Above-rafter PIR insulation on large pitched roofs: setting out and detailing

In the trial bay, check the joints, bearing surfaces and fixing positions before repeating the agreed method elsewhere. Include part of the eaves or a penetration as well as an unobstructed area. This gives the installers a chance to check the insulation where it meets a more complex detail.

Before ordering the eaves flashing, mark the levels from the top of the rafters to the finished roof surface on a section drawing. Draw the actual thicknesses of the insulation, sheathing and supporting battens. On a long eaves line, check this height at several points.

Set out the first run of insulation from verified reference points. A small deviation in the first row can extend along the entire eaves line. Check the levels of the counterbattens and terminations against the intended roof geometry. Correct uneven framing separately. Early checks let the crew lay subsequent rows quickly without repeating the same error across dozens of boards.

When work stops, trace the path rain could take from the upper edge of the open strip down to the eaves. Plan how to close the strip overnight even if the boards have a factory-applied membrane. A separate detail on the room side should maintain the continuity of the air seal.

A factory-applied membrane still needs watertight edge details. On a large roof the area laid in a day can be much greater than on a house. Define a temporary seal for every exposed edge so rain cannot run under the layers. Match deliveries and crew size to the amount of roofing the installers can protect before they leave.

On a section through a counterbatten, draw the screw passing through the insulation into the rafter. For the specified build-up, the fixing design sets out how the screw is installed, its required embedment and the spacing. Mark on the roof plan where the fixing pattern changes near the edges.

Wind actions vary across a roof. Check the design before taking the fixing pattern for the central area into an edge zone. Mark the zone boundary for the crew and allocate the appropriate quantity of fixings to each area. Inspect them during installation, while the positions are still visible. A repeatable fixing detail is useful only within the zone for which it was designed.

Start the board layout by marking the outlines of the rooflight upstands. Allow the space shown on the detail to turn up the membrane and fit the flashing around each upstand. At the foot of the slope, check where the last row of boards meets the wall insulation.

Coordinating these details reduces awkward cuts and helps maintain continuity around openings. A line of rooflights can divide an otherwise simple slope into short runs. Confirm the upstand dimensions before ordering boards and allow for the membrane terminations in the cutting plan. A combined drawing of framing, openings and insulation keeps the seals accessible and reduces clashes on site.

Mark the unloading positions approved for the structure and the board handling route on the work plan. For each shift, set out a strip the crew can close before rain arrives. Calculate how much material that strip needs.

Schedule deliveries to match the rate at which boards can be laid and protected. Before moving packs onto the roof, check the storage positions, transport routes and measures against wind. The structure limits where a delivery can be placed. Sending packs directly to sections that are ready for installation reduces handling and the time boards spend exposed.

In this example an 80 cm repeating roof bay comprises 8 cm of timber and 72 cm between rafters. One tenth of its width therefore lies over timber. A continuous above-rafter board adds resistance along both heat-flow paths. Calculating the U-value requires the resistances of both sections and data for the actual roof. The fraction of the width occupied by timber alone cannot tell you the fraction of heat lost through it.