COP v3.0:external-moisture; thermal-expansion-and-contraction

6.2 Thermal Expansion And Contraction 

All metal roof and wall 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 causing damage to the cladding, fixings, or structure and without producing undue noise.

Metals have a high thermal expansion coefficient and generate more movement than many other building materials during heating and cooling. Plastic roof lighting and non-ferrous metal cladding move even more than steel.

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

Long lengths of roof cladding often require oversized or clearance holes. Load spreading washers or clips are also necessary to avoid roof noise and damage to the sheeting or the fastening.

Where end laps are through fixed, or where the roof and wall cladding are connected by curved sheets, they must be considered as one length.

Changes of the direction of the cladding also require expansion provisions.

The amount of metal expansion given in various building related literature, N.Z Standards, and other publications can be misleading because it is calculated from theory and not real conditions.

Non-ferrous metals have a higher rate of expansion than steel. To prevent the material from being over-stressed the length of copper, aluminium, and zinc cladding or flashings are restricted. Because these materials are not as strong as steel, expansion provisions such as slip joints, sleeves, and welts are necessary.

Steel cladding does not need the same degree of expansion provisions as non-ferrous metals, and vertical wall cladding does not require the same provisions as roof cladding, because of solar radiation angle.

Horizontal wall cladding is restricted to the same length requirements and fixing provisions as roof cladding. (See 6.2.2 Roof Cladding Expansion Provisions for the indicative figures for maximum fixed lengths of roof and wall cladding.)

Oversized holes and washers an give some room for expansion and contraction, but over long spans, it is not enough to allow movement without stress or distortion. In such cases, a slip joint or expansion joint should be used. The recommended method is to form a step in the roof structure, which allows the cladding to move independently (see 7.4.4.2A Change of Pitch Junction Flashing) , or if the pitch is greater than the minimum by 2°, the roof can be 'sprung' (see 8.3.8B Watershed (b)).

 

To assure independent movement no fixing should go through both sides of an expansion joint.

With warm roof construction or composite panel systems, thermal movement should be considered in relation to the top sheet, insulating core, and the lining sheet. (See 14.5 Insulated Panels) Temperature changes from one surface to another produces different expansion rates. The resulting tendency of bowing increases the tensile and shear forces on fasteners.

Site assembled semi-composite panel systems do not pose the same problem, as each element can be dealt with independently and no shear forces are transmitted to the insulation.

Where profiled metal sheets are fixed horizontally in long lengths or to curved corners, expansion provisions should be made as in  6.2.2 Roof Cladding Expansion Provisions

The amount of linear expansion — especially in relatively short sheets — may appear insignificant, but considerable forces develop if the sheet is fully restrained at both ends and with no deflection.

Sheet lengths of polycarbonate (PC) and rigid PVC should be more restricted than other plastic roof lights because they expand far more per degree of temperature change than metal or GRP. Roof lights constructed from PC and PVC should have fixing holes larger in diameter than the shank of the primary fastener, and the fasteners should be centred in the fixing holes to allow for expansion or contraction of the roof lights.

When the outdoor temperature is significantly higher than indoors, double skin roof lights fabricated from PC and PVC expands more than a single skin roof light, because of the different expansion of the skins. That may result in ponding on roof lights installed on low pitched roofs. (see rooflights section 9.0)

6.2.1 Temperature Range 

 

Ranges of temperature likely to be experienced in N.Z. by different steel cladding are:

6.2.1A 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°

7.7.1D T Flashing shows detail for a flashing that provides for expansion in long horizontally clad walls.

Aluminium and zinc, which have twice the theoretical expansion rate of steel, do not necessarily expand to this degree because of the different characteristics of mass, emittance, and radiance which affects the temperature. They do, however, need considerably more expansion allowance than steel.

Copper and stainless steel theoretically expand one and a half times as much as steel. Figures published for metal expansion rates are given linearly per degree, which does not take into account the many other factors that affect the expansion. The theoretical expansion of steel roof cladding is 12 mm/100°C/10 m, and aluminium is approximately twice that.

Other metal expansion rates can be calculated as follows:

  • Given a length (e.g., 40 m) and that the material (e.g., a light coloured insulated roof) moves through a 75°C range ( e.g., + 60°C -15°C), the theoretical increase in length is:
  • 12 x 0.75 x 40/10 = 36mm
  • In reality, the maximum actual expansion would be 10 mm.

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

  1. The material would usually have been fixed when the ambient temperature would average 15°C. Below that temperature the metal, in theory, would contract and the amount of expansion would, therefore, be 45/75 = 20 mm.
  2. The building also expands with the ambient temperature, although to a lesser degree.
  3. The flexibility of the building can accommodate a large amount of movement, depending on the structural design, and also by deflection of the purlins. Known as purlin roll, the amount of deflection depends on the type of Purlin–structure attachment, i.e., cleat or screw fixed.
  4. The roof cladding bows between purlins when it is constrained. How much it will bow, depends on the strength and span of the profile; the stronger the profile, the more allowance is needed for expansion.
  5. 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.
  6. The sheet fixings will bend under load if the cladding is rib fixed, i.e., if the point of fixture is a distance from the point of constraint. The extent of any constraint depends on the metal and the type of fixing.
  7. If the roof cavity is ventilated, or an uninsulated building has open doors, a significant difference between the ambient and the internal air temperature is unlikely.
  8. Heat radiation from a metal roof or flashings balances the temperature at which the roof sheeting reflects as much heat as it is receiving. This temperature depends on the colour of the roof, the placement of any insulation, air movement, and the ambient temperature.
  9. When a length of sheeting is fastened at the centre and unconstrained at either end, the movement is towards the ends of the sheeting, so the actual expansion or contraction movement is only half that of a full length of roof or wall cladding fastened at one end.
Other factors—colour, metal, temperature, profile, and insulation— influence the extent of roof and wall cladding expansion.

Solar radiation striking a roof is absorbed and heats the cladding. Some heat is reflected upwards, some is radiated downwards, and some is convected upwards. The thermal performance of an unpainted Z or AZ coated steel roof may change after some years, because of surface oxidation and the accumulation of dirt.

A dark coloured surface absorbs more solar radiation than a white or light coloured surface, and the amount of heat radiated depends on the thermal emittance of the surface. Emittance should not be confused with reflectance, as shiny surfaces have a low thermal emittance and dull, rough surfaces have a high thermal emittance irrespective of colour.

The amount of wind also affects the temperature of the cladding.

If there is little wind, the heat build-up due to solar radiation can rise to an equilibrium point where emittance or radiation, equal the absorption. This seldom occurs in practice, because the heat differential sets up air convection currents that conduct heat away from the roof cladding, and for uninsulated roofs, a breeze of 3 m/s (approx 11 k.p.h.) can lower the roof surface temperature by 25°.

Statutory insulation requirements for domestic buildings have accentuated roof expansion problems, and special design considerations need to be provided in domestic skillion or curved roofs to avoid noise resulting from expansion. (See 11.1 Roof Noise)

Insulation must not be placed hard up against the roof cladding, as it causes the temperature to rise considerably higher than that of an uninsulated roof, and unvented insulated ceilings can increase the roof temperature by over 20°.

The amount of expansion that should be allowed for long length metal cladding depends on many factors, and these conditions require individual assessment, specific to the environment, roof structure, and cladding being considered.

To provide some guidelines, the following (Graphs 4.1.6 A and B) show the expansion provisions for typical favourable and unfavourable conditions

6.2.2 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 Unfavourable and conditions for expansion and suggest what these are.
 
 

 

 

 

 

Notes :

 

1) These are guidelines only and special engineering of the roof, fixing or ventilation may allow greater spans to be used.

2) These diagrams refer only to roof cladding screwed through the top. Secret or clip fixed roofs are able to move more freely if installed correctly and again may allow for greater spans to be used.

 

6.2.2.1 Profile Strength 

Corrugated steel does not need the same provision for expansion as trapezoidal or tray roof cladding because it is a weaker profile. Longitudinal expansion takes place by bowing upwards between the purlins. 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.

With stronger profiles the bowing tendency is resisted by the strength of the profile—so the stronger the profile, the greater the provision required for expansion.

The linear expansion of any roof depends on the many factors outlined in 6.1.1 Conduction. The strength of the profile should also be considered, before recommending a fixing system.

6.2.2.2 Flashing Expansion 

Before any metal roof or wall cladding or 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 7.2 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 6.2 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, i.e., every 12 m for coated steel flashings.

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.

Lengths of coated steel ridging, cappings, and apron flashings over 12 m should have a slip joint as described in 6.2.2.3 Expansion Details.

Inadequate provision for expansion can also cause Roofnoise.

6.2.2.3 Expansion Details 

Non-ferrous metals have a higher rate of expansion than steel. The need a greater provision for expansion, which can be achieved by slip joints, sleeves, butt straps, clips or welts, and although some of these details are complicated they have been used successfully for many years.

Expansion should be considered at the design stage and the flashing details should be included in the working drawings and tender documents. It is the responsibility of the designer to provide these details, but the roofing contractor also has a responsibility to install flashings to allow for expansion.

As flashings cannot move in the lateral direction without stress, they should have some provision for longitudinal expansion.

Crest fixed primary fasteners allow some movement, but oversized holes with round or shaped load spreading washers can only provide expansion for lengths up to 12 m of coated steel flashings. For longer lengths, this provision is insufficient to accommodate movement without distortion, and an expansion slip joint is required as shown in per 6.2.2 Roof Cladding Expansion Provisions.

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 drawings 6.2.2.3A Apron Slip-joint, 6.2.2.3B Ridge Slip-joint.

Soaker expansion joints relying on sealant should not be used.

A sealant is required at the slip joint, not to make it weatherproof, but to exclude dust and dirt from two close fitting surfaces, which can retain moisture by capillary action and cause corrosion. In this case only, no through rivet or screw fasteners should be used in conjunction with the sealant.

 

 

 

 

When steel flashings are joined together in lengths of more than 12 m, expansion slip joints should be formed with 200 mm laps. Longer laps than 200 mm should be avoided as these may buckle with thermal movement and require a clip or cleat fixing to hold the flashing in position without restraint.

Barge and apron flashings longer than 12 m that are fixed to the sheet ribs to prevent uplift should be free to move with the structure to avoid expansion 'creaking'. If the sheet length exceeds 12 m, flashings fixed rigidly to the structure should have oversized holes.

Roll top and square top ridging strengthen the ridge, and when used with oversized holes they provide some allowance for longitudinal expansion of the roof sheeting.

6.2.2.4 Building Expansion Joints 

Each building material component has a different coefficient of expansion, and when subjected to varying temperature changes results in different amounts of thermal movement. (see 6.2.2 Roof Cladding Expansion Provisions)

Building expansion or construction joints are used to minimize the effects of stresses and building movement. Metal flashings should be located in the same location as the structural expansion joints of the building. For new construction, it is the designer's responsibility to provide details to allow for building movement and for the placement of expansion joints.

Roof expansion joints should be provided at the following locations .

  • Where building expansion joints are provided in the structure .
  • Where fire-walls separate roof areas .
  • Where steel framing, structural steel or cladding change direction .
  • Where separate wings of L or U shaped buildings or similar configurations exist .
  • When additions are connected to existing buildings .
  • At the junction between a new and an existing building .
  • At junctions where a heated office abuts an unheated warehouse .
  • Wherever differential movement between vertical walls and the roof cladding may occur .
  • Where wind or seismic loading can cause building sway .

Expansion joints should extend across the entire length or width of the roof and continue through to the roof edge or perimeter. Parapets can be designed as expansion joints.

Where a valley occurs it can be used as an expansion joint . The use of a central roll at this point will not only provide expansion provision, but will inhibit fast discharge across the valley. 5.6D Recessed Valley

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 as acting as an expansion joint cover.

There are two main design types of expansion joint covers.

1. A one-piece design to accommodate movement by the use of a central bellows or roll that allows the flashing to be positively fixed on both sides. See drawing 5.4.8.A.

2. A two-piece design to accommodate movement by the use of hemmed edges, with sufficient clearance for the expected movement.

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. 7.4.3 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.