Structural Support Members

There are many ways to build a house, but they all use some kind of structural support members to carry the weight of the home and its contents. A few of those structural members may appear quite similar, but are often referred to by different names. For example: a joist, girder, and beam may at first glance appear to refer to the same things but serve different purposes.

A beam is the horizontal support member which gathers and distributes weight from the building, occupants, and belongings. Beams are used at all levels of the home, from the basement to the attic. Beams are the most generic term for horizontal supports, and specific applications of beams have their own names and installation methods.

A joist is technically a type of beam. It is a secondary support member that spans from wall to wall and distributes weight horizontally to rim joists as part of the wall, if not also distributed to a more durable girder beam that is not part of either exterior wall.

Girders are THE main beam in a house, which support all other beams and joists. For that reason, it is usually the thickest and most sturdy beam. Girders can be wood or steel, though steel is not common for residential homes in our area. It is able to take great weight loads without sagging, bouncing, or deflecting, and transfer the weight vertically down through posts.

Posts are any vertical support member that transfers weight loads downward, typically into the ground. In this sense, posts are the vertical cousin of horizontal beams – they both encompass other types in their same category.

Columns are a type of post – freestanding vertical support members – which are round. They are freestanding in the sense that they are not integrated as part of a wall, but they may be attached to other members. Columns are posts which, due to their highly visible location, are designed to be more ornate and decorative.

Pillars are a type of post – freestanding vertical support members. While there is no hard-set definition separating the three, it is generally acceptable to say that pillars are taller than columns or may have a lower ratio of width to height. But that differentiation isn’t easy to translate from one era or architectural type to another, so don’t put too much stock into the differences here.

Piers are a type of post – freestanding vertical support members. The most notable difference is that piers are typically foundational. They distribute weight and keep a structure in place against the forces of nature, just as a house foundation keeps the house in place. While a pillar, post, or column COULD be foundational, they do not by definition have to be. They may be set atop a pier or other kind of foundation instead, while the pier acts as both the vertical support and its own foundation.

The mighty pilaster! Or so it would like you to think. It is an engaged column – meaning that where a column is freestanding (not connected to a wall), the pilaster is integrated into the wall. While a column is round, pilasters are mostly square or rectangular cuts. While the post, pier, pillar, and column are vertical support structures – pilasters are only designed to LOOK like support structures. They are just an architectural decoration. Don’t get me wrong - you can’t simply remove them, they ARE bearing weight, just no more than the rest of the wall. They look special and get a lot of attention, while their cousins are doing just as much work next to them in obscurity. Poor things.

Too Long, Didn't Read Version: Beams, joists, and girders are horizontal support members. Posts, columns, piers, piles, pilasters, and pillars are vertical support members. They all serve similar, but different purposes – even if the purpose is appearance based.

-Marshall Tramp, CPI, RMP


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Marshall Tramp, CPI, RMP
Author: Marshall Tramp, CPI, RMP
Top Notch Inspection Services llc
Serving Eastern Washington & Northern Idaho