Greenhalgh & Co.

Chartered Building Surveyors & Valuers

 

boltonsurveyors.org.uk

Greenhalgh & Co.

Chartered Building Surveyors & Valuers

21 Towncroft Lane

Bolton  BL1 5EN

Tel: +44 (0)1204 845382

Mob: +44 (0)7813 439196

Email: greenhalghco@btconnect.com

 

RICS Company No. 002973

 

Roof coverings

Roof coverings on traditional pitched roofs

 

Natural slating

Natural slates were either random in size, and usually laid in diminishing courses or alternative sized (all the same size).  It is usually more difficult to repair or strip and re-slate a random slated roof.

 

When one sees slates held on lead straps, it usually means that the nail has corroded or broken.  If one replaces missing or broken slates/tiles one should use a non-ferrous screw instead of a nail, this removes the impact of the hammer from the repair process and thus reduces the chance of the battens breaking.

Concrete interlocking tiles

There are usually around 10 per square metre of roof area (although this can vary due to the pitch of the roof and the type of tile).  Sand faced concrete tiles usually loose there are original colour within ten years of being fitted, whereas colour through tiles are able to maintain the original colour for many years.

 

Rosemary tiles 

These are small clay tiles (although they can be made from concrete) and there are usually around 60 per square metre of roof area (this can vary with the pitch of the roof).  Rosemary tiles are often used on steeper roof pitches and thus recovering the roof tends to be more expensive because of the number of tiles and the pitch of the roof.

Clay rosemary tiles tend to be affected by frost action over the years causing the face of the tile to spall, and the fact they are relatively thin makes them easy to break when walking across a roof.

 

Hardrow Slates 

These are slates formed from concrete and they are more brittle than conventional concrete tiles and affected by the acidic nature of rainwater.  Hardrow slates dissolve slowly over many years and thus it becomes more difficult to replace damaged slates because it is very easy to break other slates when traversing across the roof.

 

Weights of typical roof coverings

 

Type

Pitch

Size (mm)

Lap (mm)

No. per sqm

Weight per Kg / sqm

Natural slates

 

30°

500 x 250

100

20.00

50.0

Natural slates

 

30°

600 x 300

100

13.30

46.5

Marley Modern tile

 

30°

420 x 330

75

9.7

44.0

Marley Roman tile

 

30°

420 x 330

75

9.9

50.0

Marley Ludlow tile

 

30°

420 x 330

75

9.8

45.0

Rosemary clay tile

 

35°

267 x 168

65

60.0

73.8

 

The above shows that in real terms the majority of slate roofs will usually be slightly heavier than the majority of tiled roofs, particularly when the double coursing at eaves is taken into account.  The rosemary tiled roof (small clay tiles) is the heaviest roof covering, simply because of the number of tiles needed to cover a square metre.  

 

Torching and under-felting

 

Before the introduction of bitumen roof felt, the slates were usually torched with mortar on their underside.  This is simply a thin coat of mortar and the purpose is to insulate the roof and prevent wind blowing through the same. 

Contractors now use roofing felt for this purpose and this is often called sarking felt, however the purpose is the same, namely to insulate the roof.

Flat roofs

A major cause of leakage has been an insufficient slope to the roof. A designed fall of 1 in 80 does not result in a finished fall of that gradient. Variations in constructional accuracy, settlement, and thermal and moisture movement, lead to lower finished falls.

 

In addition to the above, timber joists will deflect over the years causing further reduction in the fall of the roof until ‘ponding’ occurs. 

 

If a roof holds water, even a small hole in the centre of the ponding will allow all water to drain through.

 Asphalt coverings

Asphalt is a durable material but can crack if subjected to stress, usually at low temperatures. The most common causes of cracking are movement of the deck upon which the asphalt has been laid and differential movement between the deck and features such as parapets, verges and return corners.

 

Asphalt has a high coefficient of thermal expansion and, is likely to be subjected to large temperature changes unless protected from solar radiation.

 

Poor detailing at parapets and projections has been a cause of many failures. Extensive cracking can occur when asphalt has been dressed up a parapet to form a skirting, without allowance made for the differential movement between deck and parapet.

 

If the asphalt slumps and shrinks away, rain will penetrate behind the skirting into the roof deck. The finishing of flat roofs at the edges, using aluminium trim into which the asphalt is dressed has, on the whole, not been very successful and cracking has occurred, particularly at changes in direction of the trim. This, too, is basically because of differential thermal movement, leading to fatigue.

 

Exposure to sunlight over a substantial period causes mastic asphalt and bitumen to harden and to shrink, the shrinking often results in surface crazing. This effect, which is due to short-wave solar radiation, is assisted by atmospheric oxidation.

 

Being black or dark grey in colour, its absorption of solar radiation is high and the high temperatures reached as a consequence can, if the material is not of the right grade, cause softening and flow.

 

When such changes are slow, the movements may not result in damage but rapid movements through sharp changes in temperature can cause cracking, particularly in cold weather.

 

Under mechanical stress, bitumen products can flow. Asphalt and bitumen are not affected by biological agencies or by pollution but contact with oil and more particularly petrol will dissolve them.

 

Surface crazing is fairly common. Crazing is due, in general, to failure to sand-rub the asphalt with a wood float.

 

Crazing can be due, also, to the lack of a good surface reflective treatment and to ponding of water on the roof, however it is rarely of great importance.

Built-up bituminous felt roofing

The most common defects in built-up felt roofing are splitting, blistering and local embrittlement.             

 

Splitting is caused mainly by excessive differential movement between the felt and the substrate to which it is attached. Felts may be based on organic or glass fibres but none can be stretched, without splitting, by more than around 5%, and less if the felt is aged.

 

Differential movement, which could cause such an extension, occurs commonly. It is because of this that the lowest layer of the usual three-layer felt system is recommended to be only partially bonded to substrates likely to impose undue stress on the felt system.

 

Extensive movement of the substrate has been due either to its drying shrinkage or through poor fixing to primary supports, such as timber or steel joists. This has been particularly the case with chipboard and with wood-wool.

 

Blisters occur far more readily in built-up felt roofing than they do in mastic asphalt. They form either between layers of felt, commonly under the top layer, or between the felt and the deck or the insulation, if the latter is used above the deck. In both cases, they are due to the expansion of entrapped air or moisture by solar heating.

 

However, blisters should not be taken necessarily as a sign of failure. They are common in practice and often have little significance though, like blisters on asphalt, they present a thinner surface and one, therefore, more prone to damage.

Polymer roofing

In recent years, a range of polymeric materials, including notably poly­isobutylene, butyl rubber, PVC, chlorosulphonated polyethylene and polymer modified bituminous roof membranes reinforced with poly­ester fleece have become available.

 

Many are designed for use as single-layer systems, stuck to the substrate with special adhesives and the joints between sheets solvent- or heat-welded.

 

Those used in single-layer systems require a high standard of workmanship and most are installed by approved installers. These products are more flexible than the conventional bitumen felts and retain good flexibility after ageing. The main problem has been water ingress through inadequately bonded joints and mechanical damage, causing splits. Such splits are caused relatively easily by sharp objects left on the roof and human traffic across the same.

 

Roof coverings