Fasteners: Structural Wood Screws
Last updated May 1, 2022
By Ian Story
Sometimes you need a little more strength than nails can offer. Today’s article is about Structural Wood Screws. These long, partially-threaded screws are designed to replace small-diameter lag screws, offering a lot of strength but simplifying the installation. Structural Wood Screws usually do not require pre-drilling, saving a lot of labor for installation.
Our go-to product for Structural Wood Screws is the TimberLok screw by FastenMaster.
Shear Loading – Capacity
Basic Capacity (Z): 260 lbs
Adjustment Factors
| Duration Factor | CD | 1.6 | (for wind/seismic/impact loads) |
| 1.15 | (for snow loads) | ||
| Wet Use Factor | CM | 0.70 | |
| Reduction for SPF wood | CSG | 0.80 | |
| Parallel to Grain Loading | CII | 1.07 | (if load will only be parallel to grain) |
Shear Loading – Clearances
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We have chosen to specify TimberLok screws (by FastenMaster) as our go-to structural wood screw. These fasteners have several benefits: they are corrosion-resistant and can be installed in pressure treated lumber; they have a narrower shank than competitors which reduces the risk of splitting the wood; they have some of the highest capacities in the category (setting aside special-use cases like SDS screws); they have tighter allowed spacings; and they are available everywhere.
This article concerns a type of wood screw intended to replace lag screws, but with simpler installation (usually these do not require pre-drilling). These fasteners have higher capacities than nails, and are an intermediate step between nailed connections and bolted connections. Additionally, these structural screws come in longer lengths (6″ / 8″ / 10″) that allow connecting multiple plies or bundled studs – another advantage over nails.
To allow contractors flexibility to work with in stock product, we specify Structural Screws by a general specification, rather than calling out a specific product in the structural details. A typical callout might read ‘(2) rows 6″ Structural Screws @ 6″ o/c, stagger rows min. 5/8″ ‘
The general specification allows the following products
- FastenMaster TimberLok / HeadLok / LedgerLok (
- FastenMaster HeadLok
- FastenMaster LedgerLok
- GRK Washer Head Wood Screw
- SPAX PowerLag
- Simpson SDS
- Grip-Rite Structural Screw
- Simpson SDWH
| Brand | Model Name | Code Report |
| FastenMaster | TimberLok | ESR 1078 |
| FastenMaster | HeadLok | ESR 1078 |
| FastenMaster | LedgerLok | ESR 1078 |
| GRK | RSS (Rugged Structural Screw) | ESR 2422 |
| SPAX | PowerLag | ESR 1782 |
| Simpson | SDS (Heavy-Duty Connector) | ESR 2236 |
| Simpson | SDWS (Timber Screw) | ER 192 |
| Grip-Rite | Structural Screw | ESR 3933 |
Note: the SDWS screws appear to be in a separate category: they have higher capacities and greater edge distance and spacing requirements (also, they aren’t available at Home Depot…)
Note: I also considered Grip-Rite, GRK, and Simpson SDWS screws for this article. After consideration, I have decided to exclude these from the specification, for the following reasons:
- Grip-Rite are lower performing – they only have about 62% of the capacity of the rest of the screws, so including them would significantly bring down the strength of the specification.
- GRK are also under-performing, by about 10%