Skip to content

What Size Lag Bolts For Deck Posts

A well-built, well-designed deck may completely change the look of a yard. But suitable materials are necessary for a deck to be built properly. You can’t just slap one with whatever you have in your tool shed or garage and expect it to be secure for you, your friends, and your family.

Unfortunately, improperly designed decks may collapse under tremendous loads, putting everyone in danger. Almost always, decks are attached to a building, house, or structure on at least one side. The sort of fastener you pick for this connection is crucial, and the lag bolt is the safest and most widely used variety. In our guide, you can learn what size of galvanized lag bolts will serve you well.

By the end, you’ll know the type and size of lag screw you need or why a through bolt could be an option. Whatever the solution, you will anchor your screwed beans in position and be able to tell the difference from the increased deck stability. (Read Gap Between Above Ground Pool And Deck)

What Size Lag Bolts For Deck Posts

Why Use Lag Screws and Bolts for Building Decks

Never attach a deck to a rim joist with nails because nails lack the solidity necessary to support the deck, and much fewer people standing on top of it. To start, you should know that a piece of wood joins a deck to a building called a ledger board.

The conventional and advised way to attach a building to a ledger is with lag screws. The fact that lag screws are reasonably priced is a plus. For the best performance, purchase stainless steel or hot-dipped galvanized screws.

Lag screws that don’t need predrilled holes can be purchased for a little bit more. Washers are frequently affixed to the heads of these unique lag screws. Additionally, bolts can be used in place of lag screws if you have access to the area behind the house’s rim joist.

Secure the carriage bolt in position by fastening a washer and nut to the other side of the rim joist after using carriage bolts to secure your deck to the house. Which size lag screws are required? Depending on your situation, 12-inch diameter lag screws will do the trick.

Drill a 5/16-inch hole through the rim joist and the ledger first, followed by a 12-inch hole through the ledger only. Lag screws must be at least a half-inch long and reach past the band board.

Knowing what you’re drilling your lag screws into is essential. As previously said, a rim joist is the perfect portion of a building to drill them into, but when building a deck, there isn’t always a rim joist to attach to.

which screw to use

Which Bolts for Pressure-Treated Wood

If you don’t use the correct fasteners, pressure-treated wood uses chemicals to make rust occur more quickly. Galvanized screws with a galvanizing thickness of 75 and 1.5 millimeters should be used when building a deck made of pressure-treated wood. To attain this thickness, look for galvanized materials in the range of A153 to F2329.

It’s a common misperception that you require double- or triple-dipped galvanized fasteners. To be usable in the construction of decks, double- or triple-dipped galvanized fasteners need to be zinc coated.

How to Choose Bolts or Fasteners for Decks

At the very least, the structural support gear for decks should be made of hot-dip galvanized steel. Use stainless steel for the best protection. Use through bolts or lag screws at connections supporting structural stresses, such as deck ledgers or railing posts.

Wherever possible, drilled wood and through bolts should be used because they are stronger. Use 1/2-inch bolts to fasten for the strongest connection on your decking, such as where ledgers attach to the house frame or fasten to a support post.

Put large washers at the ends of through bolts and under the heads of lag screws. After the first year, retighten bolts and lags and check their tightness regularly. (Learn How Much Does A Gallon Of Paint Weigh)

Machine-Threaded Bolts for Deck Construction

Bolts provide the strongest connections when both sides of the joint are accessible. Drill pilot holes between 1/32 and 1/16 of an inch larger than the bolt’s diameter so that it can slide through easily.

Saturate the hole with the preservative after drilling. Substitute big washers for the nut and head. After the first year, tighten again because the wood might have shrunk.

lag bolt for deck

Lag Bolts for Deck Construction

For lag bolted deck connections, drill a bigger pilot hole for the unthreaded portion and a smaller hole (65 to 75 percent of the lag’s diameter) for the threaded portion. Therefore, a 5/16-inch pilot hole would be used for the threaded section of a 1/2-inch lag, and a 1/4-inch pilot hole would be used for a 3/8-inch lag.

Saturate the drilled pilot hole surface in wood preservative after drilling. The lag bolt must be threaded into solid wood for at least half its length. A weak connection will result from inserting a 5-inch lag, for instance, into a 2x joist surface with just 1 1/2 inches of anchoring.

The lag screw should instead be threaded into the thicker 4×4 after passing through the 2x. Put a big washer underneath the head and nuts and tighten again after the first year in case the materials have shrunk.

Joist Hangers for Deck Construction

Use metal hot-dipped galvanized structural fasteners as a bare minimum. The hot-dipped galvanized metal lag bolt should adhere to ASTM A153 for metal fasteners, ASTM A653, and G185 when used with pressure-treated wood for connectors.

The best protection is provided by stainless steel. For extremely wet areas, such as poolside decks, Type 304 or higher stainless steel is advised, or Type 326 for exposure to salt or seawater.

Suppliers of Joist Hangers & Fasteners

Stainless steel or galvanized steel joist hangers, connectors, lag bolt, and other nuts, bolts, and metal products can be found by:

  • Simpson Strong Tie
  • Incom, Schuler Manufacturing
  • Daytona Bolt & Nut, Hutchinson
  • U.S. Lumber, Hohmann & Barnard
  • Direct Tools & Fasteners and more

What Size Screws for Decking

A different kind of screw is required for each section of your deck. When fastening deck boards to joists, the most popular screw size is an 8 gauge, 2.5″ coated deck screw.

Structural wood screws, such as Simpson SDS 1.5″ screws, work well with post/beam brackets, joist and stringer hangers, and deck structure. While 3.5″ coated deck screws are needed for the balusters and rails of railings, 12″ diameter galvanized, or stainless steel through bolt are required for the posts of railings.

The size of the lag screw you can use for decking from the post and beams up to the railings will be discussed below. When you buy screws, you’ll see numbers. Without knowing these numbers, you’ll get the wrong screws.

Also, screws and nails have various numbers. 16d nails are not 1/4″ screws.


The screw’s diameter is the gauge. The size of the screw increases with the gauge. You are most likely looking at eight or 10-gauge deck screws. For example, a diameter of 4 mm is roughly equivalent to 8 gauge.

You’ll probably use 14″-diameter screws if you’re utilizing structural wood screws. Although they are typically marked as 14″, these are also known as 14 gauge. Any screw that is larger than 14″ in diameter is mentioned in its actual diameter rather than gauge. (Learn How Long Does Spray Paint Take To Dry)


The following number on a package of screws is the length, followed by the gauge or diameter. A screw should go halfway into the wood beneath the attached piece. Since over half the screw is hidden in the joist, a 2.5” deck screw is usually long enough to attach deck boards.

For hurricane ties and joist hangers, you may require shorter screws that don’t protrude through the ledger boards or joists. Here, 1.5″ or 2″ 1/4″ SDS structural screws work.

Thread Size

Many deck screws don’t identify the thread size because “deck” means coarser. “Coarse” thread size suggests the threads are farther apart than on a machine screw. Wood, deck, and multi-purpose treated-wood screws have identical thread sizes, which is fine.

types of deck screws

Types of Deck Screw

There are many screws to choose from, so to help, here’s a breakdown of what you’ll find in the fastener aisle.

Regular Deck Screws

A steel deck screw with zinc coating or another weatherproof coating is called a “standard” deck screw. They are green or brown and vary in length and gauge. These deck screws are cheap, effective, and most usually used.

Structural Screws

Load-bearing structural screws are thicker. These screws can replace nails for the joist-to-ledger board and beam-to-joist and attaching brackets for railing post connections. These screws are usually 1/4″ or larger. They’re made of high-quality steel and are corrosion-resistant.

Stainless Decking Screws

Rust-resistant stainless steel decking screws are perfect for external use. They’re rarer than wood decking screws since they cost more. Cedar decking requires stainless head screws. As the coating leaks into the wood, coated screws may discolor. Uncoated stainless screws will not discolor when installing cedar.

Lag bolts

Lag bolts can secure shear force applications of two pieces. Hexagonal bolts of at least 1/4″ in diameter or width are joint. Unlike a conventional bolt, a lag bolt has a pointed end like a screw.

Lag bolts are perfect for posts or ledger boards where the bolt must be buried in the lumber during deck construction.

Through Bolts Vs Lag Bolt

Bolts have a characteristic shape. The shape can help you pick. A lag bolt’s threaded segment ends at a point. This bolt part is wider. They have hexagonal head and circular necks. They’re lag screws. Because they look more like a screw than a bolt, this is simply the case.

Carriage bolts and plow bolts are other names for through bolts. Their threaded section is cylindrical and has flat bottoms. Square neck and domed head, so this shape prevents bolt rotation when attaching surfaces.

All building parts must be properly secured. If you’re installing your deck, we must check the strength of these bolts. Lag bolts are preferred for fastening large wood structures. Simply put, they lack strength. Because they rarely come with an extra washer and nut. They cannot carry more weight.

Since through bolts are fastened, they have an advantage. This strengthens your creation. It can also be tightened, and the wood shrinks over time. Overtightening may damage your DIY deck. (Read Difference Between Enamel And Acrylic)

These bolts are used in wood buildings for many applications. When a project needs greater strength, lag bolts are commonly employed. They usually build decks and rafters. Building garages, barns, and other structures are similar. Thus, before building, read the directions.

Building dock rails and frames require through bolts. This is due to their ability to avoid wood. Most bolts are needed for the swing set framework to support the weight of the frame and the person’s weight. The manual comprises carriage bolts, as seen if you read it. In the building of pergolas, carriage bolts are employed.

What Size Lag Bolts For Deck Posts (2)