Standing-Seam Metal Over Historic Skip Sheathing

The proposal is not acceptable as a complete roof assembly. Reusing the skip sheathing is plausible only if it is structurally sound, but the skip sheathing alone does not provide a continuous substrate for underlayment, and the proposed air gap is not automatically a ventilated assembly. Without a continuous underlayment, meltwater, wind-driven rain, and condensation can reach the historic framing. The 2.5 kPa snow load and freeze-thaw cycles increase the risk of ice damming and corrosion at the valleys and chimney.

Acceptable with conditions:

• Keeping the existing skip sheathing visible from below if it is sound, dry, and fire-resistive as required.
• Using purlins to support the metal panels, provided the structural load path is engineered.

Not acceptable:

• No underlayment at valleys, eaves, and chimney.
• Treating an uncontrolled cavity as ventilation.
• No snow retention above the main entry.
• No cricket or proper flashing at the masonry chimney.

A solid deck or a continuous underlayment system is needed over the skip sheathing before the metal roof.

To preserve the exposed skip sheathing, install a new structural deck above it, such as plywood or oriented strand board, sufficient for the purlin spacing and snow load. Do not install metal directly over the skip sheathing with only an open cavity.

Assembly:

• Inspect the skip sheathing and rafters for rot and structural adequacy; replace damaged members.
• Install a continuous solid deck over the skip sheathing, mechanically fastened to the rafters, with joints staggered and supported.
• Install a high-temperature synthetic underlayment over the entire deck, plus self-adhering ice-dam membrane at the eaves, valleys, and chimney. Metal roofs need a slip sheet or compatible underlayment to prevent abrasion and to control condensation.
• If the cavity between the metal and deck is to be ventilated, provide continuous eave intake and high ridge exhaust with baffles; otherwise seal it as a warm or cold assembly. Do not call an uncontrolled gap “ventilation.”
• Use thermal breaks or a separation layer to reduce condensation on the underside of the metal.
• Install the purlins and standing-seam panels per the manufacturer, with concealed clips designed for the snow and wind loads.

This approach keeps the historic skip sheathing visible from below while giving the metal roof a continuous, water-shedding substrate.

Snow retention: the 12:12 slope and metal surface will shed snow rapidly onto the sidewalk at the main entry. Install snow guards or snow fences above the entry, walkways, doors, and any lower roof or valley where sliding snow can accumulate. Space and size them for the 2.5 kPa snow load and the panel width; do not rely on a few randomly placed guards. Snow retention is a safety detail, not an optional accessory.

Chimney:

• Build a cricket on the uphill side to divert water around the masonry chimney.
• Install step flashing along the sides and a saddle or backer flashing at the rear, integrated with the underlayment and metal panels.
• Use counterflashing cut into the masonry and sealed with a compatible sealant; sealant alone is not the primary flashing.
• At the valleys, install a continuous valley underlayment and metal valley detail that accommodates thermal movement.

Because the roof is steep and freeze-thaw is frequent, all flashings must allow drainage and movement. Inspect the chimney masonry for deteriorated mortar before flashing; repointing may be needed first.