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Support For Exterior Wood Deck

By John F Mann, PE


Basic Support Requirements

Capacity of Lag Screws

Alternate Method Of Support

Design Example; Ledger Board Connection

Lag Screw Design per NDS Code

Posts & Foundations

Basic Support Requirements

Just about every summer along the New Jersey shore, at least one exterior deck collapses suddenly, usually during a party with many guests on the deck. Multiple injuries occur, some of which are severe. Fatalities have occurred.

Corrosion of connectors, wood decay and lack of maintenance can sometimes be the main cause of collapse. 

However, the primary reason for deck collapse is more often lack of adequate support, even without corrosion or decay, due to improper design and construction of the original deck. 

The following link provides listing of many deck collapse incidents throughout the US during the past several years;

Decks are often built by contractors and homeowners who are not qualified to determine support requirements. Local code officials tend to allow such incompetent "design", based on the mistaken idea that they are "helping" the homeowner to avoid the cost of professional design. After a deck collapse, the cause is all-too-often considered to be an "accident", instead of the neglect of responsibility that should be identified as the real problem.

A deck must be designed to support the same design loads as the house, as specified by the governing building code. In general, this means that the deck must have capacity to safely support uniform live load of 40 pounds per square foot (psf) on the entire deck surface. Greater live load should be considered if regular assemblies of many guests are expected.

Any walking surface that extends out from (or over) a wall, without any support along the far edge, is considered a balcony (not a deck). The balcony is considered to be cantilevered from the wall. Minimum design live load for a balcony is 60 psf per code. However, this discussion is not applicable for a balcony. 

For any deck serving a restaurant or club, 100 psf live load should be used.

Many decks are built by installing joists (such as 2x10s) perpendicular to back wall of house. Front ends of these joists (at the house wall) are supported by joist hangers nailed to a wood ledger board connected to the house. This connection must have adequate design capacity to support load from deck floor joists. All-too-often connection is made only with grossly inadequate nails or lag screws. The result of inadequate connection is a deck collapse waiting to happen.

For a house built after 1950 or so (with platform framing), and with the deck surface at same elevation as the adjacent house floor, a ledger board will be opposite a rim board (across ends of house floor joists) or an edge joist (if house floor joists are parallel to the house wall). Wall sheathing (plywood or OSB) will generally be in between the ledger board and the rim board or edge joist. Therefore, the maximum thickness of wood that a connector can be installed into is about 2 inches (1-1/2 inch rim board + 1/2-inch sheathing), unless the connector goes through the rim board into a sill plate or end of a floor joist.

Capacity of Lag Screws

Prior to publication of the IRC 2009 building code, design of connections for a deck ledger board were governed by the governing code for wood construction (currently NDS-05). Due to minimum penetration requirements, capacity of lag screws was often not adequate for a single thickness (1-1/2 inch) rim board or edge joist.

See below for design example illustrating how lack of adequate wood thickness can be a major problem for lag screws when using NDS code provisions for design.

In IRC 2009, a new table is now included which lists prescriptive requirements for ledger board connection using 1/2-inch lag screws and through-bolts into a single-thickness rim board (band joist). Design capacities of lag screws are much greater than those based on NDS provisions. These new capacities, based on testing performed several years ago (with test results reported in American Wood Council publication), are technically only valid for 1/2-inch lag screws. However, it may be reasonable to conclude that NDS provisions for other sizes of lag screws are also overly conservative.

Capacity of Other Connectors

Design capacity of typical wood screws is less than 16-penny (16d) nails, as also discussed below. 

For an older house built with balloon framing, the bottom of wall studs should be available for connectors.

Through-bolts should be considered when there is access behind the rim board, such as in a basement. 

Alternate Method Of Support

For deck joists perpendicular to house wall, a much-preferred alternate method of support is to install a beam (girder) about 2 to 3 feet away from (and parallel to) the house wall. Although this method will cost more than connecting ledger board to house, the support provided is much more secure.

Foundation piers should be installed down to depth of existing footing for back foundation wall, unless the girder is at least several feet away from the foundation wall. This is required since soil (backfill) along the foundation wall is generally very compressible due to over-excavation during construction of the house. 

Deck joists can also be installed parallel to house wall, supported by beams that are perpendicular to house wall. Of course proper support for ends of beams near house wall must be provided, which will require a new foundation element unless each beam is extended into back wall of house (supported on new column in the wall).

Design Example; Ledger Board Connection

For the most typical condition, with deck joists perpendicular to back wall of house, and with only one other support (for joists) along back edge of deck, design load is equal to 45 psf (40 psf live load + 5 psf dead load) times half the span length.

For deck joists spanning 14 feet, design load is then 315 pounds per linear foot (PLF) along back wall of house (45 psf x 14 feet / 2). For the usual joist spacing of 16 inches, design load is 420 pounds for each joist. This vertical load is also known as "reaction force" and "shear force".

Design capacity of various connectors (nails, wood screws, lag screws, bolts) to resist shear force is calculated per standard provisions of the National Design Specification for Wood Construction (NDS-2005). Capacity depends on size of connector and various other factors such as penetration into supporting ("main") member and wood species of both the supported ("side") member and the supporting member.

Plywood or OSB wall sheathing is often considered part of the supporting member.

The 3-1/2 inch length of a 16d nail is just equal to total thickness of ledger board, wall sheathing and rim board. For these conditions, shear capacity of one 16d nail is approximately 120 to 140 pounds, depending on wood species. To provide required design shear capacity (420 lbs) would therefore require three (3) 16d nails every 12 inches or four (4) 16d nails every 16 inches.

However, the building code does not allow use of nails for deck support if the nails may be subject to pullout (withdrawal) force. Such condition might be applicable for a deck without any other method of resisting outward lateral force. Such force may have to be considered for an upper level-deck without sloped bracing.

Shear capacity of typical (8 gage, 10 gage) wood screws is less than nails. Shear capacity for one 10 gage wood screw (with full penetration) is approximately 100 to 120 pounds, depending on wood species. For full capacity, minimum penetration into main member is 10D, or 1.90 inches. Total thickness of 2 inches for 1-1/2 inch rim board plus 1/2-inch wall sheathing is adequate. For the example above, 4 or 5 wood screws are required every 16 inches. 

If only a rim board is available, 8 gage screws can be used, with about 90 percent of full capacity (since full penetration of 1.64 inches is slightly greater than the 1-1/2 inch rim board). Design capacity of 8 gage wood screws then range from 70 to 80 pounds. For the example above, six (6) wood screws are required every 16 inches.

Lag Screw Design per NDS Code

For full design shear capacity, penetration of the lag screw (into supporting, or "main" member) must be at least 8 times the screw diameter (8D). For a 3/8-inch lag screw, full penetration is 3 inches. For lesser penetration (down to 4D minimum), capacity is proportionately reduced (using standard tables). Penetration is equal to length of screw (under head) minus thickness of the supported ("side") member and minus the "tip" length, which is 7/32 inch for 3/8-inch lag screw.

For typical condition, the 1-1/2 inch thick rim board and 1/2-inch wall sheathing act together as a 2-inch thick supporting or "main" member. When connecting 1-1/2 inch thick ledger board, penetration of a 3-inch long, 3/8-inch lag screw (into main member) is only 1-9/32 inches, less than the minimum required penetration of 1-1/2 inches (4D). Shear capacity for the 3-inch long, 3/8-inch lag screw is therefore zero for design purposes per NDS-2005.

For a 3-1/2 inch long, 3/8-inch lag screw, penetration of 1-25/32 inches is greater than the minimum requirement (4D) for reduced capacity. However, capacity is only about 60% of full design capacity. For typical lumber (such as Hem-Fir or Douglas Fir) reduced shear capacity is only about 70 to 80 pounds, much less than a 16d nail.

For a 3-1/2 inch long, 1/2-inch lag screw (with 5/16 inch tip), penetration of 1-11/16 inches is less than the minimum 2 inches (4D) required. Design shear capacity is zero.

The only way to obtain design shear capacity from 1/2-inch lag screw (connecting 2x ledger to rim board or edge joist, with plywood sheathing) is to use a long enough lag screw to ensure that the entire tip length extends through rim board or edge joist. This is the basic condition for use of the new table in IRC 2009, which provides design shear capacity for 1/2-inch lag screws connecting ledger board to single rim board or edge joist.

Spacing values listed for 1/2-inch lag screws in Table R502.2.2.1 (IRC 2009) are based on allowable values that are about 5 times the values obtained by using NDS code provisions.

For example, a spacing value of 18 inches is listed for deck-joist span length of 10 feet. For total design pressure of 50 psf (40 psf LL + 10 psf DL), design shear force at the ledger is 250 PLF. This means that design shear capacity of the lag screw (per new table) is 375 lbs.

For comparsion, consider that capacity of 1/2-inch lag screw using NDS code provisions is only 68 lbs for Hem-Fir ledger and SPF rim (band) joist, even with adequate lag screw length of 4-inches (8D). Plywood wall sheathing is taken to be part of rim joist. Capacity must be reduced to only 50% of the full capacity due to the limited penetration distance of 2-inches. For spacing of 18 inches listed in the table, the lag screw can resist design shear force of 45 PLF, which is only 18% of required capacity.   

If lag screws can be accurately installed into center of a 2x4 wall stud (with 1/2-inch wall sheathing), full design capacity of 128 lbs can be obtained for 3/8-inch lag screw (5-inch long) since full penetration of 3-inches can be provided in the 4-inches available. However, for 1/2-inch lag screw, full penetration of 4-inches (8D) is only available for 6-inch long lag screw that so that the tip length extends through the stud. Yet, providing minimum edge distance of 1-1/2D (in wall stud) would not be practical for 1-1/2 inch thick wall stud unless lag screw is only 5/16-inch diameter. Even for 5/16-inch lag screw, trying to maintain installation straight through the stud (without splitting stud) would be very difficult at best.

Therefore, if nails are considered unacceptable, wood screws should be used as alternate, unless through-bolts can be installed. Shear capacity of 10 gage wood screws is only slightly less than capacity of 16d nails.

Posts & Foundations

Posts (columns) are often used to support decks. Proper connection between edge beams and posts are essential. Bracing for high posts is also very important. 

Posts should preferably be 6x6s, which allows top of post to be notched to provide bearing support for a beam that is 3 inches thick while also allowing the beam to be bolted to the remaining part of post.

When 4x4 posts are used, beams are sometimes connected only to the side of post. For such condition, installation of a separate 2x4 under the beam is preferred to provide bearing support, with the 2x4 securely connected to the post. Support of a beam only by connection to post is not recommended. Over time, such connections can loosen, especially if impact on the deck is a frequent occurrence.

Foundations for posts are often undersized. Round concrete piers are most often used. Such piers should generally be a minimum of 16-inch diameter and may have to be 24-inch diameter, depending on design loads and allowable soil bearing capacity.

For the example above, with edge beams spanning 8 feet on each side of an interior post, design load is 315 PLF times 8 feet, or 2,520 pounds. For allowable bearing pressure of 2,000 psf, bearing area of pier must be 1.25 square feet or more. Minimum diameter of pier is then 1.27 feet or 15 inches. For allowable bearing pressure of 1,500 psf, which is typical for clay soil, minimum pier diameter is 18 inches.

Foundation piers should extend down into the ground sufficiently so that the pier is supported by firm, competent soil that will not compress excessively. In climates subject to winter conditions, base of pier should also be below frost depth. In many northern areas of the US, frost depth is 4 feet or more below grade. In temperate areas, frost depth is typically 3 feet.

Soil against the sides of concrete piers must be very firm to ensure adequate resistance to lateral force and movement. Undisturbed native soil is preferred. Compaction of any soil backfill is necessary, although additional concrete should be considered. 

Foundation piers must also have capacity to resist horizontal force applied from the deck. This is especially important for posts supporting back edge of deck.

For high-level decks supported by wood posts, bracing should be provided to resist horizontal (lateral) force that can cause vibration, sidesway and even collapse. Such lateral force can occur due to the movements of persons on the deck, as well as wind and seismic (earthquake) action.

Use of steel posts can eliminate the need for lateral bracing, as long as the post is securely bolted (with 4 bolts) to a foundation element that has capacity to resist lateral and overturning forces. Such foundation element must have adequate size and adequate embedment into soil. Typically this requires a 24-inch diameter pier that is embedded at least 4 feet into the ground.

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