This article outlines some of the more common building defects and issues relating to flat roofs. It is relevant to residential, commercial, and industrial buildings.
Flat Roofs and Ponding
Firstly, ‘flat roofs’ are rarely flat, and most incorporate gradient falls of around 1 in 60 to 1 in 80, in order to drain away surface water, (unless otherwise specifically designed). Over time, due to issues such as deflection, or sagging of the decking and structure, or poor initial installation, a situation termed ‘ponding’ can occur. Ponding is where parts of the roof retain standing surface water.
This standing surface water can result in thermal stresses at the edge of the pool of water, (i.e. colder wet areas adjacent to warmer dry areas), which can ultimately lead to a breakdown in the protective roofing membrane; whether it is asphalt, bitumen-felt or some other material. The full-depth splits and tears that can result have the potential to allow water penetration. Where water penetrates the build-up of the roof, if the concealed insulation is water absorbent and becomes saturated, the thermal conductivity will increase, and the performance of the insulation will be correspondingly reduced. Where timber is present within the structure, the timber is susceptible to decay. Even if the water does not penetrate the internal areas below, there can still be problems with cold bridging, condensation, and consequential mould growth.
Other flat roof issues include poor detailing to skirtings, upstands, flashings, and edge trims. These problems could be due to the selection of incorrect materials, inappropriate detailing or installation, or thermal movement. Water ingress usually results in the longer term. Some roofs may have roof screed ventilation with ‘mushroom’ vents, which can be damaged over time, either accidentally or deliberately. In general terms, damp stains on internal ceilings, are often the first indication of an external roofing problem.
Types of Flat Roofs
Flat roofs are broadly divided into three generic types: ‘Cold Roofs’, ‘Warm Roofs’, and less commonly, ‘Inverted Roofs’. Cold roofs are no longer encouraged or recommended, as the thermal insulation is positioned within the structure, below the roof decking, making this type of roof susceptible to condensation problems in situations where there is inadequate ventilation in two directions. But the good news is that many cold roofs can be upgraded to the better type of warm roof; by installing overlays of new decking, thermal insulation, and new roof membrane coverings.
Roof membranes need to be supported adequately, to avoid slumping, ridging, or other failures. Adequate support is usually provided by a roof deck, or decking material of some type. On solid roofs, such as concrete, the roof deck can be supplemented with a roof screed. Failures of a roof screed can be due to cracking, moisture ingress, incorrect materials, or poor installation. The result is that the roof membrane often sags, splits, tears; any of which could allow water ingress. A building defects report will normally be able to identify the cause, or causes, of failure.
Modern roof covering systems may often achieve the slope or roof falls that are required, by using purpose-made tapered insulation, and many also have a bonded, or a separate decking, ready for the roof coverings. Most flat roofs will require a vapour-check membrane, sometimes incorrectly termed a ‘vapour-barrier’. Positioning and detailing of the vapour-check layer is crucial to avoiding building defects, particularly condensation. Effects of the sun, or solar gain, can accelerate the ageing of exposed materials. Asphalt and bitumen felt can blister in warm weather, indicating pockets of trapped moisture.
Flat roofs incorporating metal, (such as lead, zinc, copper, or aluminium), can easily be damaged by foot traffic during maintenance, or unauthorized access, (e.g. during the installation of satellite dishes, or similar). The installation of a metal roof should only ever be done by an appropriately qualified and experienced tradesperson, and it is important that the correct thickness and grade of metal sheeting is selected. The preparation of the decking underneath is critical to avoid premature failure of the roof sheeting. Thermal movement must be allowed for, to avoid splits and tears.
Inspections by Building Surveyors
Often the building surveyor is asked to inspect a roof, where unsuccessful repairs have already been attempted. These ‘repairs’ often take the form of liquid ‘wonder’ coating that can make the problem worse. Often, the original roof covering cannot be repaired correctly after such coatings have been applied, so they are always best avoided until the situation has been clarified. Opening-up the structure in small areas with a builder or roofer, can indicate problems such as screed failure, timber rot, sagging decking, and failed or missing vapour-check membranes. Once water ingress occurs, it can cause significant and major structural damage, longer term.
Sometimes, the roof membrane has simply reached the end of its working life, due to ageing. Bitumen-felt roofs will normally only last a fraction of the time of say, a professionally installed and properly maintained asphalt or a lead covered roof, but offer short-term cost benefits.
Following the inspection of a defective roof, the client can be issued with a report that includes a description of the problem, options for solutions, and an indication of estimated costs and programme, where required. Access equipment may be required for both the survey and the works, which will then affect the budget costs.
Modern Technical Solutions and Warranties
Modern technology has introduced proprietary single-layer polymer roof membranes, polymer-modified asphalts, torch-on bitumen felt systems, and many others. Modern thermal insulation materials are more efficient and easier to install. Fire protection of new or replacement roof membranes and coverings needs to be undertaken fully in accordance with Building Regulations. Roof warranties, and insurance-backed systems are available, for peace of mind. CDM 2015 Regulations on health and safety, will need to be considered for remedial works.
Arun Associates can manage every part of the process, from a site inspection and the production of a technical defects report, to specification, tendering, and administration and inspection of the remedial works. Where insurance claims are invoked, they have the experience to assist with those projects, and in discussing claims with loss adjusters.