Rooftop Decks - Installation Considerations when Using Hardwood

By Bill Christou, 07/27/17

Many people want to build their hardwood deck closer to the ground than we recommend in our installation guidelines. Close to ground applications also include rooftop decks and any type of deck installed directly over a flat structure with less than the minimum recommended 16" of clearance.

There are a number of critical steps that must be followed in order to install decking in this manner:

Width of Material

We recommend only 1x4, 5/4x4 or narrower boards. Never use 1x6 or 5/4x6 material in close to ground applications.

Spacing between boards

You must allow space for your deck boards to expand when they take on moisture, as they will inevitably do when exposed to wet weather, rain or high humidity. The final spacing should be at 1/4" if you have material with a moisture content in the 10-12% range and relative humidity in the 35-45% range.

If you are installing during very dry conditions and the boards are measuring in the 6-8% range, then you should add 1/16" additional spacing so that your deck can handle higher humidity and rain.

If you are installing high moisture content material, such as Air Dried Ipe in the 16-18% range, then a gap of 3/16" is appropriate so that material does not gap too much in dry weather.

There is a specific science behind the required gap which is a function of the exact wood species, the beginning moisture content of the wood and the dry and wet extremes you want your deck to handle. In most cases, you should plan for 100% humidity levels and expect that the deck boards will expand to their maximum amount at the fiber saturation point of the wood.

The dry side of the equation has more variability since many areas of the country have typical minimum humidity levels. You don't need to plan for bone dry conditions in the Southeastern United States, for example; but you certainly do need to plan for bone dry conditions in Arizona and Central California.

Shrinkage and Expansion

The amount of expansion from bone dry to fiber saturation is given by tables in references such as The Wood Handbook. The rate of movement is linear with respect to the moisture content from bone dry (0%) to fiber saturation (generally around 25%). Most hardwoods can be expected to move around 8% which is 0.28" for a 3.5" board. This is the maximum movement from bone dry to fiber saturation.

Typical hardwood species such as Batu, Cumaru and Angelim Pedra (as well as many others not sold by Nova) have movement up to 8%. The ONLY hardwood species which is significantly more stable is Ipe, which has a total movement in the 5-6% range.

We always recommend Ipe as the best hardwood species when building close to ground. Ipe will exhibit less tendency to cup, warp and or check as changes in moisture content occur. However, all hardwood decking species will exhibit some degree of checking, cupping and warping when allowed to fluctuate from very dry to very moist conditions.

Weatherizing & Sealing / Finishing

The best way to minimize the movement in service of wood is to slow the flow of moisture into and out of the boards.

We recommend waxing the underside with a typical end seal wax product. Wax should be applied on the raw wood - it is not necessary to coat the material with oil finish if wax is being used on the back side.

An oil finish should be applied on the face and edges of every board during installation. Oiled boards may still be waxed on the bottom side provided that the oil allows the wax to penetrate and provide a moisture seal. Some sanding or cleaning with solvent may be required in order to get the wax to properly adhere to the previously oiled boards.

The only reason a hardwood deck fails when close to the ground is INCORRECT INSTALLATION. Nova's warranty is against rot and decay - it does not cover shrinkage and expansion.
Each rooftop deck application comes with its own unique series of challenges. Contact the experts at Nova if you have any questions www.novausawood.com

By Bill Christou, 07/27/17

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