Built-In Bookcase Planning Checklist: Measure Before You Buy Timber

Most built-in bookcases that end in trouble do not fail at the saw. They fail at the wall. A reveal that looks square to the eye can be 25mm wider at the top, an old skirting can drop 12mm across an alcove, and a ceiling can dip just enough to leave a gap you will stare at for years. None of that shows up if you measure once, in one spot, with a tape held at arm's length.

An hour with a notebook, a tape, a long spirit level and a pencil is the cheapest part of the whole job. Work through the checks below in order and you will order the right timber first time.

Start with a sketch, not a tape

Draw the wall as you see it, roughly to scale on squared paper or the back of an envelope. Mark the corners, the skirting, every socket, the radiator, any pipework, the position of the ceiling and the floor line. Include the adjacent walls too, so you can see how the unit will meet a chimney breast or a door frame.

The sketch does two things. It gives you somewhere to write numbers as you go, and it forces you to notice things you would otherwise walk past: the socket at ankle height, the pipe in the corner, the way the left wall kicks out near the ceiling.

Measure the width in three places — and check for square

Take the width at the bottom, the middle and the top. Walls lean and plaster bulges, so the three numbers will rarely match. The smallest of them is your working width; anything you build has to fit through the tightest point.

Now measure both diagonals of the opening, corner to corner. If they differ by more than a few millimetres, the reveal is out of square and one side panel will need scribing or a filler strip. Then hold a long spirit level vertically against each side wall. A wall that bows 10mm over two metres will push the whole carcass out at the top, which is why fixing back to the wall matters as much as standing the unit on the floor.

Account for skirting, cornice and picture rails

Skirting is the most common surprise. Note its height and how far it stands proud of the wall, then check whether the top edge is level across the whole opening. In older houses it seldom is. Sitting a bookcase on top of a skirting that falls 12mm from one side to the other leaves a visible wedge under one end.

You have three options: notch the side panels so they sit over the skirting, cut the skirting away and refit it around the unit, or build the base to follow the slope. The first is usually easiest. Repeat the same checks at the top for cornice, coving or a picture rail, where a scribed filler strip or a piece of matching moulding hides the join.

Deal with radiators, sockets and pipes first

A bookcase in front of a radiator is a bad idea. Radiators need moving air to work, and boxing one in wastes heat and can damage the unit over time. If the radiator has to move, that is a job for a qualified plumber — draining and refilling a system is not a learning project. The same goes for anything electrical: sockets you intend to bury or relocate need a competent electrician, and if you are unsure about the wiring in your home, get it checked properly.

Where a socket can stay accessible, measure its position from the nearest corner and from the floor, then plan a cut-out in the back panel. Cables chased into a wall, pipework running up an alcove corner and meter boxes all need the same treatment.

Take the height in several places

Measure floor to ceiling at both ends and in the middle. Ceilings sag, floors slope, and a difference of 15mm across a 1.2m opening is perfectly normal. If you are building to the ceiling, cut the frame a few millimetres short and finish the gap with a scribed moulding or flexible filler. If you are leaving a gap, decide now how big it should be — a 50mm shadow gap looks deliberate, a wobbly 20mm one looks like a mistake.

If you plan to fix the top of the unit to the ceiling, find the joists first. Plasterboard alone will not hold a loaded bookcase.

Work out shelf spans before choosing timber thickness

Books are heavy, and shelves bow. As a rough rule, 18mm MDF or chipboard shelves loaded with books will start to sag well before they reach a metre, which is why shelving of that thickness is usually best kept under about 750mm clear span. Move up to 25mm board, or use solid timber, and you can go wider — but the figures depend on the material, the load, and whether the shelf is fixed or adjustable.

Your board supplier or manufacturer will have span guidance for the product you buy; use it. Where a span is borderline, add a vertical divider rather than hoping. Fixed shelves supported by a mid-divider are far stiffer than adjustable ones sitting on pins, and a run broken into two bays of 600mm will outlast a single unsupported metre by a wide margin.

Decide what the shelves are for before you fix the spacing, too. Measure your tallest books, box files, ring binders or record sleeves, add 25–30mm of finger room, and build the design around that number. Guessing shelf heights is how you end up with a beautiful unit that will not take an atlas.

The final once-over before you order timber

Stand in front of the wall one more time with your sketch and run through this list:

• Narrowest width recorded, plus top, middle and bottom measurements.
• Both diagonals measured, so you know how far out of square the opening is.
• Side walls checked for plumb and for bowing.
• Skirting height, depth and level noted, with a plan for dealing with it.
• Cornice, coving or picture rail marked at the top.
• Radiator position and clearance decided, with any plumbing booked in.
• Sockets, switches, cables and pipes drawn on the sketch.
• Floor-to-ceiling heights taken at both ends and in the middle.
• Ceiling joists located if you intend to fix upwards.
• Shelf spans calculated, with a mid-divider where the span is marginal.
• Shelf depths and heights based on what you actually own.
• Wall fixings chosen to suit the wall — masonry, studwork or plasterboard over blockwork.

With those numbers on paper, ordering timber becomes a straightforward job rather than a gamble. Measure twice, cut once — and if the wiring, the plumbing or the condition of the wall looks beyond a straightforward DIY job, bring in someone qualified. A bookcase built to fit a wall you took the time to understand will still look right in ten years.

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