip the purlin and poke up through a hole in the sheet, finished with a washer and a nut. After years of weather those bolts rust and the holes in the sheets crack around them. This is one reason sheet roofs start to leak at the fixings long before the sheeting itself fails. When we lift old sheets, we note the type and condition of every fixing so we know how the roof was held and where it let go.

Fixings for new coverings

New covering, new fixings. For metal box-profile sheets, we use screws with sealing washers, driven through the crown of the profile into the purlin, in the pattern the sheet maker specifies for the edge zones and the field of the roof. For tiles on a pitched garage, the battens are nailed to the rafters and the tiles are clipped or nailed according to the exposure. Ridge and verge details need mechanical fixing too: BS 5534:2014 says mortar alone can no longer be relied on to hold ridge and hip tiles, and that bedded tiles and fittings should have mechanical fixings. A dry-fix system holds ridge, hip and verge tiles with clips and screws instead of mortar, and BS 8612:2018 is the standard such systems are tested to. Our page on box profile and composite sheets shows how sheet fixing patterns vary from one product to another.

Why coated and stainless fixings win

Cheap zinc plated screws in a damp garage roof can start to corrode within a few winters, particularly in contact with timber that has been treated with certain preservatives. We choose screws and washers suited to the sheet and the timber, and where a roof sits close to a drying wet area or a boiler flue, we pay extra attention to the fixing material. A fixing does not need to be showy. It needs to resist rust, stay tight as the timber moves with the seasons, and keep its seal.

The same applies to flat roofs. EPDM, a single sheet of synthetic rubber, is normally glued to the deck with adhesive and finished at the edges with trim, so wind uplift is resisted by the adhesive over the whole surface. GRP, glass reinforced plastic, laid wet over a timber deck, forms a continuous skin that is fixed by the resin itself. In both cases the deck beneath has to be fixed down properly or the roof will move under the surface. The page on garage roof materials compares the main systems.

Fixings are also where a leaking garage roof most often gives itself away. A drip that appears directly below a bolt, rather than at a lap or a join, points at a failed washer or a cracked sheet. Our guide to garage roof leaks sets out the common causes and which of them are worth repairing.

With the frame sound and the fixings chosen, the roof needs a deck or covering that will keep the weather out for the long term. Chapter four looks at the new surface itself and the choices open to you.

The edges of a garage roof take more punishment than the middle. Wind lifts at the edge, water runs to the edge, and the join between the roof and the house or the neighbouring wall is where a weak detail gets found out. Good edge work is quiet and unremarkable. Bad edge work shows up as a dark stain on a wall, a lifting corner or a drip down the inside of a lining.

Drip edges and fascia

At the lower edge of a flat roof a drip trim, a metal or plastic angle with a lip, throws water clear of the fascia and into the gutter. Without it, water can run back under the membrane or along the underside of the board, which stains the wall and rots the timber. A fascia is the board that closes the end of the roof and carries the gutter, and on an old garage it is often the first timber to rot because it is wet all the time. We check it, and we replace it where it has gone soft. Our general service page on guttering, soffits and fascias goes into the detail for the house as well.

Wall abutments and flashings

Where a garage roof meets the wall of the house, a flashing seals the junction. A flashing is a thin sheet of metal, most often lead, tucked into a joint in the brickwork or stone and dressed down over the roof covering so that water runs on to the roof and not behind it. Lead is durable and easy to work, but it needs correct detailing: a chase cut into the masonry, a step where the roof slopes, and a flexible joint that allows for movement. On a flat roof an upstand of the membrane runs up the wall under the flashing. If either is missing or failing, rain tracks down the house wall into the garage ceiling. In Bath, where many garages and coach houses sit against stone, we match the repointing to the wall, and where lime mortar belongs, we use it.

Verges and ridges

At the gable end of a pitched roof, the sloping edge is called the verge. On a tiled garage it is traditionally finished in mortar, though dry-fix verge systems are widely used now. Whichever we use, we mechanically fix the verge tiles, in line with BS 5534:2014. On a box-profile roof, the verge and ridge are finished with purpose-made trims that clip or screw over the profile, with closure pieces beneath to stop wind and insects getting in. On a flat roof the equivalent detail is the end trim or the fascia upstand.

It is common to find that previous work stopped short at the edge. A flat roof covering is turned over the fascia without a trim, a flashing is replaced with a smear of sealant, or sheets overhang the gutter by only a finger's width. Sealant will hold for a season or two and then crack. We do not rely on it as the first line of defence; we use it as a finishing detail.

If your garage sits in a conservation area or the property is listed, the visible detail at the edge is also a planning question. You can read about it on the page outbuildings and coach houses on listed and conservation properties. Planning questions for a new garage roof in general are dealt with in the answer on whether a new garage roof needs planning permission.

Every edge eventually drains somewhere, and on a garage that somewhere is often an afterthought. The last chapter takes up the gutter and what happens to the water once it leaves the roof.