How to build a floating timber deck

Floating timber decks have become necessary over exterior membrane decks since an amendment to the Building Code Clause E2 External Moisture came into effect in 2005.

Page 54 of clause E2 says: “Raised removable surfaces of tiles, pavers or timber shall be provided over the underlying weathertight enclosed deck surface for cleaning and maintenance”. The diagram below of Level Thresholds for Enclosed Decks was provided on page 55:

Drawing as found in Building Code Compliance Document E2

Key points to note:

  • A waterproof membrane must run under the joinery and down 100mm to deck surface.
  • The membrane deck surface will typically fall away from the building at 1:40 (1.5 degrees), however Auckland council usually requires 1:30 (2 degrees) of fall. Check with your council.
  • Timber joists that support the decking must be supported by an isolation pad to allow water to flow freely under the joist.
  • Timber deck structure must be H3.2 treated minimum.
  • A 12mm gap must be maintained around the perimeter of the floated timber deck.
A more efficient way to build a deck

Cutting timber packers under each joist is labour intensive and problematic. Adding to this, Clause E2 shows an additional layer of membrane placed under each packer. However, when using small packers in this way indentations into sheet based membranes can occur.

Pic3
The better way to build a floating timber deck is to use specifically designed plastic pedestals to support the timber joists. Pedestals come in two basic designs.

  • Static height pedestal
    A static height pedestal, such as the Nuralite Nurapad, is designed to support a timber joist which follows the fall of the membrane deck or is ripped to the fall of the membrane deck – creating a level trafficable surface. Joists are placed 400mm apart with the pedestal supporting the joist over the membrane. Typically a 50 x 100 joist will need to be supported at 600mm centres by the Static Height Pedestal. If the joist is ripped to the fall of the deck, the thinner the rip the closer together the pedestals will need to be. A good guide is to move the Static Height Pedestal 100mm closer together for every 20mm ripped from the joist. Remember that any H3.2 treated timber that is cut should have the cut edge treated with an appropriate treatment.
  • Adjustable Pedestal
    An adjustable pedestal, such the popular Nuralite Nurajack (as shown above), is designed to support a timber joist and create a level trafficable deck surface. Ideally the pedestal should be able to be levelled to counteract the fall of the membrane deck surface. Non levelling deck pedestals have a tendency to ride up and point load the membrane, however pedestals that can be levelled, such as the self-levelling Nurajack pedestal ensure an even distribution of load onto the membrane without point loading. Joists are still 400mm apart and the adjustable pedestals need to be at 600mm centres along a 50 x 100mm joist. If thinner joists are to be used (such as a 45 x 75mm) the adjustable pedestal should be moved closer together as per the above scale.

Advantages of adjustable pedestals
The advantage of adjustable pedestals is the time savings on site. To install, set up a joist using the smallest pedestal at the shallowest end and using the largest pedestal at the deepest end of the deck, adjusting both to create a level joist. Then at 600mm centres screwing additional Adjustable Pedestals and adjusting until they support the joist – ensuring the joist stays level. Repeat for each joist 400mm apart, minimal nogging (if any) is required. Screwing the timber decking in place ensures the decking is removeable and the membrane is always accessible.

In many instances, to fit a floating deck level with a door threshold, using a combination of Static Height and Adjustable Pedestals is required.
The finished deck surface is level, raised above the membrane, allows water to drain effectively and meets the Building Code Clause E2.

Acceptable Solution utilising Nurapad and Nurajacks

ITM is now a distributer of the Nuralite Nurajack and Nurapad range of pedestals.  If you have any questions about Floated Deck design, talk to your ITM sales rep or visit www.nurajack.co.nz.