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Plywood Roof Decks

Plywood is frequently used as roof decking for small extensions to dwelling houses but if the roof is not properly designed condensation is likely to become a major problem and is often incorrectly diagnosed as a leak in the roof. Cold roofs are those where the insulation is laid on top of a plasterboard ceiling and between the joists.

This system is to be avoided as it is almost impossible to adequately ventilate the space between the top of the insulation and the underside of the roof deck in which case condensation will occur.

It is also quite common to find roofs which have no insulation. While this is acceptable for open porches, car ports or similar, an uninsulated roof over a room of a dwelling house or office will cause sever condensation. The best choice is a warm roof where the insulation is laid on top of the deck and below the waterproofing felt and provided the insulation is sufficiently thick, condensation should not be a problem.

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Plywood Roof Deck Diagram

  1. Ensure that the fall of the roof is sufficient to avoid ponding. A small gap, say 2 to 3 mm should also be left between the sheets of plywood to allow for movement caused by changes in the moisture content. If the roof abuts a concrete or brick wall a plywood upstand should be fixed to the deck (A) to facilitate movement between the deck and the wall.
  2. Paint the plywood with Bitumen Primer (B).
  3. Cut strips 225mm wide from Casali 20050 and place over joints between plywood sheets, torching to one sheet only and leaving loose over the adjoining sheet.
  4. Torch on a layer of Casali 20050 (C) with 80mm laps and run it up the plywood upstand.
  5. Lay the insulation board (D), which should be faced with glass fibre reinforced felt to protect the insulation from excessive heat when torching. Fix the board through to the deck (A) with mechanical fastenings (E) according to the manufactures instructions.
  6. Loose lay a layer of Gruver (F) perforated felt butt jointing in preference to lapping.
  7. Torch on a layer of Dermabit 30160 (G) with 80 mm longitudinal laps and 150mm transverse laps if joining two lengths. While ensuring a good bond don't use excessive heat which could damage the insulation board (D). If the board is exposed to high winds it is advisable to run a line of mechanical fasterners (E) 40mm in from the edge of the sheet to be covered by the lap.
  8. Torch-on a cap sheet of Dermaflex 40160 (H) making sure to stagger the laps so they run down the centre of the underlay.
  9. Paint with two coats of Aluminium Reflective Paint (I) preferably 4 to 6 weeks after laying the felt.
  10. Both the underlay (G) and cap sheet (H) should be torched up the upstand and covered by a metal flashing (L) chased into the wall.

While Dermaflex or indeed Dermabit are the preferred choices for the cap sheet (H), some plywood roof decks are being built to a budget and a membrane with a lower specification may be considered adequate.
* In case of roofs exposed to high winds, we suggest to mechanically fix the underlay (G) to the deck (A) avoiding the use of Gruver (F).

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