How much to space deck floor boards:

This article explains proper gap size to leave between deck boards for drainage and drying, and extra steps to take to protect deck joists from rot.

We include a description of tightly-butted tongue-and-groove pressure treated deck boards as well as typical 5/4" thick pressure treated deck boards that should be spaced for drainage.

Discussed here: What is the proper gap to leave between deck boards?W hy is it important to space deck boards apart by a small gap? Steps to protect deck joists from rot. Best Construction Practices for decks or porches. Deck & Porch construction details & procedures.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

Spacing the Deck Boards for Drainage & Drying

This article series includes excerpts or adaptations from Best Practices Guide to Residential Construction, by Steven Bliss, courtesy of Wiley & Sons. Also see our BOOK REVIEW of that book.

[Click to enlarge any image]

When Lon Church constructed this outdoor stage for the Summerblue Arts Camp (Two Harbors MN) in 2001, we knew that the structure would have to endure severe winters and strong summer sun without the protection of a roof.

And because the surface is used as a performing stage, we used 5/4 tongue and groove treated wood for the surface - no gaps for drainage.

Sloping a solid deck floor to achieve drainage

To compensate for the drainage worry, we made sure to slope the stage (from back towards the front or audience side) - and the stage pitch became a theatrical feature.

Below is a photo of the stage in use [image file], courtesy Lon Church, Summerblue Arts Program.

The original construction, installation and long term performance of this outdoor performing arts stage floor are described in a series of articles beginning at BATTER BOARDS LAYOUT METHOD.

Leave gaps between deck boards to achieve drainage

But most decks need and can achieve spacing between the deck boards for drainage.

As discussed in Best Practices Guide to Residential Construction:

'When securing the decking, it is important to leave adequate spacing between the boards for water to drain. The goal is to have about an 1/8-inch gap (the diameter of an 8d nail) between boards after the decking has dried to its equilibrium moisture content.

If the decking is installed wet, as is often the case for pressure treated material, it is best to install the boards tight, letting gaps form as the wood dries.

When installing kiln-dried stock, use a 16d nail as a spacer to leave enough space for the boards to swell slightly and still leave an adequate drainage space.

For wood that has partially air-dried, it is a judgment call. If in doubt, it is best to err on the side of leaving a little extra space for the wood to swell when wet.

Below is the same Summerblue Arts Camp performing stage seventeen years later, in February 2018. Still standing and still in use (in the summer time).

This stage, built with 5/4 tongue and groove flooring, has no between-board drainage, but the deck is pitched slightly forward, towards the audience for performance reasons. That pitch improves its drainage.

The original construction, installation and long term performance of this outdoor performing arts stage floor are described in a series of articles beginning at BATTER BOARDS LAYOUT METHOD.

Watch out: don't leave wider gaps between still-wet treated lumber deck or porch boards during construction or you may find that as the wood dries, excessive deck board gaps can form a trip hazard, especially for people wearing high heeled shoes.

Home Inspector David Grudzinski provided the too-tight deck board photographs just below, and comments:

Here is a deck poorly installed with no deck spacing, and no ventilation. I'm sure the deck joists and structure have suffered from this installation.

Those steps look a bit slippery too. These deck boards are Trex™ or another synthetic decking material. That may explain part of the too-tight boards and no drainage - someone didn't realize that synthetic deck boards, unlike (usually wet) treated lumber decking, will not not shrink and open up later.

Mr. Grudzinski adds: Not to mention when it snows and the snow melts, the added weight on the deck by a non draining deck will be immense [... if trapped water results in a build-up or accumulation beyond the anticipated design of the deck. - Ed.]

Editor's note: the weights of a given volume of snow and of the water produced when the snow melts will be identical. There is no change in the total mass when snow (or ice) melts to liquid water form. The density of water in solid form is slightly less (0.9167 g/cm³) than in liquid form but the total number of water molecules does not change when snow melts.

However if the snow pattern on a deck traps water that cannot drain through the deck boards, those conditions increase the risk of leaks at the building wall and in changing weather conditions, can lead to an accumulation of a combination of snow and ice to increase weight on the deck.

David Grudzinski, Advantage Home Inspections, ASHI cert # 249089, HUD cert# H-145, is a professional home inspector who contributes on various topics including structural matters.
David Grudzinski, Cranston RI serving both Rhode Island and Eastern Connecticut can be reached at 401-935-6547 fax- 401-490-0607 or by email to Davidgrudzinski@aol.com

Protecting the Deck or Porch Joists from Leaves, Rot

In decking applications subject to continuous wet conditions or the buildup of leaves and tree debris, it is best to protect the tops of the joists from moisture.

This can be achieved by laying strips of felt paper or self-adhesive membrane (one approved for UV exposure) over the tops of each joist prior to laying the decking boards.

Photo (left) courtesy of Mark Morsching, Everflashing, shows an Everflashing stainless-steel flashing product installed over the deck ledger.

Additional metal flashing is in use to protect the deck joist top from rot. Additional details about deck flashing and more deck flashing images are at Deck Flashing at Building.

To avoid deck joist top rot Morshing applies peel-and-stick flashing membrane material to joists and ledgers during deck construction.

Another proprietary product for this application, Vycor Deck Protector™, is available from Grace Construction Products. Membranes such as Vycor Deck Protector can also be used as a barrier between pressure-treated joists and joist hangers, flashings, or other metal hardware to reduce corrosion.

-- Adapted with permission from Best Practices Guide to Residential Construction.

This article series discuss best porch & deck construction practices, including choice of framing materials, decking or flooring choices & installation, how to select and use deck and porch structural and flooring fasteners, actual deck & porch framing construction details & connections, deck joist & beam span tables, how to build leak-proof rooftop decks, construction of covered & screened porches, deck & porch railing construction & materials, choices of finishes and stains for decks & porches, and past & current deck lumber preservative treatments with related health & environmental concerns.

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DECK BOARD GAPS & SPACING at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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