If you want to know exactly what happens when a flat roof is replaced or repaired in Fulbourn, this page walks through every stage of the job in the order it actually happens.
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Most homeowners commissioning a flat roofing job have never watched one done. That makes it hard to know whether the roofer on your garage or extension is doing the work properly or cutting corners. Understanding the sequence - what should happen and in what order - means you can ask the right questions and recognise a thorough job when you see one.
What follows is the process for a flat roof replacement on a typical domestic garage or single-storey extension. Repairs follow a shorter version of the same process. We cover each stage from the initial inspection through to the point where the site is left clean and the roof is watertight.
Before any price is agreed, the roof needs to be looked at properly. That means getting up onto or close to it, not just looking from the ground. A roofer assessing a flat roof from a ladder or a garden is giving you an educated guess, not a survey.
A proper inspection looks at the condition of the existing covering - whether it is blistering, cracking, shrinking or pooling water. It also looks at the perimeter details: the drip edge or fascia, any upstands where the roof meets a wall, and the lead or alternative flashing at those junctions. All of these are potential failure points that a ground-level look cannot assess accurately.
The inspection should also identify whether there are signs of water having penetrated into the deck below. Soft spots underfoot, staining on fascia boards, or rot at the edges of the roof all suggest the deck has been wet and may need partial or full replacement. This affects the cost and scope of the job, so it needs to be identified before work starts, not halfway through.
A flat roof replacement starts with stripping the old covering back to the deck. This is not optional - laying a new membrane over an existing one traps any moisture between the two layers and prevents a proper bond. It also hides deck problems that will cause the new covering to fail prematurely.
Stripping felt generates a significant amount of waste - rolls of old felt, used nails and screws, and sometimes sections of rotten board. This material needs to be removed from the roof and disposed of properly. A skip or a loaded van is what you should see at the end of day one, not old felt piled in the garden.
Once the deck is bare, the roofer can see exactly what is there. This is the point at which any rotten, soft or damaged decking boards are identified. If problems were suspected at the inspection stage, this confirms them. If the deck looked sound but turns out to have hidden rot, it needs to be dealt with now - not covered over.
The deck is the structural base that everything else sits on. For most domestic flat roofs, it is made from timber boards or, more commonly today, WBP plywood sheets. Plywood that is 18mm thick is the standard specification for a flat roof deck - it is stable, holds fixings well and does not swell and move the way chipboard does when it gets wet.
Any sections of decking that are rotten, soft or delaminated are cut out and replaced with new plywood at this stage. The roofer should use treated timber for any new joists or noggings if the structure below also needs attention. All new boards should be screwed down firmly - a deck that flexes or has loose boards will cause the covering above it to crack at the same points over time.
If the roof has had a persistent drainage problem - water pooling in a low spot - this is also the stage at which a tapered insulation system or a repositioned outlet can be introduced to improve the fall of the finished roof. This is more involved and adds cost, but it solves the underlying problem rather than just covering it.
With a clean, sound deck in place, the new covering goes on. For torch-on felt, this means laying a vapour control layer first, then an underlay layer, then the cap sheet on top, each one bonded to the last using a gas torch. Each layer is taken up the upstands at the edges and overlapped carefully at all seams. The cap sheet - the layer you see when the job is done - has a mineral granule finish that provides some UV protection.
For EPDM rubber, the process is different. The deck is primed, then the adhesive is applied to the deck in sections and the rubber sheet is rolled out and pressed into position. Large sheets can cover a whole garage roof with very few joins. Once the field of the roof is bonded, the edges are folded up the upstands and secured using EPDM-compatible tape and trim. The join between any two sheets is made using seaming tape applied under firm pressure.
In either case, the roofer should not be rushing this stage. Seams made in a hurry - or joins made when the adhesive or torch work has not fully set - are the most common source of early failure on a new flat roof.
The perimeter of a flat roof is where most leaks start. Getting the field of the roof watertight is important, but if the edges are poorly finished, water will find a way in anyway.
At the drip edge (the perimeter where the roof meets the fascia), a metal drip trim is fitted before the cap sheet or rubber is taken over it. This directs water away from the fascia board and into the gutter rather than letting it run back under the covering.
Where the roof meets a wall - at an upstand - the covering is taken up the vertical face and secured, then a lead flashing (or lead-effect alternative) is dressed into a chased mortar joint in the brickwork above and pointed in. This flashing sits over the top edge of the roof covering and prevents water from tracking behind it. A flashing that has not been properly chased and pointed will eventually pull away from the wall and let water in at that junction.
Rooflights, pipes and any other penetrations through the roof surface are sealed and flashed at this stage. Each penetration is a potential leak point and should be treated individually, not just mastic-sealed and left.
Once the covering and all edge details are complete, the finished roof should be walked and visually checked. Any low points where water might pool should be noted. Gutters that were disturbed during the work should be refitted and checked for correct fall. Any debris - offcuts, adhesive tubes, old nails or screws - should be cleared from the roof surface and from the gutters before the roofer leaves.
The site around the building - paths, garden, drive - should be left as it was found. Flat roofing work is not a particularly disruptive job from a homeowner's perspective, but it does generate waste, and clearing it is part of the job.
You should be able to walk around the finished roof (if it is safely accessible) and see neat, tight detailing at every edge, upstand and penetration. Any areas of concern should be pointed out to you and explained before the roofer leaves.
For a repair job, the process is shorter but follows the same logic. The damaged area is identified and exposed, the cause of the failure is understood (not just patched over), and the repair is made using materials compatible with the existing covering. The edges of any repair patch should be bonded and sealed, not just laid on top. A repair that is not properly adhered at its perimeter will lift and let water in at those edges within a season or two.
We carry out flat roofing work in Fulbourn and across the surrounding area from our base in Haverhill. We cover Linton, Saffron Walden, Steeple Bumpstead, Sturmer, Little Walden, Thaxted and the neighbouring villages. Call us to arrange a survey of your flat roof and we will tell you honestly what the job involves and what each stage will look like.
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Call any time. Calls are answered day and night and passed straight to our roofers.
