Loft conversion ridge beam

Question

I’ve currently got a loft conversion going on and my structural engineer has stated a ridge beam of 203x203x46 for a span of approx 5 meters but the builder has installed a ridge beam of 152x152x37. This has been queried with the structural engineer (waiting for reply) but I want to know from builders if this ridge beam is a common size

Daniel · Accepted Answer

Yes it is the structural engineers spec is a little over the top however your building inspector will want it done to the engineers specification

Check with him if obviously you have one to pass the works off

Dhimiter · Answer

I think 203x203x46 is over the top but the builder has to follow enginers calculation . Spam is 5m and depends on the load which will carrying on.

Michael · Answer

Hi, 203x203x46 is a standard size. The 46 on the end is the weight per meter, making the smaller beam lighter. If the builder alters the SE requirement in construction, the builder takes full responsibility. Therefore, any costs or delays because building control will not sign your job off, sit with the builder.

Phillip · Answer

Well you're builder must use the engineers guide and use the that was calculated. This was out into the engineers design and excepted by building control and planning. Your builder will have to adhere to the drawing and plans . Engineers today always well engineer belt and brasses.

Alan · Answer

There is no real common size.  Always follow engineers calcs and spec which will also satisfy your building control officer.  Your builder should know this IMO. The enginner has given him the beam size for good reason and to maintain the structural integrity of the new imposed loads.

Jeevan · Answer

The 152x152x37 UB is a commonly used size, but for a 5-meter span, it really depends on the load it's carrying—roof structure, finishes, and any point loads. If your structural engineer specified 203x203x46, it's likely due to specific load requirements. I'd wait for their confirmation before proceeding, as undersizing could lead to deflection or structural issues down the line.

Craig · Answer

Structural engineers tend to over calculate beams not really thinking of the builders that have to install them however it's not down to the builders to override the engineers calculations if that's the size the engineer has calculated that's what building control will want to see

Julian · Answer

Hi,
As previous answer, the building inspector will always want to see the Structural Engineers spec met.
However, this beam size is unusual for a ridge, normally, for this application, the depth is greater than the width so 203 x 103 x (whatever weight is specified).
So, do check with your engineer, as their calculation is essential, but to my experience 203mm wide seems unusually wide for a ridge.

Dariusz · Answer

"Yes it is the structural engineers spec is a little over the top however your building inspector will want it done to the engineers specification

Sidrit · Answer

The ridge beam specified by your structural engineer (203x203x46 UC) is significantly larger and stronger than the one your builder has installed (152x152x37 UC).

Here’s what this means in practical terms:
	•	203x203x46 UC
	•	A much stronger beam with a higher load-bearing capacity.
	•	Likely specified to handle structural loads, roof weight, and potential point loads from purlins or rafters.
	•	152x152x37 UC
	•	Still a universal column but considerably weaker than the original specification.
	•	May not be adequate for the span, depending on load calculations.

Common Ridge Beam Sizes

A 152x152 UC isn’t unheard of for a small loft conversion, but for a 5m span, it’s less common unless additional supports (such as posts or intermediate steelwork) are in place.

Next Steps
	•	Wait for your structural engineer’s response—they will determine if the smaller beam is acceptable or if it needs replacing.
	•	If in doubt, insist on the correct beam—changing it later will be costly and disruptive.
	•	Check if Building Control has inspected the steelwork—they might flag issues.

It’s crucial that the beam meets the engineer’s specifications to avoid structural risks. Keep pushing for clarification before further work continues!

Loft conversion ridge beam

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