Flat roofing

At one time, the general rule of thumb was that a roof covered with felt to BS 747 would last about 15 years before re-covering was necessary. However, with the advent of high performance materials, life expectancies of 25 years or more can be routinely expected. Like most systems though, the success of a roof depends not only on workmanship, but the design of the roof in the first instance.

Early roofing felts employed an asbestos base, although these performed poorly and had poor tensile and tear resistance. In the 1970s, asbestos was replaced with polyester giving rise to a significant improvement in performance. For example, BRE fatigue tests on samples of BS 747 felt failed after around 32 cycles, while polyester samples failed after 3,200 cycles. By using polyester based felt and modified bitumen over 30,000 cycles were attained.

Contemporary construction has adopted the nomenclature of RBMs (reinforced bitumen membranes); guidance for the selection and use of these materials is given in BS 8747:2007. BS 747 was amended in 2000 but withdrawn in 2004 to be replaced by BS EN 13707:2004. Generally speaking the range of RBMs and their performance characteristics are now far superior to anything available in the early 1970s.

Whereas the norm for BUR roofs was at one time 3-layer felt, the tendency in current practice is to use 2 or more layers or alternatively a single-layer system using a specifically designed RBM. In a 2-layer system the lower sheet provides the flexibility and the ability to accommodate movements in the deck, while the top sheet provides a protective function against static or impact loads. Many top sheets are self-finished with either mineral chippings or foil to provide solar reflectance. These sheets are generally referred to as cap sheets.

To prevent the effects of oxidisation embrittlement, modern felts are usually compensated by the addition of either:

• APP (atactic-polypropylene): usually for torch-on felts. These plastomeric modified bitumen membranes are produced on polyester, glass fibre, and glass/polyester mixed bases. The base is saturated and/or coated with bitumen, modified with polyolefin or a polyolefin copolymer.
• SBS (styrene butadiene-styrene): usually pour and roll, bonded or torch-on applications. These elastomeric modified bitumen membranes are produced on polyester, glass fibre, and glass/polyester mixed bases. The base or carrier is saturated and/or coated with bitumen that has been modified with thermoplastic rubbers.

As with asphalt coverings, the success of an installation depends on the suitability of the deck to which it is laid and to the standard of detailing of abutments and roof penetrations. A deck that is likely to move and deflect under load or moisture changes is also likely to impart significant stresses to the covering, which offers less tensile strength as it hardens on ageing and exposure to UV light.

No matter how high the specification of RBM, it will be let down by poor attention to detail. Here, a membrane has been bonded over an older, asphalt membrane. The detailing of felt is almost impossible to achieve properly in these circumstances, with the result that attempts have been made to seal the top edge of the felt using an incompatible mastic sealant.

Problems with polystyrene insulation

Polystyrene insulation is generally considered to be unsuitable for asphalt roofing. Equally it can cause problems with BS 747 type felts because of their poor flexural strength. Boards should preferably be adhesively bonded to the deck rather than mechanically fixed; they must be tightly butted and secured to prevent wind uplift. Some insulation boards are intended for use with BUR roofing systems and sheet sizes are limited. Large sheets exhibit correspondingly greater movement capabilities at joints.

Felt used as a patch over existing failed asphalt. This may be satisfactory as a short-term measure, but here trapped moisture has formed severe blisters. The felt is also cockled and split.

If the boards are mechanically fixed, their propensity to move is greater, and cyclical movements can stress the covering, possibly causing it to rupture. Similarly, changes in level at board joints brought about by inadequate or partial fixing can damage the roofing membrane. Modern high performance materials may be better able to withstand the amount of movement than older BS 747 felts.

Blistering

Cockling or blistering of felt roofing is usually a symptom of water entrapment within the interlayers or beneath the complete system. The problem can occur wherever there is a lack of adhesion between the cap sheet and the base layer. Blisters could be due to residual water from construction or other detail failures elsewhere on the roof. Similarly, application of a membrane over an already damp surface will cause problems, as will work in cold weather if the material has been laid in unfavourable weather conditions.

A blister is an entrapped pocket of air and moisture (sometimes water filled) that can range over a few square centimetres to several square metres in area. Research by IRC-NRC (Liu, K.K.Y., Paroli, R.M. and Smith, T.L., Blistering in SBS Polymer Modified Bituminous Roofs, Construction Technology Update No. 38, June 2000, National Research Council Canada) suggests that the problem of blistering can be more prevalent with SBS type felts than APP types. If the interface between sheets is voided, any moisture present within the void can vaporise during hot daytime temperatures. The volume change associated with the conversion of water to water vapour is in the region of 1,250 times greater, so even a minute quantity of moisture can exert considerable expansive forces on the felt. Given the felt will be warm and flexible, the expansion forces may be sufficient to overcome the bond strength of the membrane around the blister, tending to enlarge it.

Blistering in built-up felt roofing

At night the vapour condenses but since the void is now slightly larger, the felt is cooler and stiffer and may not return to its original shape. More air is drawn into the partial vacuum than was present at the start. The cycle is then ready to commence again.

Blisters do not generally affect the tensile strength of felt, but a rupture will make a large difference in tensile strength of the cap sheet. Small blisters can therefore be left alone so long as they are not in areas subject to foot traffic. Larger, particularly water-filled, blisters should be cut out and repaired. (Paroli, R.M. and Booth, R.J., Ways to Reduce Blistering Built-Up Roofs, Construction Technology Update No. 4, May 1997, National Research Council Canada.)

Woodwool cement boards

Woodwool slabs were very common in the 1960s and 1970s but are less so now, at least within the UK. Roof decks of (usually) pre-screeded and edge reinforced slabs are common in commercial buildings and schools - the latter being particularly popular when used in conjunction with 'Metsec' type lightweight steel beams.

Although a somewhat maligned material, woodwool could or can perform well if used in appropriate circumstances. In older buildings the felts used were generally unable to accommodate the cyclical changes in moisture and temperature that brought about dimensional strains. Commonly, woodwool slabs are believed to lose their integrity when wet, although this is by no means certain - indeed they have been used in external situations for sound attenuation purposes by the power industry. However, in common with many construction materials, consistently damp conditions are unsatisfactory and can lead to softening and warping of the slabs. Warping of the slabs produces stress in the felt covering, with the risk of splitting along board joints.

Removal of old coverings during remedial works can be problematic, particularly where the felt has been fully bonded to the screeded finish of the slab. In these circumstances the screed can be damaged easily, leading to delamination problems and disintegration of the slab, necessitating a deck replacement in addition to the roof covering.

A further problem in 1960s and early 1970s buildings was a general lack of vapour control. If the woodwool was used in effect as a warm roof, it would need a vapour barrier directly beneath it. Such a provision would be rare, with the result that the dew point would occur at some point within the thickness of the slab giving rise to a risk of interstitial condensation. In severe cases, the slab could warp.

One particularly vulnerable form of deck (now rarely found due to life expectancy) is Stramit compressed strawboard - an insulation board material made from straw sandwiched between layers of building paper. Stramit boards had to be laid in a direction corresponding to the grain of the individual straws; failure to do this could result in excessive deflection. Condensation occurring within the board could lead to rapid decay.

Deck failure (chipboard): the outline of roof joists is clearly visible

Fibreboard

Bitumen impregnated fibreboard is vulnerable to moisture movement and sagging over time, particularly when it gets wet. In contemporary construction it is unlikely to be used in isolation. However, it was a common form of roofing when used in conjunction with profiled metal decking supported by lattice beams - the fibreboard being laid over a felt vapour barrier on top of the decking and covered with 3-layer BS 747 felt. If condensation is allowed to form on the fibreboard it will sag between ribs, giving a characteristic multiple ridging of the covering above the profile ribs. Regular stripped pattern staining along the rib lines is a good visual indicator of this type of construction. (Maintenance and renewal in Educational Buildings. Flat Roofs – Criteria and methods of assessment, repair and replacement, Design Note 46, Department of Education & Science Architects & Building Group, 1985.)

Effects of condensation on the chipboard deck in cold roof construction: the chipboard has lost its integrity

Ponding

Ponding of roofs of BUR construction can be problematic as water can gradually penetrate at lap joints and find its way into the roof structure. BRE identified ponding due to undersized roof joists as one of the main causes of failure in a survey of rehabilitation work on housing (BRE building elements: roofs and roofing – performance, diagnosis, maintenance, repair and the avoidance of defects, BRE Report 302, revised 2000).

However, in a separate study into the performance of roofing on Crown Estate buildings, no evidence was found that the life expectancy of roofing was compromised by the existence of ponding (Asphalt and built-up felt roofing, BRE Digest 144). As with asphalt, the existence of ponding does mean that there is a larger reservoir of water available to enter the building should a leak occur.

Deflection due to undersizing (or bowing as a result of thermal expansion being constrained by the surrounding structure) may not be the only cause of ponding. Movement in decking materials (particularly organic materials such as plywood and chipboard) can cause more localised depressions and any roof that exhibits a regular pattern of such depressions must be regarded with caution until the integrity of the decking material has been established. A frequent problem in older roofs is condensation occurring on or within the decking material causing it to expand, sag or decay.

Solar control

BUR roofs benefit from solar protection in much the same way as asphalt. Mineral flakes or chippings are often incorporated in the cap sheet to provide solar control - very light or white finishes are possible. A common alternative to reduce reflectance is the use of mineral chippings bedded in hot bitumen. However, as with asphalt, these can be subject to wind scour and can provide a useful base for the formation of moss and algae.

Promenade tiles or a screed material called Paropa were sometimes used. Deterioration of the felt in lines between concrete tiles could occur due to their increased exposure and small cyclical temperature movements in the tiles.

Wind scour

Loose laid insulation in inverted roof systems must be secured in place with either loose ballast, paving slabs or by using composite sheets incorporating a cementitious finish. Some insulation slabs contain an edge interlock such that adjacent boards engage with each other and help to provide resistance to wind uplift. However, edge interlock by itself is unlikely to provide sufficient restraint, particularly when the size of the pressure waves travelling across a roof are at a similar dimension to the individual boards.

In certain circumstances, wind scour of ballast systems can lead to the migration of the ballast into heaps and even blow individual stones off a roof - a possible risk to passers by or adjacent glazing systems, vehicles and the like. Much depends on exposure and roof profile, topography and location of adjoining buildings. Small chippings, such as those used as a solar reflective coating, are unlikely to be particularly harmful, but larger stones have the propensity to cause serious damage. Further information can be found in BRE Digest 311, Wind Scour of Gravel Ballast on Roofs, 1986.

Understanding wind uplift   There are 2 main influences on a permeable covering to a roof: the global pressure field; and the local velocity field. The global pressure field is the pressure that forms around the building or roof as a whole, influenced by topography, exposure, proximity of other buildings and of course wind strength.   The local velocity field is the zone of pressure around a subsidiary part of the building, such as the tiles on a pitched roof, insulation slabs, ballast. Essentially, a rough projecting surface (for example, individual tiles) disturbs the air flow and can make a large difference to the air pressure immediately adjoining the element. Conversely, a smooth finish permits uninterrupted airflow and so the local velocity field will be less significant.   The effects of either the local velocity field or the global pressure field depend on the size of the gaps between elements and the volume of air beneath it. So, as pressure fluctuations affect the external air, small gaps and a large void will mean a time lag before the pressure can equalise. During this time the pressure beneath the element may be higher than above it, causing wind uplift.   A larger gap and smaller void will achieve pressure equalisation much faster and so the effects of uplift will be reduced. In effect this is why felt or timber sarking is placed immediately below tiles of slated roof coverings - it reduces the volume of the void and enables rapid pressure equalisation.   Roofs with loose laid insulation are vulnerable to wind uplift by this mechanism and so it is necessary to provide additional ballast or loading sufficient to overcome the forces generated by wind uplift.