Flat roofing

Flat roofs evolved from the influence of the modern movement and were consistent with the fashion for clean lines and abstract designs. The room layouts of these modern homes in the 1900s were in sharp contrast to the layouts of traditional homes, and the resulting footprints made it difficult to install pitched roofs.

The concrete flat roof and the system-built flat roof heralded a new outlook for the design and construction of housing, but were unfortunately to leave a poor impression on the buying public, as they failed time and time again due to a lack of tolerance to thermal movement and the poor performance of early felts. Flat roofs from this era are more than likely to have been upgraded or re-covered in modern materials, and while some may remain troublesome, most are performing well.

Roofs coated with asphalt outlasted many of their felt counterparts, and if recoated should be performing well. Concrete roofs are known to have had quality problems, and these usually need the early intervention of a structural engineer to access structural integrity. Condensation problems caused by cold bridging in concrete roofs can also be a problem, as no insulation was included on such roofs.

Common problems with flat roofs:

• Roofing felts that bubble and split: Patch repairs can be undertaken, but once an old domestic roof begins to fail it is unlikely to respond well to temporary repairs such as liquid sealants.
• Leaks: Mainly caused by membrane damage, which in turn soaks the decking and starts the decay process. If patching, ensure that it is a large patch, and if the problems are suspected to be the upstands then it is important to lift flashings or remove renders to see the detail close-up. Adding sealants on top of upstands has minimal success.
• Blocked gutters and outlets: These cause the roofs to fill and breach upstands. Wire balloons can be more of a problem than a cure on inaccessible high roofs. Sometimes it is easier to remove them and place access traps lower down the pipe run, so that any debris can be removed quickly at ground level. Outlets become detached from an internal downpipe. In poorly accessible situations there is little option but to use liquid sealants. This defect can be the cause of difficult-to-diagnose leaks, because where a pipe run has a horizontal section to it, water that leaks out of an outlet joint tracks on the outside of the pipe for some distance, before it forms a drip and falls. It is often an unfortunate feature of horizontal rainwater pipe runs that the reason it is horizontal is to carry it over an important area like a foyer or shop unit. The introduction of overflows and double downpipes can avoid the majority of problems associated with roofs becoming blocked.
• Thermal movement: Keeping a flat roof still is a good philosophy when assessing ways to keep a roof in good order. The use and maintenance of solar reflective paints and protective layers will stop stresses building up on roof coverings. The shadow cast by a taller adjacent building moving across a roof on a hot day can have a dramatic effect in terms of the temperature difference between dark and light, and can set up stress cycles which can tear open otherwise sound joints.
• Flush eaves boards are prone to rotting where the back face was not prepared sufficiently prior to fixing. Splice repairs of such items are not cost effective and complete replacement in many case is quicker. If it can be done easily it is best to re-detail these to stand proud of the wall on spacer blocks.
• The roof is being used as a balcony: This happens by degrees, where bedroom windows are converted to doors, or a roof designated as a fire escape is used by residents. These surfaces can be easily damaged by chairs and cigarette burns, and should be preserved from such use by tenants. In private accommodation the use of lightweight tiles can help protect the surface. Items such as roof decking or plant pots should not interfere with the water run-off route to the outlet, as this can cause the roof to flood in severe weather and where there is slow water run-off.
• Leaks in skylights: These mainly arise due to poor design, where the cloaking over the upstand either does not allow for easy access to dress the felt up during a re-roof, or the cloaking does not cover the upstand sufficiently to protect the detail against wind-driven rain. It is unwise to assume that skylights 'lift off'. They may come off, but fixing them back can be difficult and the use of adhesives to fix frames is often relied on too heavily.
• Staining to ceilings: A lack of insulation in early roofs can make roof voids very cold, and with no vapour-check, condensation can occur within the roof space, which can cause damp staining. It is common to find that very simple constructions, that were originally only intended to be sun-rooms or potting areas, have been converted into a living space. Any attempt to improve the performance of such areas usually results in a major rebuild as the scope to add on all the necessary insulation, ventilation and vapour-checks is limited.