Roofs In Middleton And Failsworth
Decades of rear extensions have left a great deal of housing with a flat roof tucked against the back of the original building. Those extensions were often built by general builders rather than roofers, and it shows at exactly one place: the line where the new roof meets the old wall.
Key takeaways
- The abutment where an extension roof meets the original house is the defining weakness of this house type.
- Extensions built in different decades used different flat roof systems, each with its own lifespan.
- A flat roof below a pitched one receives everything that comes off the slope above, including grit and moss.
- Where the extension roof is below a window, the detail becomes considerably more demanding.
- The damage usually appears inside the original house rather than inside the extension.
Why the abutment is the problem
An extension roof has to be joined to a wall that was previously an external wall of the house. Doing that properly means cutting a chase into a mortar joint, fitting a flashing that dresses down over the roof covering with an upstand of adequate height behind it, and pointing it in. Done that way it lasts for decades. Done as a bead of sealant against the brickwork, which is extremely common, it lasts a couple of years and then lets water into the wall. Because the wall it lets water into is the original house wall, the damp then appears in the original rooms rather than in the extension.
Upstand height, the detail nobody checks
The vertical part of the covering that turns up against the wall is called the upstand, and its height matters. Too low and water driven against the wall, or ponding on the roof, or a build up of debris and snow, can get over the top of it. Established practice calls for a meaningful upstand, typically at least one hundred and fifty millimetres above the finished roof surface. A great many domestic extensions have far less, particularly where the builder was trying to keep the roof below a window. That is a design fault rather than wear, and no amount of resealing the top edge fixes it.
The window problem
Extensions are frequently built up to just below a first floor window, which leaves very little room between the roof surface and the sill. That squeezes the upstand, complicates the flashing, and often means the detail has been fudged. It is also the arrangement most likely to fail when water ponds, because there is no height in reserve. If your extension roof sits close under a window and you have damp appearing in the room above or beside it, that geometry is the likely explanation and it is worth having somebody look specifically at how the covering, the upstand and the sill relate to each other.
What is on top, by decade
Extensions built through the middle of the twentieth century generally carry built up bitumen felt, which has a service life of roughly fifteen to twenty years and is likely to be on its second or third covering by now. Later work used torch applied felt, which performs better. More recent extensions use EPDM rubber, single ply membrane or glass reinforced polyester, all of which last considerably longer when detailed correctly. Knowing roughly what is up there tells you whether you are looking at something near the end of its life or something that should still have years left and is therefore failing for a reason.
Everything from the slope above lands here
A flat extension roof set below the main pitched roof receives all the water from that slope, plus whatever it carries: grit washed off ageing tiles, moss fragments, leaves and general debris. That material collects at the low points and around the outlet, which is why extension roofs block and pond more readily than a freestanding flat roof of the same size. It is also why a gutter above, discharging onto or beside the extension, is worth checking whenever the extension roof is being looked at, since the two problems are frequently the same problem.
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Where the water actually shows up
The counterintuitive part of this house type is that a failed extension roof usually presents inside the original house rather than inside the extension. Water entering at the abutment goes into the wall, travels within the masonry, and emerges on the inside face of what used to be the outside wall, which is now an internal wall or the wall of an upstairs room. Homeowners consequently report damp in a bedroom and never connect it to the flat roof outside, because the extension below is perfectly dry. Damp on any wall adjoining or above a single storey extension should send you straight to that abutment before anything else is considered.
Checking it yourself
From an upstairs window you can usually see the whole extension roof, which is the easiest inspection available. Look for standing water and note where it sits. Look at the line where the roof meets the wall: you want a metal flashing disappearing into a mortar line, not a bead of sealant on the surface. Look for debris piled against that junction, for splits, blisters or crazing in the covering, and for a lifted or curling edge. Note whether there is an outlet and whether it is clear. Photograph it, because that record is what makes change visible next year.
Fixing it so it stays fixed
If the covering has life left and the fault is at the junction, the work is to cut a proper chase, fit new flashing with an adequate upstand and point it in. If the covering itself has failed, replacing it is usually better value than patching because the areas are small and the cost is dominated by access and preparation. Either way the question worth asking directly is how the abutment will be formed and how high the upstand will be, because those two answers determine whether this is the last time you deal with it or the next instalment.
Building regulations and extensions
Extension roofs are subject to building control, and the requirements have tightened considerably over the decades, particularly around insulation. That matters in two ways. An extension roof being replaced rather than repaired may need to meet current standards for thermal performance, which can affect the build up and the finished height, and that in turn affects the upstand and any threshold below a door. It also means that work carried out without the appropriate approvals can surface later during a sale, when a buyer's solicitor asks for the paperwork. Where an extension roof is being fully replaced rather than patched, it is worth establishing at the outset whether building control is involved, because retro fitting compliance is considerably harder than designing for it. Where an existing extension is being reroofed rather than rebuilt, the position may well be different, which is exactly why the question is worth asking at the outset rather than assuming either answer.
Extensions built on extensions
A good many properties have been extended more than once, with a later addition built against or over an earlier one. Each round of work creates another junction, and the junction between two extensions of different ages is frequently the least well considered detail on the building, because neither builder was designing for the other. Levels rarely align, coverings differ, and the flashing arrangement is often improvised. If a house has been extended in stages and has a persistent damp problem, the boundary between the phases is the place to look, and it is worth establishing when each part was built, since that sequence usually explains the geometry.
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David Boardman founded Boardman Roofing in 2018. Dave has 25 years on the tools. Read 150+ Google reviews or call 07879 736629.
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