Expansion should be considered at the design stage, and the flashing details should be included in the working drawings and tender documents. As flashings cannot move in the lateral direction without stress, they should have some provision for longitudinal expansion when their length exceeds 18 m, or for lesser distances for visual flashings such as fascias and facades.
When using an expansion joint, independence of movement should be assured by the omission of any fixing through both sides of the joint. Preferred and acceptable slip joint expansion details are shown in 8.10A Apron Slip-joint and 8.10B Ridge Slip-joint. Soaker expansion joints relying on sealant should not be used.
Slip joints require sealants, not to make them weatherproof, but to exclude dust and dirt from two close fitting surfaces, which can retain moisture by capillary action and cause corrosion. Rivets and other fasteners must not prevent movement of the slip joint.
Before any transverse flashings are fixed the framing timber must have a maximum moisture content of 18%.
Transverse flashings, such as ridging, are sometimes prone to excessive buckling which is blamed on metal expansion but is usually due to timber shrinkage, and a phenomenon known as compression timber. Abnormal growth causes this defect in timber, and it can shrink up to 10 times more than normal. It is not easy to recognise compression timber and roofers are advised to measure the moisture content, particularly of ridge purlins, before fixing.
An alternative is to use steel top hat purlins.
Fixing roof cladding should be treated in the same way as internal linings, i.e., do not fix transverse flashings when the moisture content of any timber is more than 18%. The thickness of flashings should always comply as specified in 8.1 Flashing Materials
If flashings are positively fixed, framing timber that does not meet this requirement can cause failure of ridging and flashings due to timber shrinkage when drying.
The metal expansion allowances quoted in many publications can be misleading because the information is based on theoretical metal expansion values and is not related to real-world conditions.
Figures published for metal expansion rates are given linearly per degree, but it does not take into account the many other factors that mitigate the theoretical figure. (See 7.3 Thermal Expansion And Contraction.)
It is necessary to make provision for cladding and flashing movement; when long lengths are used and positively screwed or riveted together, they should be regarded as one length.
The maximum length before expansion provision should be made for either cladding or flashings will vary according to colour, micro-climate, ventilation and fixing spacings. It is , however, possible to provide indicative figures based on a study of empirical data over time. The maximum recommended flashing length without any expansion provision is similar to that of roof cladding. (See 7.3.2 Roof Cladding Expansion Provisions.)
Aluminium rivets, which have a low shear value, will fail when there is no provision made for expansion in flashing lengths of over 12 m. Using aluminium joints is only acceptable if they are used at the prescribed distances, and are not used to replace expansion joints.
Expansion joints should be designed to accommodate contraction and expansion. Expansion joints should be detailed and constructed to a minimum height of 100 mm above the roof cladding, and curb-type expansion joints should be designed and installed to ensure drainage of the roof and to prevent any damming of water.
Wood curbing secured to the substrate on both sides of an expansion joint should be flashed with a metal capping capable of acting as an expansion joint cover.
Both of these designs are shown as a parapet following the pitch of the roof. Where this is not the case, the top of flashing should have a 10˚ slope as for all other parapet flashings. (See 8.5.3 Parapet Cappings).
Metal wall construction joint flashings that are embedded in the wall should be made with a bellows or other means of accommodating movement without fatigue and have a durability of 50 years.