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Under One Roof

Home > Roofing Technical Guidance from CRCA > Manuals > Canadian Roofing Reference Manual > Chapter 3: Roofing Preparation > 3.1 Roof Deck Preparation > 3.1.5 Separation Sheets

Canadian Roofing Reference Manual

3.1.5 Separation Sheets

Separation sheets are used in roofing systems to separate the deck from the main roof components and serve to:

  • Prevent bleed-through of asphalt, adhesives or other liquid roofing materials.
  • Provide a levelling surface.
  • Act as a slip sheet.
  • Provide ventilation.

The type of separation sheet installed in a particular roofing system depends upon project specifications, the type of deck and the type of roof system that is being installed.

3.1.5.1 Wood Decks (BUR)

The following details the installation of separation sheets on the following:

  • Plywood decks, having a separation sheet membrane constructed with organic felt.
  • Dimensional lumber decks having a separation sheet membrane constructed with organic felt.
  • Plywood decks having a separation sheet membrane constructed with inorganic felt.

Uninsulated Roofs –Organic Felt

On plywood decks, having a membrane of uninsulated organic felt, a 2 ply, #15 perforated felt separation sheet is to be installed as follows:

  1. Lay a 480 mm (19 in) wide roll of felt.
    1. Start at the low point, at right angles to the slope, and lay a 480 mm (19 in) wide roll of felt.
    2. Position the felt so that it is flush with either the roof edge or the top of the cant strip.
    3. Tack the felt sufficiently so that it is held in place.
  2. Lay a 900 mm (36 in) wide roll of felt.
    1. Lay the 900 mm (36 in) roll of felt over the previously laid 480 mm (19 in) roll of felt.
    2. Position the 900 mm (36 in) roll of felt so that it is flush with either the roof edge or the top of the cant strip. Ensure that the felt is flat to the deck and that there are no buckles or voids.
    3. Fasten the 900 mm (36 in) roll of felt into place using at least 25 mm (1 in) galvanized roofing nails along the toe of the cant strip and along the outside edge, 300 mm (12 in) o.c.
  3. Continue to lay 900 mm (36 in) wide rolls of felt until the opposite wall is reached.
    1. Continue to install 900 mm (36 in) rolls of felt until the opposite wall is reached. Overlap each roll by 480 mm (19 in) to ensure 2 ply coverage over the entire roof deck.
    2. Nail the edge of each lap a minimum of 300 mm (12 in) o.c. The roll end laps must be at least 150 mm (6 in).
  4. Lay a 480 mm (19 in) wide roll of felt to complete the 2-ply coverage.
    1. Lay the 480 mm (19 in) roll of felt.
    2. Fasten the felt into place, 300 mm (12 in) o.c. along both edges and the toe of the cant strip.

Remember: Never install separation sheets over an area of the roof which is not going to be completely covered with the main membrane on the same day.

Dimensional Lumber Decks

The procedure for installing separation sheets on dimensional lumber decks is very similar to the one described above for uninsulated decks with an organic felt. The only exception is that one layer of dry sheathing paper is laid over the entire deck before the organic asphalt felts are installed. This is necessary to prevent the asphalt felts from bonding to the deck, because dimensional lumber will dry out and shrink. This could cause the felts to tear.

Uninsulated Decks – Inorganic Felt

On uninsulated decks having a membrane of inorganic felt, separation sheets consisting of one (1) ply dry sheathing paper and one (1) ply Type IV or VI glass felt can be use as follows:

  1. Lay a roll of dry sheathing paper.
    1. Cover the entire deck with one layer of dry sheathing paper.
    2. Lap the sides a minimum of 100 mm (4 in) and the ends a minimum of 150 mm (6 in).
    3. Tack the sheathing paper into place.
  2. Lay a roll of glass felt.
    1. Lay a roll of glass felt over the previously installed dry sheathing paper.
    2. Stagger the side laps (they must be offset from the sidelaps on the dry paper).
    3. Lap the sides a minimum of 100 mm (4 in) and the ends a minimum of 150 mm (6 in).
    4. Nail the glass felt to the deck 150 mm (6 in) o.c. along the lap, and 300 mm (12 in) o.c. in rows 300 mm (12 in) apart along the middle of the roll.
    5. Extend all plies to the top of the cant strip on all curbs or walls.

Remember: Only install separation sheets over an area that can be covered with the main membrane the same day.

3.1.5.2 Steel Decks

The separation sheets or deck covering installed on steel decks are generally either 12.7 mm (1/2 in) plywood, cementitious or gypsum board. Gypsum board and cementitious board can be used to provide a thermal barrier (fireguard) when combustible insulations are being used. The major steps for installing separation sheets or deck covering on a steel deck as follows:

  1. Position the separation sheets or deck covering on the steel deck.
    1. Positioning the separation sheets or deck covering so that their edges are resting on the flutes on the steel deck.
    2. Laying the sheets so that their long side is perpendicular to the flutes, and the joints are tight together.
  2. Fasten the separation sheets or deck covering to the deck.
    1. Fasten the sheets to the steel deck using either mechanical fasteners or an adhesive as specified.
    2. Fastener type and density and adhesive type and application rates will be provided in specifications and/or manufacturer’s instructions based on uplift resistances requirements.
  3. Prime the top surface if necessary.
    1. If plywood is being used, the top surface of the plywood must be primed with an asphalt primer.

Remember: Only install separation sheets or deck covering over an area that can be covered with the main membrane the same day.

3.1.5.3 Concrete Decks

Concrete roof decks typically contain moisture even when the deck surface appears dry and is dry enough to permit roof installation. Separation sheets used on concrete roof decks are often intended to facilitate the ventilation of this moisture from beneath the roof system.

Vented base sheets can take one of several forms including;

  • Button Vent Type – A heavy granular surfaced based base which is perforated with uniformly spaced 19 mm (3/4 in) +/- holes. These sheets are laid loose over the primed roof deck and extended over the perimeter parapets to the building exterior. The width side and end laps consistent with the manufacturer’s instructions. The vapour retarder or roof membrane (uninsulated construction) is then installed into a full mopping of asphalt bitumen with hot asphalt bitumen and a portion of the bitumen flows through the perforations spot adhering the vented base sheet to the deck and the base sheet mechanically fastened at the parapets. The space between spots of adhesive allow the venting of moisture from the deck to the building exterior.
  • Spot or ribbon adhered pressure sensitive base sheet – A heavy base sheet with a pattern self-adhesive surface and venting surfaces (fleece or granules). The sheet is positioned on the primed surface of the roof deck extending up and over the perimeter curbs to the building exterior. The release film is removed, and the sheet is pressed into contact with the primer with a weighted field roller or a hand roller on the parapets. The venting surfaces between the adhered areas allow the venting of moisture from the deck to the building exterior.

Reminder: Only install separation sheets over an area that can be covered by the main membrane the same day.