75 x 150 C24 Treated Timber vs 75 x 200 C24 Treated Timber Comparison
Picking between 75 x 150 C24 treated timber and 75 x 200 C24 treated timber usually comes down to one thing: how much strength and depth the job really needs. They look similar on the rack, they’re both chunky structural timbers, and they’re often used in the same types of projects. But that extra 50 mm of depth can make a big difference.
The short version is simple. 75 x 150 C24 treated timber is easier to handle, often cheaper, and suits plenty of everyday structural jobs. 75 x 200 C24 treated timber is deeper, stiffer, and better for longer spans or heavier loads.
The right choice depends on the span, load, fixing method, exposure, and building requirements. Let’s walk through it in plain English.
What both timber sizes have in common
Both 75 x 150 and 75 x 200 C24 treated timbers are structural softwood sections. The C24 grade means the timber has been strength graded and meets a higher structural strength class than C16. In day-to-day terms, C24 is often chosen when you need stronger, stiffer timber for framing, joists, beams, rafters, decking structures, and general construction.
The “treated” part usually means the timber has been pressure treated with preservative to help resist rot and insect attack. That makes it suitable for many outdoor and damp-prone jobs, depending on the treatment class and where it’s being used.
They also share a few practical points:
Both are commonly used for structural work.
Both need correct fixings, supports, and end treatment after cutting.
Both should be kept as dry and well ventilated as possible on site.
Both may be slightly smaller than the nominal size once machined.
Both should be selected with the right span tables or structural guidance.
That last point matters. A bigger timber is usually stronger, but timber design isn’t guesswork. If the timber is supporting a roof, floor, raised deck, opening, or anything safety-critical, check the span tables, building control guidance, or ask a structural engineer.
The main difference is depth
The numbers look close, but the difference is bigger than it first seems.
Both timbers are 75 mm wide. The difference is the depth:
75 x 150 mm
75 x 200 mm
That extra 50 mm of depth gives the 75 x 200 section more stiffness and strength in bending when installed upright. For joists and beams, depth is a major part of performance. A deeper timber resists sagging better than a shallower one of the same width and grade.
Here’s a simple comparison.
Feature | 75 x 150 C24 treated timber | 75 x 200 C24 treated timber |
Width | 75 mm | 75 mm |
Depth | 150 mm | 200 mm |
Typical feel on site | Heavy, but manageable | Noticeably heavier and bulkier |
Stiffness | Good for many medium-duty jobs | Better for longer spans and heavier loads |
Cost | Usually lower | Usually higher |
Cutting and handling | Easier | More effort, especially overhead |
Best suited to | Smaller beams, short joists, framing, garden structures | Larger joists, beams, raised decks, roof work, heavier framing |
The main takeaway is this: if span and stiffness matter, the 75 x 200 has the advantage. If access, cost, and ease of handling matter more, 75 x 150 may be the better fit.
When 75 x 150 C24 treated timber makes sense
75 x 150 C24 treated timber is a strong, useful size. It’s not lightweight by DIY standards, but it’s still easier to work with than the 200 mm deep option.
It often suits jobs where the span is moderate and the loading is not extreme. Think garden structures, smaller decking frames, pergola beams, short joists, framing work, and general bracing. It can be a good choice when you need a solid section but don’t want the weight, cost, or bulk of a deeper beam.
Common uses include:
Decking subframes where spans are suitable
Pergola and gazebo framing
Raised beds and garden retaining details, where properly designed
Shed bases and workshop floors
Short-span floor or roof joists
Timber lintel-style support in non-critical applications
Heavy-duty fencing, rails, and gate framing
One of the big benefits is workability. A 75 x 150 timber is easier to lift, line up, clamp, drill, and fix. If you’re working on your own or doing a weekend build, that matters more than people admit.
It also gives a neater and less bulky look in exposed garden builds. A pergola made with 75 x 150 sections can look solid without feeling overbuilt. For visible work, proportions can be just as important as strength.
The downside is that it won’t resist deflection as well as 75 x 200 over longer spans. Deflection is the bend or sag you see when timber is loaded. A timber can be strong enough not to fail but still flex more than you want. That’s where deeper sections earn their keep.
When 75 x 200 C24 treated timber is the better choice
75 x 200 C24 treated timber is the stronger and stiffer option in most like-for-like situations. The extra depth helps it handle longer spans and heavier loads better than the 150 mm deep section.
This makes it useful for more demanding structural work, including:
Larger deck joists and beams
Raised decking frames
Roof rafters and purlins, subject to design
Floor joists, subject to span tables
Heavy pergola beams
Structural framing where deeper timber is needed
Carport and canopy framing
Timber work where bounce or sag needs to be reduced
If a deck is raised well off the ground, or if people will gather on it, the extra stiffness can be worth it. The same applies to workshop floors, platforms, or storage areas where the load may be higher than normal.
The trade-off is weight. 75 x 200 timber is harder to handle, especially in long lengths. It can be awkward to lift into position, and it may need two people for safe fitting. Cutting takes a little more time, drilling through it takes more effort, and fixings may need to be longer or heavier-duty.
It can also look chunkier. That’s sometimes a good thing. A large pergola, carport, or garden room frame can look better with deeper sections because the proportions suit the scale. But on a smaller build, 75 x 200 can look oversized if it’s not needed.
Strength, span, and stiffness are not the same thing
This is where timber choice can get confusing. People often ask, “Which one is stronger?” The easy answer is 75 x 200. But the better answer is that timber has to be checked for several things.
Strength
Strength is about whether the timber can carry the load without failing. C24 gives both sizes a solid structural grade. The deeper 75 x 200 section has more capacity in bending when used the right way up.
Span
Span is the distance between supports. The longer the span, the harder the timber has to work. Small increases in span can make a big difference to how much a timber bends. That’s why you shouldn’t just copy a beam size from a different project unless the layout, load, and spacing match.
Stiffness
Stiffness is about how much the timber flexes under load. This is often what people notice first. A floor that feels bouncy, or a deck with visible sag, may not feel right even if it hasn’t failed structurally.
This is where 75 x 200 C24 treated timber has a clear edge. It’s deeper, so it tends to deflect less than 75 x 150 over the same span and loading.
If the job involves people standing, walking, sleeping, parking, or storing heavy items above the timber, don’t size it by eye. Use span tables or get proper structural advice.
How treatment affects your choice
Both sizes being treated is a good start, but treatment is not a magic shield. The exposure level still matters.
For indoor structural work in dry conditions, treatment may not always be required, though treated timber is often used anyway for extra protection. For outdoor jobs, treatment becomes much more important.
Common outdoor risk areas include:
Decking frames close to the ground
Pergolas exposed to rain
Garden rooms and sheds
Fence rails and posts
Carports and canopies
Timber near masonry, soil, or damp surfaces
If you cut treated timber, the cut end exposes untreated inner wood. Brush on a suitable end-grain preservative after cutting, especially for outdoor jobs. It’s a small step that can add years to the life of the build.
Good detailing also helps. Keep timber off standing water, allow airflow, slope surfaces where possible, and avoid trapping moisture between boards, walls, and brackets.
Cost and value are not always the same
75 x 150 C24 treated timber will usually cost less per length than 75 x 200 because it uses less material. If the smaller section meets the structural requirements, it can be the sensible buy.
That doesn’t mean cheaper is always better.
If you undersize timber, you may end up with bounce, sag, callbacks, repairs, or wasted material. In that case, the saving disappears quickly. On the other hand, using 75 x 200 everywhere “just to be safe” can make a project more expensive, heavier, and harder to build than it needs to be.
A good way to think about it is this:
Choose 75 x 150 when | Choose 75 x 200 when |
The span is short to moderate | The span is longer |
Loads are modest | Loads are heavier |
You want easier handling | You need more stiffness |
The structure is smaller | The build is raised, large, or load-bearing |
Budget matters and the size is suitable | Reducing deflection matters more than saving material |
For trade work, the best value is often the size that passes design requirements with a bit of practical margin, without making installation painful. For DIY work, ease of lifting and fixing can be just as important as the timber price.
Fixings and installation details matter
The timber size is only part of the structure. Fixings, supports, and spacing can make or break the job.
For both 75 x 150 and 75 x 200 treated timber, pay attention to:
Correct joist spacing for the deck boards, flooring, or roof covering
Suitable joist hangers, brackets, bolts, or structural screws
Proper bearing at each end of the timber
Timber installed upright, not laid flat, for beams and joists
Bracing to stop twisting and movement
Preservative treatment on cut ends
Stainless steel or suitable coated fixings where required outdoors
Laying a 75 x 200 beam flat might seem like it gives a wide fixing face, but it loses much of the benefit of the deeper section. For bending strength and stiffness, joists and beams usually need to stand on edge.
Also watch for twisting. Deeper timber can move as it dries, especially if stored badly. Stack it flat, support it evenly, and keep it covered but ventilated before use.
Which one should you pick?
If you’re building something small to medium-sized, with short spans and normal loads, 75 x 150 C24 treated timber is often the practical choice. It’s strong, available, and easier to live with on site.
If the project has longer spans, heavier loads, a raised platform, or a stricter need to reduce bounce, 75 x 200 C24 treated timber is usually the better option. It gives you more depth, more stiffness, and more confidence when the design calls for it.
Here’s the plain-English verdict.
Project type | Better starting point |
Small pergola | 75 x 150 |
Large pergola beam | 75 x 200 |
Low-level decking frame | 75 x 150, if spans suit |
Raised decking frame | 75 x 200 is often more suitable |
Shed base | 75 x 150 for many builds |
Workshop floor with heavier use | 75 x 200 may be better |
Short roof members | 75 x 150, if designed for the span |
Longer rafters or purlins | 75 x 200, subject to design |
The phrase “subject to design” matters. Timber size depends on spacing, span, load, support, and use. A beam that works at one spacing might not work at another.
Final takeaway
The 75 x 150 C24 treated timber vs 75 x 200 C24 treated timber comparison comes down to balance.
Go with 75 x 150 when the design allows it and you want a strong, manageable, cost-conscious timber for general structural work. Go with 75 x 200 when you need longer spans, better stiffness, or more load capacity.
If the timber is carrying anything important, don’t rely on guesswork. Check span tables, follow local building rules, and get structural advice where needed. The best timber isn’t always the biggest one. It’s the one that suits the job, fits safely, and performs well for years.
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