August, 2011
Feature
Inspection News and Views from the American Society of Home Inspectors



The Word: Decks

BRUCE BARKER

Part 1 of 2

Once again, The Word invites you to travel into the dark realm of issues that sometimes are misunderstood by home inspectors. The Word hopes you will find this trip informative and maybe a little entertaining.

The Word's term this month is decks. The Word finds this term interesting because deck construction has become a lot more complex since he built his first deck some 25 years ago. When construction gets more complex, inspecting that construction also gets more complex.

Deck construction is far too complex for one column. We'll begin this month with basics including flashing, deck ledgers and hardware. We'll continue next month with more advanced issues.

We'll discuss single-level decks designed for the standard 40 psf uniform live load. Multi-level decks and decks supporting concentrated loads such as spas should be designed by a qualified engineer.

What's the fuss?

Let's first review two of the most significant problems with decks. The headline grabbers are the deaths and injuries caused when decks collapse and when guards fail. Decks collapse and guards fail for many reasons, but two of the most common reasons are improper fastening of decks and guards, and deterioration of deck fastening to the home due to water infiltration. Water infiltration is usually caused by improper drainage where the deck is attached to the home. Because water infiltration causes many other problems, let's look at deck drainage first.

Control that water!

The area where the deck ledger board is attached to the home is one long, potential water infiltration area and one of the longest such areas around the home. A properly installed drainage plane to control water is essential. Figure 1 shows the ideal arrangement of multiple materials that constitute the drainage plane. You won't be able to see most of the details shown in Figure 1 during a home inspection, but here are a few things that you might be able to see.

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Figure 1

The deck ledger board shouldn't be attached directly to any wall cladding material, including brick and stucco. This is both for structural and for water infiltration reasons. The wall cladding material should be removed and the deck ledger should be attached directly through the sheathing into an appropriate band or rim board. The sheathing shouldn't be more than 1 inch thick, and the distance between the band or rim board and the deck ledger shouldn't be more than 1 inch.

You may see a gap of ½ inch or less between the deck ledger board and the sheathing. This gap allows the deck ledger to dry and allows any water that gets behind the deck ledger to drain off. This gap isn't required and some controversy exists about whether a gap between the ledger and the sheathing is wise. The controversy involves whether the benefits of the gap are offset by the reduction in structural integrity caused by loss of contact between the deck ledger and the sheathing.

You should see some type of drip-edge flashing, usually galvanized steel, lapping over the deck ledger board and under the deck flooring. You probably won't see how the drip-edge flashing is installed under the wall cladding and how it's integrated with the other drainage materials, but drip-edge flashing installed on top of the wall cladding is a reportable deficiency.

Deck ledger boards

The deck ledger board should be treated or decay resistant dimension lumber that often is treated #2 southern pine or redwood. The deck ledger should be at least a 2x8 and at least as deep as the deck joists, so 2x10 deck joists should be attached to at least a 2x10 deck ledger. The deck ledger should not be deeper than the home's band or the rim board, so a 2x12 deck ledger shouldn't be attached to a 2x10 band board.

The deck ledger board should be attached to either a nominal 2x dimension lumber band board or to an engineered 1-inch or thicker rim board. The deck ledger should not be attached to I-joist type trusses or to open web (metal-plate connected) trusses without written engineer-approved plans. Figure 2 shows a summary of these typical requirements.

The type and location of fasteners are essential to ensure sound structural attachment of the deck to the home. Bolts or lag screws of ½-inch diameter are the usual minimum size. Hot-dipped galvanized is the usual minimum coating material for all hardware including nails, screws and joist hangers. Stainless steel may be required in severe-exposure areas such as near salt water.

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Figure 2

Figure 3 shows the typical location of deck ledger board fasteners. The spacing of fasteners depends on the length of the deck joists, whether attachment is to a dimension-lumber band or to a rim board, and on the fastener type (bolt or lag screw). Typical fastener spacing for deck ledgers attached to engineered-rim boards is between 9-12 inches on center and between 11-24 inches for deck ledgers attached to dimension-lumber band boards. The more distant spacing applies to shorter deck joists and the closer spacing to longer deck joists. Lag screws need closer spacing than bolts. Refer to your local codes and to the references cited at the end of this column for more information about ledger fastener spacing.

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Figure 3


Attachment of the deck ledger board is intended to accommodate vertical loads and lateral loads. Vertical loads are the weight of the deck materials (dead load) and the weight of occupants and belongings (live load). Lateral load is the force that wants to rip the deck out of the wall in the horizontal direction. You may see hardware such as that shown in Figure 4 as an alternate method of accommodating lateral loads. There should be at least two of these brackets per deck. Note that these brackets do not replace lag screws or bolts unless allowed by an engineer or by the bracket manufacturer's instructions.

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Figure 4


Hardware corrosion

Almost all decks built before 2004 used wood treated with an arsenic-based compound known as CCA. The Federal government outlawed CCA as of December 31, 2003. CCA is somewhat corrosive to deck hardware, although hardware corrosion due to CCA isn't widespread. Hardware corrosion in CCA-treated decks is more likely due to severe environments and due to substandard hardware.

Most decks built beginning sometime in 2004 use wood treated with compounds such as ACQ and CBA. These compounds are more corrosive to deck hardware and hardware failure has occurred, particularly in decks built from 2004 to around 2007, or so when the corrosion problem became more widely known. You should look extra carefully at hardware on decks built since 2004, and especially at decks built in the early years of the conversion from CCA-treated wood.

It's pretty easy to tell the difference between galvanized and non-galvanized hardware. Galvanized hardware usually has a dull gray appearance. Non-galvanized hardware usually has a bright metallic appearance. It's more difficult to tell the difference between the different types of galvanized hardware unless you look at the box.

Hot-dipped galvanized hardware has a thicker corrosion-resistant zinc coating compared to other galvanized hardware such as electrogalvanized (EG) and mechanical-plated. The thinner coating of other galvanized hardware does not provide the necessary protection from corrosion caused by the new wood-treatment compounds. Some deck builders are using membranes to isolate hardware such as joist hangers from deck lumber. This is a good idea, although it's not required.

Here are some other hardware warnings. Never use aluminum hardware with the new wood-treatment compounds. Never use stainless steel and galvanized hardware together. For example, never use galvanized nails with a stainless steel hanger. The galvanic reaction will increase the corrosion rate. Never use copper flashing with the new treated wood unless the copper flashing is made with an isolation membrane.

The bottom line
For many years, deck building was lightly regulated and decks often were not carefully inspected. The 2009 IRC has a prescriptive section on decks and the new 2012 IRC expands on the 2009 code coverage of decks. Two other excellent guides to deck construction are DCA 6 Prescriptive Residential Deck Construction Guide 2009 IRC Version, available at awc.org/codes/dcaindex.html, and the Deck Framing Connection Guide by Simpson Strong-Tie, available at www.strongtie.com/ftp/fliers/F-DECKCODE09.pdf.

Decks are frequently built by weekend handymen, often with little more than a "how to" book, if they even have that. Given the risk of poor construction and the risk of personal injury and property damage, decks deserve special attention during our inspections.

Memo to the deck gods: The Word does not reside on Mt. Olympus (just at its base) and welcomes other viewpoints. Send your lightning bolts or emails to inspectorbruce@cox.net. The thoughts contained herein are those of The Word. They are not ASHI standards or policies.