Simple Membrane Roofing

A common way to waterproof flat, or nearly flat roofs is to use single-ply membrane coverings, which either adhere straight to the roof deck, or are fixed with special mechanical fasteners. A major manufacturer of these is Sarnafil - lots more information can be found on their website Membrane roofing is ideal for medium to large flat roofs, as well as being particularly useful for covering awkward geometry such as curves.

The simplest way of using a membrane roof is illustrated below, with an AutoCAD drawing of the same detail available to download at the end of this article.

Membrane roofing.jpg

  1. 12.5mm plasterboard, screwed to joists with special screws, painted to create the ceiling.
  2. Vapour barrier, stapled to joists. Required for all timber construction to prevent internal moisture affecting the wood.
  3. 135mm rigid insulation, fixed between 175 x 50mm softwood joists (the main structural element in the roof). Insulation should be lapped with insulation in walls to ensure no gaps where heat can easily escape.
  4. Ventilation gap above insulation, typically 50mm.
  5. Softwood firrings, fixed to joists, to create a slight slope to the roof to allow rainwater to drain away.
  6. 18mm WBP (weather and boil proof) plywood, fixed to firrings, to create roof deck.
  7. Sarnafil membrane, fully adhered to plywood, forms outer waterproof layer to roof.

Care must of course be taken at the edges of the roof, to create a fully waterproofed construction - see this article, which details a possible edge condition, or the construction details on the Sarnafil website.


Source: Professional architect's drawings

Additional downloadable files

Membrane roofing.dwg
Submitted by Nick Howlett82 Articles
Published on Saturday 10 October 2009View Profile