If you have never had a flat roof replaced or repaired in Eriswell, knowing exactly what the job involves helps you plan, ask the right questions and understand what you are paying for.
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Before any work is priced or planned, the existing roof needs a proper look. For a straightforward single-storey extension or outbuilding, this means getting up and walking the surface - checking the membrane condition, pressing on any suspect areas to test the deck beneath, clearing back moss and debris to see what is underneath, and inspecting every flashing, edge trim and outlet.
For a roof that is actively leaking, the internal inspection matters just as much as the external one. Looking at the underside of the deck from inside the loft or room below often shows where water has been tracking. The damp patch on a ceiling is rarely directly below the entry point - water follows the line of least resistance along joists and boarding before it drops.
A good assessment will tell you whether the problem is limited to a specific detail (a split flashing, a cracked upstand, a blocked outlet) or whether the membrane itself has reached the end of its useful life. This distinction drives everything that follows.
On a full replacement, the existing membrane and any insulation board are stripped back to expose the deck. This is not a step that should be skipped or minimised. Laying a new membrane over an old one might seem like a way to save time and cost, but it buries any moisture in the existing layers, prevents proper deck inspection, and adds weight. It also makes future repairs harder to carry out.
Stripping is physical, noisy work. Felt and insulation boards are lifted and bagged for disposal. On a typical single-storey flat roof of around 20 to 30 square metres, stripping takes the best part of a morning. The waste needs to go somewhere - a skip or a trailer load, depending on the volume - and that disposal cost should be included in any quote you receive.
Once the old covering is off, the deck is inspected properly for the first time. On older properties this is often tongue-and-groove timber boarding; on more recent builds it is typically OSB (oriented strand board). Either can absorb moisture over time, particularly if the previous roof had been leaking for a while before work was commissioned.
Soft spots, dark staining and any boards that flex more than they should are all marked up. Those sections are cut out and replaced with matching material before anything else goes on. This is the stage at which the actual scope of the job becomes clear - most of the time the deck is in reasonable condition, but occasionally a significant portion needs replacing. You should be shown what is being removed and why.
The deck is then cleaned down and checked for any fixings that are proud of the surface, since a nail head or screw tip that is not flush will eventually work through a membrane. The deck is also checked for adequate fall - the slope that allows rainwater to drain to the outlet. Minimum recommended fall for a flat roof is 1 in 80, though more is better in practice.
On a warm roof build-up (the standard approach for a replacement flat roof today), rigid insulation board is fixed to the deck before the membrane goes on. The insulation keeps the deck warm, which prevents condensation forming within the structure. The thickness of insulation is determined partly by the building regulations requirements in force at the time of the work and partly by the practical constraints of the roof - threshold heights at doorways, the height of existing upstands and the position of any roof lights all affect what can be achieved.
Insulation boards are cut to fit neatly, joints are staggered to avoid continuous cold bridges, and the boards are mechanically fixed or adhesively bonded depending on the system being used.
The choice of membrane - most commonly EPDM rubber or torch-on bitumen felt for domestic properties - determines the exact process, but the principles are the same for both.
For EPDM, the sheet is rolled out across the roof and positioned so there is sufficient overhang at every edge. The adhesive is applied to both the insulation board and the underside of the membrane, left to tack, and then the sheet is folded back over itself and carefully bonded down, working out any air from beneath. On a simple rectangular roof this can be done with a single sheet, which means no seams across the main field of the roof. Any joins that are needed at corners or penetrations use compatible tape or liquid flashing products.
For torch-on felt, the process involves heating the underside of the bitumen sheet with a gas torch as it is unrolled, bonding it to the layer beneath. A two or three-layer system builds up the waterproofing with each layer overlapping the previous one at specified intervals.
This is the stage that separates a flat roof that will last from one that will be leaking within a few years. The membrane needs to be dressed up every vertical surface it meets - walls, parapets, chimneys, the sides of roof lights - to a minimum height of 150mm above the finished roof surface. This upstand is what stops water getting behind the roof covering at the edges.
Edge trims are fitted around the perimeter to give the membrane a neat, secure termination and direct water into the guttering. At wall junctions, a separate flashing - typically lead or a compatible metal - is dressed over the top of the membrane upstand and pointed into the mortar joint of the masonry. This is where a lot of corners get cut. Skimping on the lead, using a thin bead of sealant instead of a proper stepped flashing, or failing to point the flashing into the wall are all shortcuts that lead to early failure at the junction.
The roof outlet is checked, cleared and - if it is old or damaged - replaced. The outlet needs to sit flush with the finished roof surface so water does not pond around it. Where a roof drains to a parapet outlet or an internal downpipe, the membrane is carefully dressed around and into the outlet housing.
Once the membrane and all flashings are in place, the roof is checked over in full. Every lap, every edge, every penetration and every corner is inspected before the team leaves site. Any debris from the job - off-cuts of membrane, old felt, fixings - is cleared away.
A properly finished flat roof should have a consistent, unbroken membrane surface with no visible blisters, wrinkles or ridges. Edge trims should be straight and neatly fixed. Flashings at wall junctions should be bedded in and pointed. The outlet should be clear and free-draining. The roof surface should visibly fall towards the outlet or outlets rather than sitting level.
If you can see any of those details missing on a roof that has just been done, raise them before the contractor leaves. Brandon Roofers covers Eriswell and the broader Brandon area. Call us to arrange a proper survey and we will walk you through exactly what your roof needs before any work starts.
We also carry out repairs to pitched roofs, slate and tile replacement, chimney repairs and gutter repairs.
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Call any time. Calls are answered day and night and passed straight to our roofers.
