IDELCO α-therm SARKING on a large pitched roof: handling and setting out the boards

Draw the rafters, insulation above them, the underlay specified for the roof covering and the supports for the covering. Standard SARKING has a multilayer aluminium laminate on both faces. Specify the underlay separately, including how to join its laps and turn it up at obstructions.

With thermal conductivity of 0.022 W/(m·K), the over-rafter insulation also covers the upper face of the rafters. Its performance at the perimeter depends on the junction with the wall insulation. On an enlarged wall-plate section, trace a line through both insulation layers and examine where they meet.

The data sheet covers thicknesses from 100 to 160 mm. If a thicker board is proposed, compare sections at the same scale. Carry the height change through to flashing levels, access to fixings and the insulation connection at the roof window reveal.

A 1200 × 2400 mm board covers 2.88 m² before cutting. Allow room to turn the board and support it securely during cutting. Protect all four profiled edges while lifting it to the roof: damage to a long tongue can prevent the joint from closing along its length.

At a valley, mark the cut angle and the side to keep for each slope. Before counting on reusing an offcut on the opposite slope, check the direction of the bevel and the tongue-and-groove orientation. A matching angle alone does not ensure a fit in the next course.

Add the new covering level and the underlay drainage path to the existing eaves section. The two levels may need different flashing details. Check them against the fascia, gutter bracket positions and the designed ventilation inlet beneath the covering.

At a roof window, read the frame installation level relative to the finished covering from the design. Before cutting boards for every opening, check one window for flashing clearance, underlay connection and insulation around the frame.

The SARKING sheet lists at least 150 kPa at 10% deformation. Fixings require a separate design taking account of supports, roof layers and applied loads. Mark the rafter lines on site before the boards cover them.

The board has reaction-to-fire class F. Transfer the required clearances and materials from the specific chimney system documentation and the project design onto the junction drawing. Establish where the PIR ends and how the flashing fits before cutting boards.

Count rows of full boards for rectangular fields. Draw a separate cutting layout for valleys and dormers. Alongside insulation area, measure eaves, ridges, valleys and junctions with vertical walls. These lengths determine the extent of the flashings.

Two roof slopes of equal area can cost different amounts if one has four windows and the other only one. Count flashing kits, allow time for fitting the underlay and plan lifts to the work area separately. Comparing quotations against these quantities makes omissions easier to spot.

Start with a strip running from the eaves past one window. Record the time needed to lift the boards, fit the edges and complete the underlay connection at the frame. Use that trial strip and the weather forecast to decide how much of the next area can be made weathertight by the end of the shift.