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A common instinct when planning a library wall is to go deep: a shelf of 300mm or more, so that whatever you buy next will fit. The belief behind it is that spare depth is spare storage. On a shelf of books it is spare board, and it takes room from the floor to provide it. The better approach is to read the depth from the books themselves, shelf by shelf, and to let a deeper cupboard at the bottom carry the few things that really are large.
Why Spare Depth Stops Being Storage
A book stands on its bottom edge with the spine facing out, so the depth it needs is its width from spine to fore-edge. Nothing holds it at the front of the shelf. Every time you slide a neighbour out or push a row back into place, the books settle towards the back panel, because that is the only thing that stops them. On a shelf 100mm deeper than the books, the row ends up with 100mm of bare board in front of it.
That bare strip causes three small problems. The spines sit recessed under the shelf overhead, in its shadow, so the row reads darker than it should. Your fingertips have to reach in to hook the top of a spine, which is harder with a recessed row and tips the neighbouring books into the gap when one is pulled. And the strip becomes a collecting place for small objects and dust, which then hide the books further.
Tip: Stand a few of your widest books on a shelf you already own, push them to the back and run a hand along the front edge. The gap you feel is depth that would take floor space for nothing.
A shelf cut to the books avoids all three. The spines sit within a finger’s width of the front edge, the row is lit evenly, and each book can be pulled out with one hand.
Reading Depth From the Book, Not the Bookcase
Paperbacks and hardbacks are cut to a short list of trim sizes, and the width of each one determines the minimum shelf depth for the shelf that holds it. The table below sets the common sizes against a sensible shelf depth, which adds a small allowance at the front so the books can be taken out without catching on the edge.
| Book type | Typical size (width × height) | Shelf depth that suits |
|---|---|---|
| Mass-market paperback (A-format) | 111 × 178mm | 130 to 150mm |
| Standard paperback (B-format) | 129 × 198mm | 150 to 170mm |
| Larger non-fiction paperback (A5) | 148 × 210mm | 170 to 190mm |
| Hardback (Royal) | 156 × 234mm | 170 to 190mm |
| Illustrated and art books (A4) | 210 × 297mm | 240 to 260mm |
A boxed set or a book in a slipcase is wider than its trim size. The figure to take from the table is the width, because that is the dimension that runs from the front of the shelf to the back panel. The height decides how far apart the shelves sit, and it is a separate decision.
The depths in the right-hand column leave a little room in front, so spines sit close to the edge without a book overhanging it.
Hardbacks, Paperbacks and Art Books on One Wall
A mixed collection has a width range of about 100mm or more between the smallest paperback and the biggest art book. A single depth chosen for the biggest book leaves the paperbacks standing in the back half of the shelf, which repeats the bare-strip problem on every row. A single depth chosen for the paperbacks shuts out the art books altogether.
Sort the collection into stacks before any measurements are drawn up. Put the paperbacks in one pile, the hardbacks in another, and the large illustrated books in a third, then measure the widest book in each pile. Those three numbers give you the depths for three groups of shelves.
The arrangement that follows puts the shallowest shelves at the height where you reach and read spines every day, which is the band between hip and shoulder. Hardbacks sit comfortably at a similar height or a little lower. The art books, which are heavy and wide, belong low down, where the weight is carried near the floor and lifting one out does not mean holding it at arm’s length.
Some books are wider than any shelf you would reasonably build: the big coffee-table volumes that can measure 300mm or more across. Treat these as a separate category. They store well lying flat with the spine facing out, and a flat stack needs a surface rather than a deep standing shelf.
Mixed-Depth Runs in One Bay
Putting shelves of different depths in the same bay is not simply a matter of cutting some boards shorter. If the carcass behind them is one depth, a shallower board leaves a gap against the back panel, and the books on it slide back into that gap.
Mixed depth works when built into the unit’s structure. There are two ways to do it.
Stepped sections: The carcass is made in vertical sections of different depths, so that a deeper central section sits between two shallower ones, or a shallower upper part sits on a deeper lower part. Each section has its own back panel, so every shelf in it can be cut to the right depth with no gap behind. A unit whose centre section stands proud of its two sides is called a breakfront.
Aligned fronts, deeper centre: Where the wall behind steps back, as at a recess, the centre section runs deeper into it while every shelf front stays on one line, so the face of the unit reads as flat. Where the wall is uneven, the back of the unit is scribed to it on site so that the row of books sits squarely and the shelf edges stay level.
Either way, the point at which the depth changes is a decision made on the drawing before the workshop cuts a single board. It sits at a fixed height or at a fixed distance along the wall, and every shelf on either side of it is sized to its own section.
A Deeper Cupboard Under Shallower Shelves
The simplest stepped arrangement is the one best suited to a collection that includes a few large books: a deeper base cupboard with shallower shelves on top of it. The cupboard depth is chosen for what it holds, which can be boxes of papers, board games, lever-arch files or the oversized books, and the upper shelves are chosen for what they hold, which is the books you read.
Because the shelves are shallower than the base, the top of the cupboard extends in front of them as a ledge. Take a base cupboard 400mm deep under shelves of 190mm. The ledge in front of the shelves is then 210mm wide, which is enough for a lamp, a stack of books in current use or a few of the big art books lying on their sides. That ledge is a working surface at a height you can reach, formed by the difference between the two depths, so it is free: no extra material is needed to create it.
The base is the right place for the wide books for two reasons. Their weight sits low, and the top of the cupboard can take them flat, with their spines facing out, directly under the shelves they sit in front of. Anything that has to stand upright and deep, such as a record collection or a run of folders, can live behind the cupboard doors instead.
Doors on a deeper cupboard swing into the room, so the clear space in front of the unit has to take both the open door and a person standing at it. Check the base depth against the doorway and the furniture facing it, as well as the books going into it.
What Depth Takes From the Floor
Every millimetre of shelf depth comes out of the room. In a living room 2.4m wide, a 300mm shelf run on one wall leaves 2.1m between the shelf fronts and the opposite wall. A 170mm run on the same wall leaves 2.23m. In a narrow room, those 130mm can determine whether a reading chair or a footstool fits in front of the shelves, leaving space to sit.
A run of shallow shelves can go in a hallway or a landing where a deep bookcase would narrow the walkway. A deeper bay is better placed along the wall of a larger room, with an armchair or desk in front of it.
Tip: Count how many of your books are wider than 190mm before choosing any depth. If they would fill less than one shelf, give them a flat stack on the cupboard top and build every shelf above to the shallower depth.
Where the unit sits in an alcove, the alcove walls fix the width, and the depth of the recess sets the most a shelf can take. Whether a chimney breast projects or the wall is flat, the same method works: set the shelf depth from the books, and use the rest of the available space as a cupboard or ledge rather than as empty board.
Setting the Depth for Your Collection
Begin with the books, not the wall. Sort them into piles, measure the widest in each, and let those three numbers decide how many shelf depths the unit needs. A single depth suits a collection of one kind, such as a run of novels. Two depths suit a household that mixes novels with hardbacks. Three, with a deeper cupboard at the bottom, suits a household that owns art books and keeps adding to its collection.
A bookcase fitted to the books keeps its floor space for the people who read there. The depth is a small number on a drawing, and it sets how the finished wall looks and how much of the room is left.
Frequently Asked Questions
Two B-format rows need at least 258mm before any allowance at the front, because each book is 129mm wide. The back row is then hidden behind the front one, so you have to remove a book to find anything behind it. A shelf that deep works for a rarely visited reserve collection, not for the books you reach for daily.
A shelf for records and lever-arch files can run to about 350mm deep, with about 340mm between shelves for the height of a file. That depth belongs in the cupboard below the shelves, behind the doors, where the files stand on their ends and the cupboard top beside them remains a ledge.
A common upright picture-book size is 203mm wide by 254mm tall, so a shelf of around 220 to 230mm holds one standing. Picture books printed wider than they are tall, or square, need more depth, and they sit better face-out on a low ledge or in a box than spine-out on a shelf. The height of 254mm means the gap between shelves has to be taller than for a paperback, so these books are better on their own low shelf, where a child can reach, and the taller gap does not force the whole bay to be spaced the same way.
The step can sit at roughly 760 to 810mm from the floor, which puts the ledge at about waist height. At that height you can set a book down on it without bending, and the shallower shelves begin within easy reach. A lower step gives a taller shelf section; a higher one gives more cupboard storage.
A skirting board stands proud of the wall, so a unit with a flat back panel loses some depth on its lowest shelf, because the panel stops against the skirting while the wall behind it is further back. The carpenter either cuts the skirting back behind the unit, notches the back of the unit around it, or sets the lowest shelf a little shallower to match, which is decided at the design visit.
Stand each book upright with its spine facing you, close it, and measure from the spine to the far edge, the fore-edge, including any slipcase or the thickest dust jacket flap. Measure the widest three or four books in each pile rather than every volume, because the widest one sets the depth, and a book that is much wider than the rest can be laid flat instead.
Pick out your widest art book and your most-read paperback for a free design visit — each shelf can then be cut to the books it will hold, with a deeper cupboard planned beneath where large volumes need it. Humphries Cabinets serves London. Call 020 4538 7044.
