A home inspector comes to the house of a past client for whom Before The Architect designed, among other structures, a large deck all along the back of the house. The inspection is for clearing a sales contract. The inspector was supposed to enter the house for interior inspection once he'd done with the exterior. Thirty minutes passed and our client couldn't find the guy anywhere, that is, until he looked under the deck. There was the inspector taking picture after picture of footings, connectors, knee braces, posts, bolts, flashing, washers, post fortifiers, beams . . . the works. The inspector notices our client, apologizes for not coming in sooner. Says that he couldn't help himself. Says he only reads about a deck structure like this - the way it should be done and never is. The pics are going back to the office to show his co-workers how it's done right and then straight into his classroom teachings on home inspection and residential structure. Home run.
INTRODUCTION
How to stiffen a wood deck design to counter the long-term stresses of wibble-wobble and the short-term stresses of wondering whether or not to go back inside where the floors are steadier? Consider knee braces.
LET’S START WITH THE LUMBER
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Lumber
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shall be identified by the grade mark of a lumber grading or inspection bureau or agency approved by the American Lumber Standards Committee (a/k/a ALSC)
- shall not be less than #2 grade
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shall be naturally continuous for a given member
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shall be fully dimensioned members on application
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shall not be less than 2 linear inches in nominal thickness
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shall be preservative-treated wood products which shall contain not more than 19% moisture content
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shall be delivered to a site on other than a rainy day and
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shall be stored on a site sheltered from natural elements
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if graded, shall not be ripped to structural application
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shall not be
KNEE BRACE FOR DECK STRUCTURE
Comment: Let’s clear the air on this last dimensional statement.
Example: the post is 4"x6" and the beam is 3-2"x12"; therefore, the faces to which the knee brace is to be fastened are 4" at the post and 4 ˝" at the beam. Thus, the knee brace shall be 4"x4" to match the post. 4" x something greater than 4" is ok with this deck designer.
Example: the post is 4"x6" and the beam is 3-2"x12"; therefore, the faces to which the knee brace is to be fastened are 6" at the post and 4 ˝" at the beam. Thus, the knee brace shall be 4 ˝" x4 ˝" to match the beam. 4 ˝" x something greater than 4 ˝" is ok with this deck designer.
Deck Design Knee Brace, Elevation
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Key to abbreviations: NLT=Not Less Than; NTS=Not To Scale
Comment: Here’s an illustration of these knee brace deck design standards. Note that a structural knee brace need not be linear.
If there’s any doubt whatsoever as to the firmest fastening, add a deadman. (Same goes for having to notch some joists or beams to maintain a level deck platform – a deadman, every time.)
DECK DESIGN KNEE BRACE DISTRIBUTION
Comment: Here’s an example of this custom home designer’s approach to deck design with knee braces distributed wherever he can site ‘em.
Deck Design Knee Brace, Plan View
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Comment: As a distant, though crucial aside, remember to fasten the outside beams to the ledger with concealed flange hangers for full-strength support.
. . . . . . .
POSTSCRIPT
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On further review, this deck designer thinks he owes you more explanation about his parenthetical comment earlier about notching joists and applying deadmen.
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Following is an illustration of his intended meaning, in order to save us both the thousand words and more he's eager to key in.
Deadman Brace to a Notched Deck Joist (or Beam)
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Key: JST = Joist (and could as easily have been a beam); TOF = Top Of Face. |
