How to Keep Plywood Shelves from Sagging

Plywood is the go-to material for anybody building anything ever-from Ancient Egypt to your next home decor inspiration, humanity has been relying on the unique sturdiness of plywood for thousands of years. Even so, it is not invincible to strain, and a plywood shelf needs to be properly supported if you want to prevent damage in the long-term.

Sagging, for example, is not only an unattractive phenomenon but also an inevitable one. Over time, a sagging shelf left unrepaired may end up permanently bent, cracking, or even collapsing. So whether you are building a new shelf or fixing an old one, sagging shelves are unsafe shelves—keep reading to learn how to set them (and keep them) straight!

Is Plywood Good for Shelves?

Most plywood sold for home projects is softwood plywood, usually made out of a varied combination of pines, firs, and spruces, and stronger than any of them alone. This makes it great for shelves, however, not all plywood is made equal, and you definitely should not build shelves out of the plywood project board at your local hardware store!

Plywood quality is measured in two ways: by the strength of the adhesive between each layer, and by the grade of each layer. However, for most home shelving projects, you will not need to look too closely into these unless you need plywood with an environmentally friendly adhesive or are unable to see the face of the boards in-person.

Instead, choose your plywood by the appropriate number of layers for your task. This is important, as plywood strength is directly correlated to the number of layers used to create it. Generally, plywood comes in three standard plies:

• 3-ply is the most basic ply category because plywood must always have an odd number of layers. This is also the least structurally resilient plywood, but potentially the most cost-effective since even the thickest 3-plies are sold in sheets. If you have a saw and are building a shelf for lightweight, decorative objects only, this might be the ply for you. 

• 5-ply is the next ply category, stronger than 3-ply and therefore much more common for home building projects. Available as sheets or boards, this plywood preserves some cost effectiveness if you have a saw, but is still pricier than 3-ply. It is also not for large-scale construction, so do not use it for large utility shelves that you intend to climb.

• Multi-py is the last category, and is also the most vague, encompassing plies of seven or higher. Strength will generally increase with the number of plies, as will the price. Additionally, the availability of sheets or boards depends on your desired thickness, and thicker plywood does not necessarily mean more plies, either.

Alright, now maybe your shelf does not use your choice of plywood. This might be the case if you are trying to prevent or repair sagging in a pre-made shelving unit. However, your favorite plywood can also make great shelf support, no matter what kind of wood was used to make the shelf.

Designing with Sag in Mind

Sag is relatively easy to prevent if you know two things: what the shelf is used for, and how it is already being supported by the environment. For example, a floating trinket shelf may not need much support, but a floating bookshelf supported only along a single edge may need considerable support to prevent sagging.

The first is something only you, as the shelf user, can answer. Remember that shelf versatility is determined by its strength, and that a shelf made of weak plywood can still sag or break while supported if pushed past its weight limits. Again, do not use plywood project boards!

The second can be determined from the environment. A shelf will begin to sag when it can not distribute weight evenly, so consider the shelf as separate from any other furniture it may comprise and look at how it is related to the greater whole. Often, the more sides a shelf has touching something else, the more supported it will be.

For example, a floating shelf is supported along one edge by the wall it is mounted to, and is therefore limited in how much weight it can distribute before it sags. Conversely, a shelf mounted in a bookcase is supported along at least three sides, which is why we tend to put books on these and not floating shelves.

Anti-Sag Support by Shelf Type

Use this handy table to see what kind of shelf you have by how much support it already has! You can purchase supports in plastic and metal, or make them yourself out of plywood. Then secure your shelves and supports using nails, screws, wood glue, or fitted joints according to your skill level.

(Shelf is assumed to be of a regular shape, possess at least the noted number of edges, and have adjacent edges meet to form right angles).

Number of Supported Edges	Description
1	Floating/Freestanding Shelf: supported along one long edge
2	Floating/Freestanding Corner Shelf (long): supported along one long edge and one short edge, adjacent
2	Floating/Freestanding Corner Shelf (equal): supported along two adjacent edges of equal length
2	Ladder Shelf: supported along two nonadjacent, short edges
3	Bookshelf: supported along two short edges parallel to the other, and one long edge perpendicular to the short edges
3	Cubicle Shelf: supported along three edges of equal length
4	Bookshelf (External Lip edging): supported along two short, parallel edges, and two long parallel edges, where an external lip has been added to one long edge
4	Bookshelf (Flush edging): supported along both short and long edges, where a lip has been included along one long edge and cut to fit the frame of the bookshelf
4	Bookshelf (Recessed edging): supported along both short and long edges, where a lip has been included along one long edge and cut to fit inside the frame of the bookshelf

Single-Edge Support

If your plywood shelf has only one supported edge, then center an L-shaped support on the underside of the shelf, securing to both the shelf and the adjacent wall such that it sits flush with both. If the shelf is long or continues to sag, secure additional L-shaped supports at each end.

Two-Edge Support

If your floating shelf has two supported edges, you can center an L-shaped support to the underside of the shelf’s short edge, securing to both the shelf and the adjacent wall such that it sits flush with both. Secure an additional L-shaped support at the opposite end of the shelf, or to the adjacent edge if the shelf edges are equal lengths.

A ladder shelf also has two supported edges, so you can center one or more small L-shaped supports under each short end, securing each to both the shelf and the adjacent frame; Or, you can secure an I-shaped support the same width as the shelf under the shelf midpoint, and then secure it to the shelf below.

Three-Edge Support

Bookshelves and cubicle shelves can be strengthened by any of the previously described methods, where the frame of the shelf takes place of the wall. Be sure to account for the length of your shelf and add supports accordingly to prevent sagging, and remember that cubicle shelves tend to be smaller, leaving less room for additional support.

Fourth Edge Support

Lip edging adds support along the external fourth edge, rather than anchoring the shelf and supporting it from underneath. It is often an addition to a shelf that does not yet have a lip, and must be added before the shelf is inserted into the frame. However, you can cut the shelf to include this lip so that you do not have to install it later.

• External Edging: Secure a strip of wood to the fourth edge of the shelf such that it extends onto each side of the frame of the bookshelf and is level with the surface of the shelf. Then secure the lip to the bookshelf frame. The lip must be as long as the bookshelf (shelf and frame) and twice as wide as the shelf is thick.

• Flush Edging: Secure a strip of wood to the fourth edge of the shelf such that it overlaps with the inside of the bookshelf frame and is level with the surface of the shelf. Then secure the lip to the bookshelf frame. The lip must be only as long as the shelf, and twice as wide as the shelf is thick.

• Recessed Edging: Secure a strip of wood to the fourth edge of the shelf such that it overlaps with the inside of the bookshelf frame and is level with the surface of the shelf. Then secure the lip to the interior of the bookcase frame. The lip must be only as long as the shelf, and twice as wide as the shelf is thick, and the depth of the shelf must leave room for the lip to be attached.

Replacing Shelves

In the worst-case scenario, no amount of support or reinforcement is going to save your sagging shelf. If your shelf is left, permanently sagging even after you have removed all items from it, or has begun to split or crack, it would be safer to replace the shelf. 

However, you can still use these methods to prevent sagging for your next shelf!