A Guide to Carcassing Timber UK

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Look around you. Unless you’re standing in a field or a particularly futuristic glass dome, chances are you’re surrounded by it. It’s hidden behind your plasterboard, tucked beneath your floorboards, and holding up your roof tiles. It’s the strong, silent skeleton of your home, your office, and countless buildings across Britain. We’re talking about carcassing timber, the unsung hero of the construction world.

You’ve probably never given it a second thought. It isn’t glamorous like exposed oak beams or a sleek kitchen worktop. It’s the humble, workaday wood that does all the heavy lifting. But without it, our homes would quite literally fall down. From the timber-framed new builds popping up from Cornwall to the Cairngorms, to the loft conversion giving a Victorian terrace in Manchester a new lease of life, carcassing timber is the backbone.

So, what exactly is this stuff? Why does it have such a funny name? And how do builders know which piece of wood to use where? This guide will pull back the plasterboard and reveal everything you need to know. We’ll explore how a simple piece of pine or spruce becomes a precision-engineered structural component, decode the mysterious stamps you see on its side, and explain why choosing the right piece of wood is one of the most important decisions in any building project. Get ready to meet the hidden framework that keeps a roof over your head.

What Exactly is Carcassing Timber? The Building Blocks Explained

At its heart, the idea is simple. Carcassing timber is wood that has been cut and prepared for use in a building’s frame, or ‘carcass’. Think of it like a skeleton. It’s the collection of bones—the joists, studs, and rafters—that gives a structure its shape and strength before the ‘skin’ of plasterboard, cladding, or bricks is added.

It’s almost always softwood, which might sound a bit flimsy for holding up a house. But don’t let the name fool you. ‘Softwood’ doesn’t mean it’s soft and squishy like a sponge. The term comes from botany and refers to wood from coniferous trees—the ones with needles and cones, like pine, fir, and spruce. These trees grow relatively quickly, which makes their timber more affordable and sustainable than hardwood from slow-growing trees like oak or ash. Thanks to modern processing and grading, this softwood is more than strong enough for the job.

Why It’s Not Just Any Old Bit of Wood

You can’t just pop down to your local woods, chop down a tree, and start building a house. The timber used for a building’s carcass goes through several important processes to make it fit for purpose.

Kiln-Drying: Getting the Moisture Just Right

Freshly cut wood, known as ‘green’ timber, is full of water. If you were to build with it, it would shrink, twist, and warp as it dried out over time. Imagine building a perfectly straight wall, only for the timber inside to bend like a banana a year later. It would crack the plaster and could even make the structure unsafe.

To prevent this, carcassing timber is kiln-dried. It’s stacked in huge ovens, or kilns, where hot air is circulated to gradually reduce its moisture content to a stable level, usually around 18-20%. This makes the wood much more stable and predictable. It’s a bit like toasting bread to stop it from going floppy.

Regularised for a Perfect Fit

Have you ever tried to build something with pieces that are all slightly different sizes? It’s a nightmare. To ensure consistency, carcassing timber is often ‘regularised’. This means it’s passed through a planer to make sure all the pieces have the same dimensions. So, a piece of 47mm x 100mm timber is exactly that size, making it much easier for carpenters to build straight, true frames.

Eased Edges: A Small Detail That Matters

If you look closely at a piece of carcassing timber, you’ll notice its corners aren’t sharp. They’re slightly rounded. This is called having ‘eased edges’. It’s a small but important detail that makes the timber easier and safer to handle on a building site, reducing the risk of nasty splinters. It also helps paints and preservatives to stick better.

Decoding the Stamp: C16 vs C24 Explained

Pick up any piece of structural timber at a builder’s merchant and you’ll see a stamp inked onto its surface. It looks like a confusing jumble of letters and numbers, but it’s actually the wood’s CV. It tells you everything from where it was grown to, most importantly, how strong it is.

The most critical part of this stamp is the strength grade, usually C16 or C24.

This ‘C’ stands for coniferous (the type of tree), and the number tells you its bending strength. Think of it like this: if you placed a plank of wood between two supports and stood in the middle, the number represents how much force it could take before it snapped. The higher the number, the stronger the wood.

So, What’s the Real Difference?

Both C16 and C24 are approved for use in UK construction, but they have different characteristics and are used for different jobs. The main differences come down to the quality of the raw timber.

A piece of wood’s strength is affected by natural imperfections, mainly knots. Knots are the dark, circular marks where branches once grew. While they can look nice in furniture, in structural timber they are weak points. The bigger the knot, the weaker the wood.

C16 Timber: The Everyday Workhorse
- Appearance: C16 timber is allowed to have more and larger knots. The grain might not be as dense or straight.
- Strength: It’s strong enough for most standard jobs in a typical house build.
- Cost: It’s more common and therefore cheaper.
- Best for: Most internal jobs like stud walls, floor joists, and roof rafters in standard domestic projects. It’s the bread-and-butter of the UK building trade.
C24 Timber: The Premium Performer
- Appearance: C24 timber is graded to have fewer and smaller knots. It looks ‘cleaner’ and has a denser, more uniform grain. This is because it’s usually sourced from areas where trees grow more slowly, like Scandinavia, resulting in tighter growth rings.
- Strength: It’s significantly stronger than C16. It can handle heavier loads or span longer distances without needing extra support.
- Cost: It’s more expensive due to its higher quality and the fact it’s often imported.
- Best for: Projects where strength is critical. This could be for floor joists spanning a large open-plan room, rafters for a heavy slate roof, or in buildings where the design demands a stronger frame, like a mezzanine floor or a balcony.

How Do They Know It’s C16 or C24?

There are two ways timber is graded for strength:

Visual Grading: A trained expert inspects every piece of wood. They look at the size and location of knots, the slope of the grain, and any other defects, then assign a grade based on strict rules. It’s a craft that takes years to perfect.
Machine Grading: The timber is fed through a machine that bends it slightly and uses sensors to measure its stiffness and density. A computer then calculates its strength and automatically stamps it with the correct grade. This is the most common method today as it’s fast and incredibly accurate.

An architect or structural engineer will calculate the loads a building needs to withstand (from the weight of the roof tiles to the furniture and people inside) and specify which grade of timber is needed for each part of the frame. Using C16 where C24 is specified is a massive no-no and could lead to structural failure. You can, however, always use C24 where C16 is specified—it’s just more expensive, a bit like using a racing bike for your weekly shop.

What About TR26?

You might also come across TR26. This is a special, high-strength grade used almost exclusively for making trussed rafters—the factory-made ‘W’ shaped wooden frames that hold up the roofs on most new-build homes in Britain. It’s engineered to be very strong and reliable for this specific, high-stress job.

Staying Power: Why Timber Treatment is Crucial

Britain is a wonderful country, but it’s also a damp one. This combination of rain and humidity is the perfect environment for two of timber’s greatest enemies: fungal rot and wood-boring insects.

Left unprotected, a timber frame exposed to moisture would eventually start to decay, weakening the structure of the building. To stop this from happening, much of the carcassing timber sold in the UK is pressure treated with preservatives.

How Pressure Treatment Works

The process is more than just giving the wood a quick lick of paint.

The timber is loaded into a large, sealed cylinder.
A vacuum is created, sucking all the air out of the wood’s pores.
The cylinder is flooded with a liquid wood preservative (you might hear builders talk about brands like Tanalith E).
High pressure is applied, forcing the preservative deep into the grain of the wood.
The timber is left to dry, with the preservative locked inside its structure.

This process gives the wood long-lasting protection from the inside out. Treated timber often has a greenish or brownish tint, and you might see little incisions on its surface, which help the treatment to penetrate even deeper.

Understanding Use Classes: The Right Treatment for the Job

Not all timber needs the same level of protection. You wouldn’t need the same heavy-duty treatment for a stud wall inside a warm, dry house as you would for a fence post that’s going to sit in wet soil for 25 years. This is where Use Classes come in. They are a British and European standard that defines where the timber can be used.

Use Class 1 (UC1): For internal use in a dry environment. Think furniture or internal joinery. Carcassing timber is rarely sold for this class.
Use Class 2 (UC2): This is the most common class for internal carcassing timber. It’s for wood that’s inside a building’s shell, so it’s protected from the weather but is still at a small risk of getting damp from condensation. This is the standard for most floor joists, roof timbers, and internal stud walls.
Use Class 3 (UC3): For wood used outdoors, but not in contact with the ground. It’s for timber that will get rained on. Think fence panels, shed walls, and decking boards.
Use Class 4 (UC4): The heavy-duty stuff. This is for wood that is in permanent contact with the ground or fresh water. It has the highest level of preservative treatment. This is what you need for fence posts, decking joists that sit on the ground, and submerged timbers.

When you buy treated timber, it should be clearly marked with its Use Class. Using UC2 timber for a decking subframe (a UC4 job) is a recipe for disaster, as it will rot away in just a few years.

Where It’s Used: The Carcass of a British Home

So we know what it is, how it’s graded, and how it’s protected. But where does carcassing timber actually go? Let’s take a tour of a typical house and find the hidden skeleton.

1. The Floor Joists

These are the horizontal beams that form the floor of each storey. They span from wall to wall, and your floorboards (or chipboard) are fixed on top of them. The size and spacing of joists are carefully calculated by an engineer. They need to be strong enough to support the weight of everything in the room, from your bed and wardrobe to you and your family, without bouncing or sagging. For a large, open-plan living room, you’d likely need deeper C24 joists to cover the distance.

2. The Stud Walls

Most internal walls in modern homes aren’t made of bricks. They are timber frames, known as stud walls. These are made from a grid of vertical timbers (studs) and horizontal timbers (noggins), which are then covered with plasterboard. It’s a quick, light, and flexible way to divide up the internal space of a house. C16 timber is perfect for this job.

3. The Roof Structure

This is where carcassing timber really shows off its strength. There are two main types of roof construction in the UK:

Traditional Cut Roof: A carpenter builds the roof frame on-site, piece by piece. The main angled timbers are called rafters. They are supported by horizontal beams called purlins and a ridge board at the very top. This is a skilled job and often requires larger sections of C24 timber, especially for big roofs.
Trussed Rafters: As we mentioned, these are the factory-made ‘W’ frames made from high-strength TR26 timber. They are delivered to the site and lifted into place by a crane. This method is much faster and more cost-effective, which is why it’s used for the vast majority of new-build homes.

4. Decking Subframes

When you build a garden deck, the boards you walk on are only half the story. Beneath them is a sturdy subframe, a grid of timber joists that supports the whole structure. Because this frame is outdoors and close to the damp ground, it’s absolutely essential that it’s built from timber treated to Use Class 4 (UC4).

5. Other Structures

Carcassing timber is incredibly versatile. It’s also used for building garden sheds, pergolas, flat roof structures (firrings), and the frames for dormer windows in a loft conversion. It is, quite simply, everywhere.

A Practical Guide for DIYers and Self-Builders

If you’re taking on a project yourself, from a simple partition wall to a full extension, getting the timber right is key. Here are some practical tips.

Choosing the Right Timber

Check the Plans: If an architect or engineer has drawn up plans, they will specify the exact size and strength grade (C16 or C24) for every piece of structural timber. You must follow this. Don’t be tempted to save a few quid by using C16 if C24 is specified.
Know Your Use Class: Think about where the wood will be. Inside a warm, dry house? UC2 is fine. Building a deck subframe? It must be UC4.
Inspect Before You Buy: When you’re at the builder’s merchant, have a look at the timber. Avoid pieces that are badly twisted, split, or have huge knots right on the edge, as these are weak points. A good piece of carcassing timber should be relatively straight and true.

Storing Timber On-Site

Don’t just dump your timber delivery on a muddy lawn. If it gets soaked with rain and then dries out too quickly in the sun, it can warp.

Keep it Dry: Store it under a waterproof cover, but allow for some air circulation to prevent sweating.
Keep it Flat: Stack it on level bearers (offcuts of timber) to keep it off the damp ground. Place bearers every metre or so to support the whole length and stop it from sagging.

Common Mistakes to Avoid

Notching Joists Incorrectly: Sometimes you need to cut a notch in a floor joist to run a pipe or cable. There are very strict rules about how big these notches can be and where you can put them. Cutting a big notch in the middle of the span can catastrophically weaken a joist. Always check UK Building Regulations or consult a professional.
Using the Wrong Fixings: The nails and screws you use are just as important as the timber. Use proper structural screws or galvanised nails, especially for outdoor projects where they need to resist rust.
Ignoring End Grain: When you cut a piece of treated timber, you expose the untreated wood inside. This ‘end grain’ is like a straw that can suck up moisture. You must treat any cut ends with a brush-on end-grain preservative to maintain the timber’s protection.

Timber’s Green Credentials: Building for the Future

In an age of climate change, the materials we build with matter more than ever. Concrete and steel have a huge carbon footprint. Timber, on the other hand, has a secret weapon: it’s a carbon sink.

As a tree grows, it absorbs carbon dioxide (CO2) from the atmosphere. When that tree is harvested and used in a building, that carbon stays locked up in the wood for the life of the building. A typical timber-framed house stores several tonnes of CO2.

Sustainable Sourcing: Look for the Label

Of course, this is only true if the timber is sourced responsibly from well-managed forests where new trees are planted to replace the ones that are cut down. To be sure you’re buying sustainable timber, look for certification marks on the wood. The two main ones you’ll see in the UK are:

FSC (Forest Stewardship Council): A global organisation that sets high standards for forest management, covering everything from environmental impact to workers’ rights.
PEFC (Programme for the Endorsement of Forest Certification): Another global non-profit that promotes sustainable forest management.

Nearly all structural timber sold by reputable merchants in the UK today will have one of these certifications.

The Future is Engineered

The world of carcassing timber is also evolving. Engineered wood products are becoming more popular. These take timber and process it to create components that are even stronger and more predictable than solid wood.

I-Joists: These look like a capital ‘I’. They have a thin web of oriented strand board (OSB) with solid wood flanges at the top and bottom. They are very light, strong, and can span huge distances, making them perfect for open-plan floors.
Glulam (Glued Laminated Timber): Layers of timber are glued together to create massive, strong beams that can be used to create spectacular vaulted ceilings and curved structures.

These materials reduce waste by using smaller pieces of wood and offer incredible performance, pushing the boundaries of what’s possible with timber construction.

The Unsung Hero

Carcassing timber might not be the most exciting part of a building, but it is the most important. It’s a natural, sustainable, and incredibly strong material that forms the invisible framework of our daily lives.

From the simple C16 stud in a partition wall to a mighty C24 joist holding up a floor, every piece is graded, treated, and chosen for a specific job. It’s a testament to centuries of building knowledge combined with modern engineering. So next time you walk across a floor or look up at a ceiling, spare a thought for the humble, hidden skeleton of wood that’s working tirelessly to keep your world from falling apart. It’s the true backbone of Britain’s buildings.

Further Reading:

TRADA (The Timber Research and Development Association): https://www.trada.co.uk/ – The UK’s leading authority on wood, providing extensive technical information.
Timber Decking and Cladding Association (TDCA): https://www.tdca.org.uk/ – Offers best-practice guides, especially for outdoor timber use.
FSC UK (Forest Stewardship Council): https://uk.fsc.org/ – Information on sustainable timber certification in the UK.