Ledger Connection for a Deck with Heavy Snow Load
The ledger connection will accomplish two purposes. Mainly, it will support the ends of the deck joists near the main structure by transferring the heavy downward load into the rim joist and on down to the foundation. Secondarily, it will resist any horizontal shear load parallel to the ledger, say, from a wind from the side, or some back and forth ground motion (earthquake) parallel to the ledger. This design assumes the heavy snow load case `governs’. The horizontal shearing load will be checked later.
The downward load from the snow (and deck material weights themselves) is determined by,
w = Ã x S,
where
w is the downward load on the ledger, in pounds per linear foot (plf),
à is the weight of the snow and the deck materials, in pounds per square foot (psf), and
S is the width of deck contributing load to the ledger.
From the earlier article, the snow load is 250 psf (about 20 feet of snow … the cabin is in a real snowy area), and the deck `dead’ weight load is 10 psf.
The width of deck contributing to the ledger is taken to be half the joist spans, or ½ of 11, or 5.5 ft.
Thus,
w = (250 psf + 10 psf) x 5.5 ft = 1430 plf.
(Yeah, every foot of ledger needs to carry about 1500 pounds.)
To transfer the force from the joists to the ledger we will use joist hangers. The hangers are not part of this conversation. We are going to deal with the connecting of the ledger to the rim joist of the floor system. Options to do so include: 1) through bolts; 2) lag screws; 3) nails (arghhh); and 4) manufactured screws. Let’s go with manufactured screws; by this I mean screws manufactured and `rated’ for transferring relatively high loads in this particular application. In particular, we will investigate using the Simpson Strong-Tie `Strong-Drive’ SDS, SDWS, and SDWH Structural Wood Screws.
In this example the exterior wood sheathing will be bonded to the rim board with a structural adhesive to make it act as one solid piece adjacent to the ledger. Thus, the receiving member thickness for the screws will be 1-1/2 in. rim + ½ in. sheathing = 2.0 in. thickness. The ledger piece will be `2 x’ dimension lumber also of thickness 1-1/2 in. The SDS ¼ x 3-1/2 should work perfect!
All the lumber used will be Douglas fir, or similar, the allowable shear load for the SDS ¼ x 3-1/2 is 340 lb. This value is for `Normal’ load duration. For resisting snow loads the value is multiplied by 1.15 (General Instructions, Simpson Strong-Tie Wood Construction Connectors Catalog). Thus, the allowable load for the fasteners for the deck under snow load is …. 340 lb x 1.15 = 391 lb. To determine the number of these screws needed per foot of ledger, we divide the load per foot by the load per bolt to get …
… 1430 lb per foot / 391 lb per bolt = 3.66 bolts per foot.
Alternately, this could be expressed as … 1 / 3.66 ft per bolt, or 3.3 in. per bolt.
In our earlier work we found that 2 x 12 deck joists could be used at 16 in. o.c., or 2 x 10 @ 12.
For joist spacing at 16 in., the number of screws per joist spacing is,
3.66 screws per foot x 16/12 ft per joist space = 4.88 screws per joist space (16 in.).
Now let’s look to the screw manufacturer for spacing and edge distance requirements. For ledger applications, Simpson Strong-Tie provides the following for the SDS Screws:
End Distance: “4 to 5 in. from end of ledger”
Top Edge Distance: 1-1/2 in. min.
Bottom Edge Distance: 1-1/2 in. min.
Spacing of Rows: 3 in. min.
Spacing of Screws in a Row: NO MIN. PROVIDED.
In the absence of a bare-bones minimum spacing provided we will assume 3 in., same as for the screws. (For specific loading applications Simpson Strong-Tie shows spacing values as low as 2 in. o.c.) Let’s use the language `evenly spaced’ between the joists and see if everything fits. (See accompanying sketches.)
The ledger will be attached to the rim joist, just a bit low, to accommodate the difference in floor and decking thickness, and to provide a bit of a lip. To avoid a strange detail of the ledger hanging below the rim, let’s use the 2 x 10 framing @ 12 in. o.c. for the deck joists, so the ledger closely matches the rim for the floor framing, giving us about 9 in. to work with, vertically.
Using 2 screws high and 2 screws low we see we can easily get the needed number of screws in the spaces between the joists.
Solution: attach ledger to rim joist through structural sheathing using 4 – SDS ¼ x 3-1/2 Simpson Strong-Drive screws. Install screws in two rows: one row of two screws 3 in. apart located 2 in. from top of ledger; other row of two screws each, 3 in. apart, 2 in. min. from bottom of both rim and ledger. Space screws equally distant from adjacent joists. Bond wall sheathing to rim with approved construction adhesive.
NOTES
1. It is likely that all of the screws will be installed prior to the deck joists. As such, the joist locations should be marked on the ledger to avoid later conflict with joist attachment.
2. Simpson Strong-Tie manufactures the SDW screws specifically for ledger applications. The SDWS has a low-head profile which may provide some relief with joist and screw conflicts, however, proximity of joist fasteners to the ledger, and fasteners for the ledger to rim joist, may cause wood damage and compromised fastener capacities.
3. In this application a single `2 by’ (1-1/2 in. thick) rim joist has been assumed. In a later article we will check to see if the rim joist is thick enough to carry the heavy load without crushing the narrow bottom edge of the rim, or the face of the mud sill.
References
Structural Framing for a Deck for a Mountain Cabin, Jeff Filler, Associated Content.
Strong-Drive SDWS and SDWH Structural Wood Screws, Simpson Strong-Tie Company, Pleasanton, CA.
Wood Construction Connectors (Catalog), C-2011, Simpson Strong-Tie Company, Pleasanton, CA.
