COP v3.0:roofing; thermal-expansion-and-contraction

Thermal Expansion And Contraction

7.3 Thermal Expansion And Contraction 

All metal cladding and flashings are subject to expansion and contraction caused by changes in temperature, and their design should allow for this movement. The energy produced should be absorbed without damage to the cladding, fixings or structure. The recommendations in this section are specific to preventing damage and leaks through thermal movement. Thermal movement can also cause disturbing noise levels in dwellings with shorter member lengths than those recommended in this section. (See 12.1 Roof Noise.)

The ribs of metal trapezoidal or corrugated roof and wall cladding absorb expansion across the width of the sheets, but special provisions are needed over the sheets' length.

Much of the longitudinal expansion is taken up by the bowing of the sheet between fastened supports. The extent to which this happens depends on the profile strength and support spacings.

Failure by thermal expansion normally results in shearing of the fastener. Fasteners into lightweight steel purlins up to 3 mm in thickness are less vulnerable as they tend to rotate rather than be subjected to repeated bending resulting in fatigue failure. Fasteners into hot rolled steel sections or timber are far more vulnerable to this mode of failure and in all run lengths over 20 metres provision for expansion must be made when fastening into such supports.

Where overlapping sheets are fastened through the ends, they must be considered as one length to calculate thermal movement. Unfastened end laps are not recommended.

Wall cladding does not require the same provisions as roof cladding, because of solar radiation angle.

Oversized holes and washers give some room for expansion and contraction, but it is not enough to allow movement without stress or distortion over long spans. In such cases, a step joint should be used. (See 8.4.4.3B Stepped Roof Flashing)

Temperature Range

7.3.1 Temperature Range 

 

Ranges of temperature likely to be experienced in NZ by different steel cladding are:

7.3.1A Steel Cladding Temperature Ranges

  Max/Min Roof Temp °C No Wind
InsulatedLight colour+60° -15°=75°
InsulatedDark colour+80° -15°=95°
UninsulatedLight colour+50° -10°=60°
UninsulatedDark colour+65° -10°=75°

Aluminium and zinc, which have twice the expansion rate of steel, do not necessarily expand to this degree because of the different characteristics of mass, emittance, and radiance which affects their temperature range. Copper expands one and a half times as much as steel, and stainless steel can expand up to 1.5 times as much as steel depending on composition.

The theoretical expansion of steel roof cladding in mm is 12 x temperature change x length in metres/1000.

Steel expansion rates can be calculated as follows:

Given a length (e.g., 30 m) and that the material (e.g., a light-coloured uninsulated roof) moves through a 60°C range (e.g., + 50°C -10°C), the theoretical increase in length is 12 x 60 x 30/1000 = 21.6 mm.

This amount of movement of roof cladding and components does not have to be provided for in practice, because:

  • The building also expands with the ambient temperature, although to a lesser degree.
  • Fasteners into light gauge purlins will roll rather than bend. The purlin flange may also roll to a degree.
  • The roof cladding bows between purlins when it is constrained. Sighting down a corrugated steel roof on a warm sunny day will show an undulating line compared to a straight line when the roof is cool. The forces created by expansion and contraction are self-levelling, i.e., each component moves under load until the resisting force is more than the expansion force.
  • When a length of sheeting is solid fastened at the centre and unconstrained at either end, the movement is towards the ends of the sheeting; meaning the actual expansion or contraction movement is only half that of a full length of roof or wall cladding fastened at one end. Special design of the ridge or head barge flashing is required in these cases to allow free movement. Alternatively, sheets can be solidly fixed at the upper region, so all expansion takes place in the lower part of the sheet towards the eaves.
Roof Cladding Expansion Provisions

7.3.2 Roof Cladding Expansion Provisions 

 

The expansion of roof cladding depends on the materials, the constraints imposed by the fixing, the heat paths in the building and the actual temperature. The following graphics are indicative of favourable and unfavourable conditions for thermal expansion and suggest what these are. They show the lengths under both sets of conditions above which special provision needs to be made to accomodate thermal expansion.
 

7.3.2A Favourable Circumstances for Controlling Expansion

Favourable Circumstances for Controlling Expansion

 

 

7.3.2B Unfavourable Circumstances for Controlling Expansion

Unfavourable Circumstances for Controlling Expansion

Notes:

  1. Where a roof requires oversize holes, only the portion of the roof outside of the recommendations require provision for expansion.  Normally this takes place towards the eaves
  2. These are guidelines only and special engineering of the roof, fixing or ventilation may allow greater spans to be used.
  3. These diagrams refer only to roof cladding screwed through the top. Secret or clip fixed roofs can move more freely if installed correctly and allow for using greater run lengths.
  4. The recommendations are based on preventing damage to the fasteners and are not a recommendation to prevent roof noise.