Structural Screws (Lags & Bolts)

Lag Bolt vs Lag Screw

A bolt is properly assembled and tightened by rotating the nut. A screw, on the other hand, is a fastener that is properly installed by rotating the head of the fastener with a wrench or a driver bit in a recess of the head. Since lags don not require a nut and installed by turning the head of the fastener, the proper technical term would be “lag screw”

Lag Screws are most commonly used to attach the Ledger Board of a Deck to the Sill Plate of the house where there is usually not an easy way to put a bolt and nut to hold the two together.

Lag Screw

Lag screws are designed for the really big jobs – building docks, decks and wharfs. When installed properly, they have outstanding holding power and do not need to be retightened frequently, as is sometimes the case with bolted connections.

Traditional Lag Screws have a Hex Head were more of the modern Lag Screws have a either a Washer Hex Head or a large Washer Head.

Unlike other lag screws, some SPAX lag screws don’t require pilot holes You know the SPAX screws you’ve come to love, with their patented serrated threads and 4-cut point that cuts through wood like a knife through butter?

Lag Bolt

Lag Bolts and used with washers and nuts to connect heavy lumber to create a joint that are bearing an intense load. The head will have a Hex Head that will use a wrench to hole it from turning while a wrench will be used on the nut side to tighten the joint. Washers will be applied between the head and the timber as well and the timber and the nut to spread out the compression forces and prevent the head and the nut from sinking into the wood. Unlike Lag Screws, a hole needs to be drilled all the way trough the joint for the bolt to go though.

Carriage Bolt

A Carriage Bolt is like a Lag bolt in almost every aspect except that under the head is a square protrusion that will dig into the wood to hole the head from turning while turning the nut on the other end. It does not require a washer under the head as that would defeat the purpose of the square under the head to hold it from turning. The reason they are called “Carriage” Bolts is because they were used in the construction of carriages and it was easy to punch a square hole to hold the square portion under the head so carriage makers would only need one wrench to tighten it up.