For a fitted wardrobe in a UK home, the carcass should be 18mm thick. That applies to the sides (gables), top, base, fixed shelves and drawer boxes. Thinner board, usually 15mm, is sold as an economy option, and it is where sagging shelves, racking frames and drifting doors tend to begin. The back panel is different: it is normally much thinner, and that is fine.

This guide explains what the carcass is, which thickness belongs where, how far a shelf can span before it bows, and what changes for sliding doors, sloping ceilings and heavy storage. It is written for London homeowners comparing quotes, so it ends with the questions worth asking any fitter.

TL;DR

• 18mm is the benchmark for wardrobe carcasses, shelves and drawer units in the UK, in MFC (melamine-faced chipboard) or veneered MDF.
• 15mm is the economy step down. It can work for low-load parts, but it is a common cause of shelves bowing and frames going out of square.
• Sliding doors are the exception: doors are often 12mm to 15mm to keep weight on the track under control, while the carcass stays at 18mm.
• Span matters as much as thickness. Keep shelves around 800mm between supports, or add a divider or a deeper front edge.
• Ask for the spec in writing: board thickness, back panel, edge sealing and hardware, not just the finish.

Table of Contents

• What the carcass is and why thickness matters
• Carcass thickness by component
• 18mm versus 15mm: what you actually give up
• Shelf span: the number that matters as much as thickness
• MFC, MDF or plywood: thickness is only half the spec
• Sliding wardrobes: the carcass stays strong, the doors get lighter
• Sloping ceilings, alcoves and lofts
• Depth, height and how they interact with thickness
• How the carcass shows up in cost
• Our view: what we put behind every carcass
• Questions to ask any wardrobe fitter
• Bespoke wardrobes built to the right spec
• FAQ
• Sources

What the carcass is and why thickness matters

The carcass is the box behind everything you see. It holds the hanging rails, the shelves, the drawer runners and the hinges or door tracks. Doors, handles and finishes are the part you choose from a sample. The carcass is the part that decides whether the wardrobe still looks right in ten years.

In a fitted wardrobe the carcass is built to the exact shape of your opening. It sits against walls that are rarely square, on floors that are rarely level, so it has to be stiff enough to hold its geometry while being scribed and adjusted on site. Thickness is the simplest measure of that stiffness.

Three things depend on it directly:

1. Rigidity. Thicker board resists bending, so shelves stay flat and tall gables stay straight.
2. Screw-holding. Hinge cups, runner brackets and cam fittings bite into the board edge and face. Thin chipboard strips out sooner.
3. Squareness over time. A frame that racks pulls doors out of alignment, which shows up as uneven gaps and doors that catch.

MFC Direct, a UK board supplier, describes 18mm as the standard thickness for most UK wardrobes, cabinets and fitted furniture. It lists more rigidity, better screw-holding and greater stability for gables and carcasses as the reasons. It also reports that tradespeople are most often called back for sagging shelves, shaky carcasses and flexing doors caused by the wrong thickness. That matches what we see: most problems trace back to a decision made on the specification sheet, long before anything is installed.

Carcass thickness by component

Not every panel in a wardrobe needs to be the same thickness. A good specification matches the board to the job. Here is how it usually breaks down.

The door guidance comes from a plywood supplier’s wardrobe guide, which recommends 18mm for hinged doors and a reduced 12mm to 15mm for sliding doors to manage weight. The guide is written for plywood, but the weight logic applies to any sheet material.

Why the back panel is allowed to be thin

A back panel that looks flimsy next to an 18mm gable worries many homeowners. It should not. Its job is to hold the carcass square and close the back, not to carry clothes. In a fitted wardrobe it is also fixed to the frame, which stiffens the whole box. What matters is that it is properly fixed and that the gables and shelves are the right thickness.

Why drawers are a separate question

Drawer boxes are judged on the runner and the base, not just on side thickness. A full-extension, soft-close runner is rated for a load, and the carcass has to be solid enough to hold the runner fixings without flexing. That is another reason the gables beside a drawer stack should be 18mm.

18mm versus 15mm: what you actually give up

A 3mm difference sounds trivial. Across a 900mm shelf loaded with knitwear, it is not.

Bending stiffness of a board rises with the cube of its thickness. In plain terms: 18mm board is about 1.7 times as stiff as 15mm board before you add anything else. That is the physics behind why a 15mm shelf looks fine on day one and bowed by year two. Chipboard and MDF also creep: they keep bending slowly under a constant load, which is exactly what a shelf of clothes is.

The UK Workshop forum, where joiners and serious DIYers compare notes, shows the trade-off clearly. One experienced woodworker says he uses 15mm for cabinet sides and 18mm for tops, bottoms and shelves, because the thinner sides work in some jointing methods. That is a defensible choice for a small, lightly loaded cabinet. A full-height wardrobe holding a family’s coats, shoes and suitcases is a different job.

Where 15mm can be reasonable:

• Small, lightly loaded units such as a shallow shoe cabinet.
• Internal partitions that are not carrying a shelf span.
• Budget builds where the owner understands the trade-off.

Where we would not use it:

• Full-height gables over about two metres.
• Shelves wider than around 600mm that will carry folded clothes.
• Anything that takes drawer runners or heavy hinges.
• Any wardrobe you expect to live with for a decade or more.

Shelf span: the number that matters as much as thickness

The most common sagging complaint is not about the carcass at all. It is a long shelf with no support. Even 18mm board will bow if you ask it to span too far.

A material supplier’s guide gives a sense of the scale: 18mm MDF at spans of 600 to 800mm holds roughly 20 to 25kg before sag becomes visible, and capacity falls as the span lengthens to 800 to 1,000mm. Treat those figures as a guide, since board type and how the load is spread both change the result. The direction is what counts: past about 800mm, shelves need help.

The practical rules we work to:

1. Keep shelf runs to around 800mm or less between vertical supports for clothes storage.
2. Add a vertical divider in wide bays. It also gives you a neat way to split hanging from folded storage.
3. Use a stiffening front edge or a doubled-up shelf where a wide unbroken span is wanted.
4. Put heavy items low. Shoes, boxes and bags belong on the base or low shelves, not at head height.
5. Plan the load. Books, files and photo albums weigh far more than T-shirts. A home office wardrobe or pantry needs a tighter span.

If you are planning storage for books or kitchen goods, our guide on 18mm pantry shelving materials and how to stop shelves sagging covers the same principle for heavier loads.

MFC, MDF or plywood: thickness is only half the spec

A board’s thickness tells you about stiffness. The material tells you about everything else.

• MFC (melamine-faced chipboard). The workhorse of UK fitted furniture. Hard-wearing surface, consistent colour, good value. Edges must be sealed with edging tape.
• MDF. A smooth, dense core that takes paint and veneer beautifully, which is why it suits sprayed shaker doors. It is heavier than chipboard and needs sealed edges in damp rooms. Manufacturers such as EGGER describe decorative MDF faced on both sides with impregnated decorative paper for furniture and interior use.
• Plywood. Strong and good at holding screws. Its value shows up in damp spaces and for heavy-duty shelving, but it is more expensive and the edge needs finishing.

The usual starting point for a bedroom is 18mm MFC or veneered MDF. If you want help choosing between them for your room, read our fitted wardrobe materials review and the guide to choosing wardrobe materials. For painted finishes, the page on 18mm panels and sealed edges for MDF or wood wardrobes explains why edge sealing matters in a London bathroom-adjacent dressing room or a steamy en-suite wall.

Sliding wardrobes: the carcass stays strong, the doors get lighter

Sliding doors hang from, or run on, a track. Every extra kilogram of door puts load on the rollers and the top rail. That is why the door panels are usually thinner than hinged doors: 12mm to 15mm, as the guide above notes.

The carcass behind them does not follow suit. In a fitted sliding wardrobe the frame, the top and bottom track fixings and the shelves still want 18mm board, because the tracks screw into the structure and the whole unit has to stay square for the doors to glide. A carcass that moves by a few millimetres will make sliding doors bind or jump the track.

What to check when comparing sliding wardrobe quotes:

• Door panel thickness and weight, and the roller and track rating.
• Whether the track is fixed to an 18mm frame or to a thinner liner.
• Door height. Floor-to-ceiling doors in a London Victorian terrace can be over 2.5m, and weight rises quickly.
• Soft-close or soft-stop as standard, rather than a paid extra.

If you are weighing sliding against hinged or bifold doors, a New Zealand renovator has written a comparison of bifold and sliding doors for homeowners. It is written for NZ homes and building practice, but the trade-offs around space, operation and long-term wear are useful. For our own range, see our sliding wardrobes and the page on whether sliding wardrobes save space in bedrooms.

Sloping ceilings, alcoves and lofts

In loft conversions and eaves rooms the geometry works against you. Gables are cut at an angle, shelf depths step down, and the carcass is often split into several smaller boxes.

Thinner board is a particular temptation here, because smaller units feel less demanding. A forum builder describing a built-in wardrobe in the eaves asked whether 15mm would do, having believed 18mm was correct but finding mostly 15mm on sale. The question comes up often, and the answer is the same as elsewhere: if the board is carrying shelves, drawers or hanging weight, choose 18mm.

There is a second reason to prefer a bespoke carcass here. Sloped sections need cuts made to your actual ceiling line, not a stock size trimmed on site. That keeps the cut edges clean and sealed. If you are thinking about doing it yourself, our DIY fitted wardrobe sloping ceiling guide lays out what is involved. For chimney breasts, odd recesses and under-stairs voids, see the awkward spaces guide.

Depth, height and how they interact with thickness

Board thickness takes space from the room. Two 18mm gables take 36mm from the internal width of a bay, and an 18mm shelf takes 18mm of height each time. In a narrow alcove that matters, and a good designer plans around it before the first cut.

Standard hanging depth is the other planning figure. A 600mm depth is the common working measure for hanging full-length clothes on standard hangers, and we explain how we measure for it in our wardrobe depth guide. Where the room is tight, a shallower depth with a shelved or drawer-based interior can replace hanging space without weakening the carcass.

As a quick check on a typical double bay:

1. Measure the clear opening width at the top, middle and bottom. Walls vary.
2. Subtract gable thickness (2 x 18mm) and any internal dividers to find the usable bays.
3. Split the bays so no shelf exceeds roughly 800mm between supports.
4. Allow for the skirting board and cornice, which a made-to-measure unit is scribed around.

How the carcass shows up in cost

The carcass is where a wardrobe can be made cheaper without anyone noticing at handover. Thinner board, a lighter back, fewer dividers, plain cut edges instead of sealed ones: none of it shows in a showroom sample. It shows in year three.

We do not publish fixed prices because every wardrobe is measured and priced to its room. What we can tell you is where to look when two quotes differ:

• Board thickness for gables, shelves and doors.
• Edge finish: sealed and matched, or left raw on hidden faces.
• Hardware: runners and hinges, and whether soft-close is standard.
• Interior layout: how many dividers, shelves and drawers are included.
• Installation and removal: whether fitting, making good and disposal of the old wardrobe are included.

Two quotes that look similar on the front can differ a lot on these items.

Our view: what we put behind every carcass

We build bespoke wardrobes for West London homes, from Putney and Fulham to Richmond, Chiswick and Wimbledon, and the carcass is the part of the job we are least willing to compromise on.

Our view is simple. A client sees the door finish, the handle, the lighting and the layout. They do not see the board inside, so they are trusting us. That trust is why we offer a 10-year guarantee on workmanship, and why we think carefully about what sits behind the doors. A guarantee is only worth giving when the structure is built to last, and 18mm board with proper spans is the starting point our published material guides point to.

We also think thickness gets overrated as a single number. A well-planned 18mm carcass with sensible shelf spans, sealed edges, a solid back and quality soft-close hardware will outlast a badly planned thicker one. So when we visit, we measure the room first, then talk through what you will store: coats, knitwear, shoes, a suitcase stack, a steamer, a safe. The load shapes the layout, and the layout shapes the carcass.

If you have a quote in your hand, ask the same questions of us as of anyone else. We would rather you compare like for like.

— Aureliu

Questions to ask any wardrobe fitter

Take this list to your design visit. A good fitter will answer each one without hesitation.

1. What thickness is the board for gables, shelves, dividers and doors?
2. What is the carcass material: MFC, MDF or plywood?
3. What is the maximum shelf span, and where are the dividers?
4. How are the exposed and cut edges sealed?
5. What are the runners, hinges and sliding gear, and are they soft-close?
6. How is the unit fixed to the walls and floor, and how is it scribed to uneven surfaces?
7. What does the guarantee cover, and for how long?
8. Are removal and disposal of the old wardrobe included?

If an answer is vague, that is information in itself.

Bespoke wardrobes built to the right spec

We design, make and install bespoke fitted wardrobes, sliding wardrobes, walk-in wardrobes, under-stairs storage and media walls across West London. Every project starts with a free design visit, where we measure your room and talk through what you want to store.

What you can expect from us:

• Made-to-measure designs for alcoves, lofts, sloping ceilings and awkward spaces.
• Soft-close mechanisms as standard.
• Installation typically within seven days.
• A 10-year guarantee on workmanship.
• Removal and disposal of your old wardrobe.

See our fitted wardrobes or book a free design visit. You can also call 07468 150807 or message us on WhatsApp. Our studio is at 124 City Road, Kemp House, London, EC1V 2NX.

FAQ

What thickness should a wardrobe carcass be?

For a fitted wardrobe, 18mm is the standard UK thickness for gables, tops, bases, shelves and dividers. It gives the best balance of stiffness, screw-holding and weight. Thinner 15mm board is sold as an economy option and is more likely to bow or rack.

Is 15mm board good enough for a wardrobe?

It can work for light, small units, but it is not what we would choose for a full-height wardrobe carrying clothes and drawers. Shelves bow sooner and hinge or runner fixings hold less well. If a quote uses 15mm, ask exactly which parts and whether the spans are shortened to compensate.

How thick should sliding wardrobe doors be?

Sliding door panels are often 12mm to 15mm, because lighter doors put less load on the track and run more smoothly. The carcass and track fixings behind them should still be solid 18mm board. Check the roller rating against the door height and weight.

How far can a wardrobe shelf span without sagging?

As a working rule, keep 18mm shelves to around 800mm between supports for folded clothes. Beyond that, add a vertical divider or stiffen the shelf. If it will hold heavy items such as books or files, shorten the span further.

Does the back panel need to be 18mm?

No. The back panel squares the unit and closes the rear, but it does not carry clothes, so a much thinner board is normal. What matters is that it is fixed properly to a solid 18mm frame.

Sources

• Why 18mm MFC Is the Standard for Furniture, MFC Direct
• Which MM Plywood Is Best for Wardrobe? Complete 2026 Guide
• How Much Weight Can MDF Hold?, MDF Direct
• 15mm vs 18mm MFC or MDF, UK Workshop
• Built in wardrobe carcass size, UK Workshop
• EGGER Decorative MDF E1E05 TSCA ST

Recommended

• Homeowners: 18mm Panels and Sealed Edges for MDF or Wood Wardrobes
• How to Choose Wardrobe Materials: A 2026 Guide
• Best Material for Fitted Wardrobes: The 2026 Bespoke Selection Guide
• Wardrobes: What to Check Before You Choose