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This article explains critical safe-construction details for decks and porches, including avoiding deck or porch collapse and unsafe deck stairs and railings. This article includes excerpts or adaptations from Best Practices Guide to Residential Construction, by Steven Bliss, courtesy of Wiley & Sons.
Also see our review of that book. Also see DECK COLLAPSE Case Study (collapse of a new code-approved deck) and DECK FLASHING LEAKS, ROT Case Study for an example of an older deck with rot and collapse due to improper construction and missing building flashing.
Photos & Construction Details of Unsafe Compared with Safe Deck & Porch Connections to the Building
Deck Ledger Board Connection to Building Band Joist
Most residential decks are supported on one side by a ledger that is bolted or lagged to the home’s band joist. This connection is critical, since a failure here can cause a deck to collapse. Deck Collapse Case Study describes several improper connections between a residential deck and the building that led to the catastrophic deck collapse shown here.
Most residential decks are supported on one side by a ledger
that is bolted or lagged to the home’s band joist.
This connection
is critical, since a failure here can cause a deck to
collapse. Failure of the ledger can be caused by too few or
undersized fasteners, or by decay in the ledger or band joist.
Lags or bolts provide little support when fastened to rotted
wood. So proper flashing of the ledger and band joist area is
critical.
It is also important that the band joist be nailed adequately to the surrounding structure, since the ledger is only as strong as the structural members it is attached to.
Though deck failures are rare, ledger connections are typically at fault when they occur. Through-bolts make the strongest connection, but adequately sized lag screws can also work. With either, it is critical to flash the ledger area and to only use metal components that are compatible with pressure-treated wood.
Our photo (left) shows what can happen if the deck ledger board is simply nailed to the structure. Deck Collapse Case Study includes additional photos of improper connections between a residential deck and the building that led to a catastrophic deck collapse.
In new construction, if a deck is planned, make sure the band joist is pressure treated and adequately nailed to the sole plate above and the sill or top plate below, using stainless-steel or double-hot-dipped galvanized nails.
In new construction,
if a deck is planned, make sure the band joist is pressure
treated and adequately nailed to the sole plate above
and the sill or top plate below, using stainless-steel or
double-hot-dipped galvanized nails.
Fastening with 16d
common nails at 8 inches on-center is recommended. If the
nailing cannot be confirmed in a retrofit, extra toenails
driven through the exterior can help to reinforce this connection
(see Figure 4-6 at above-left).
The table immediately below gives the bolt spacing requirements for connecting a deck ledger to the building when the ledger board is bolted directly to the structure.
Table of Bolt Spacing for Deck Ledger Spaced Away from the Building
Spacing for bolts and lags used to connect the deck ledger to the building are shown in the table at above-right (ledger bolted directly to the building), and at in the figure at left for deck ledgers that are spaced away from the building.
Through-bolts are the most reliable connection, but lag bolts are adequate (photo, above left) as long as they are long enough to fully penetrate the band joist.
For through-bolts, drill holes 1/16 inch larger than the bolt. For lags, drill a full-diameter hole for the unthreaded portion and a smaller hole (65 to 75% of the lag’s diameter) for the threaded portion.
Use washers under the head of the lag bolt or at both ends of through-bolts to keep the head from crushing the wood. Soak the holes with a preservative before inserting the bolts.
Soak the holes with a preservative
before inserting the bolts. Spacing for bolts and
lags are shown in Table 4-5 below.
In humid climates, some deck builders prefer to leave
a 1/2-inch air space between the ledger and house to assist
with drying in the event that the ledger area gets wet. Because
this weakens the connection, through-bolts should
be used, as shown in Table 4-6 below. [Click any table or image to see an enlarged, detailed version.]
To prevent splitting, lag
screws or bolts should be held back two inches from edges
and staggered as shown in Figure 4-7 just below.
Alternative to Deck Ledgers: Self-Supporting Decks & Porches
Given all the problems inherent
in supporting a deck with a ledger, one approach is to
forgo the ledger altogether and support the deck on posts
on all sides, keeping it structurally independent of the
house.
In the no-deck-ledger approach, the self-supporting deck still may be braced to the
house to help it resist racking, but all vertical loads are carried
to the ground by posts (Figure 4-10 at left).
A conservative rule-of-thumb for cantilevered structures states that joists can
cantilever one-fourth of their total length, assuming that
the cantilevered end is not carrying any loads other than
the normal uniform floor loading.
Details about selecting the proper fasteners for constructing decks and porches and how they are installed can be read atDeck Nails, Screws, Hidden Fasteners. Also see critical deck flashing details for the ledger board, found at Deck Flashing at Building.
Steven Bliss served as editorial director and co-publisher of The Journal of Light Construction for 16 years and previously as building technology editor for Progressive Builder and Solar Age magazines. He worked in the building trades as a carpenter and design/build contractor for more than ten years and holds a masters degree from the Harvard Graduate School of Education.
Excerpts from his recent book, Best Practices Guide to Residential Construction, Wiley (November 18, 2005) ISBN-10: 0471648361, ISBN-13: 978-0471648369, appear throughout this website, with permission and courtesy of Wiley & Sons. Best Practices Guide is available from the publisher, J. Wiley & Sons, and also at Amazon.com.
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