RSJs, or rolled steel joists, are essential for structural support for your extension. They can also be used as support in place of load bearing walls, for example if you want to remove the wall between your living and dining rooms to create an open-plan space.

So how much does this crucial part of building work cost? We’ll explain everything in this guide.

How much does an RSJ cost?

RSJ costs vary depending on several factors including size, length, and type. On average, expect to pay between £2.50-£4.50 per kg for an RSJ. You’ll then need to pay for installation, which ranges from £200-£950 depending on the length and weight of the beam.

Item	Average cost
RSJ beam between 1-5m, up to 46kg/m	£575
RSJ installation based on a 1-5m beam, up to 46kg/m	£500
RSJ supply and installation	£1,075
Structural engineer	£250

RSJ installation cost

The installation cost of an RSJ depends on various factors such as the complexity of the installation, accessibility of the site, and whether any additional structural work is required. Typically, installation costs can range from £300 to £750 per beam.

How much RSJ installation costs will vary depending on your location. Labour costs vary across the country, with London and the south east commanding the highest prices. In other areas of the UK, you may pay less than the average. Take a look at our location cost table further down the page to get an idea of how much RSJ installation will be in your local town or city.

Types of RSJ

I beams

I beams, also known as universal beams, are one of the most common types of RSJ. They are shaped like the letter ‘I’ and are ideal for supporting heavy loads over long spans. I beams are usually used for supporting floors or roofs, so are laid horizontally.

H beams

H beams, wide flange beams or universal columns, are similar to I beams but have wider flanges, so they look like the letter ‘H’. They are suitable for applications where additional strength and stability are required.

T beams

T beams, also known as T-sections, have a T-shaped cross-section and are commonly used in construction projects extra strength needs to be added to concrete.

C-section beams

C-section beams, also referred to as channels, have a C-shaped cross-section and are often used for framing or structural support in buildings. They come in a range of sizes and thicknesses, making them ideal for lots of different applications.

How much weight can an RSJ support?

How much weight an RSJ can support depends on its size and the type of RSJ. They can all support different loads, but all are designed to support the heaviest load with the minimum amount of steel. A structural engineer or builder will be able to tell you what kind of RSJ you need based on the weight it needs to support.

Do I need a structural engineer for an RSJ?

Yes, it’s a good idea to consult a structural engineer before installing an RSJ. A structural engineer can assess the structural requirements of your project, determine the appropriate size and type of RSJ needed using the right calculations and ensure that it is installed correctly to meet building regulations and safety standards.

Is there an alternative to RSJ?

While RSJs are commonly used for structural support in construction, there are alternative options available depending on the specific requirements of your project. Some alternatives to RSJs include timber beams, concrete beams and galvanised joists. However, it's essential to consult with a structural engineer so they can determine the best type of structural support for your building.

What is the difference between a universal beam and an RSJ?

In real terms, there is no significant difference between a universal beam and an RSJ. Both terms refer to the same type of structural steel beam commonly used in construction work. The term ‘universal beam’ is the technical term that’s been used in recent years, but the term RSJ is still used among both builders and property owners. Both refer to beams with a characteristic I-shaped cross-section.

How long does it take to fit an RSJ?

How long it takes to fit an RSJ depends on what it’s being used for, how big the RSJ is and whether any additional structural work is required in the process.

Generally, the installation process can take anywhere from a few hours to a full day, including preparation, lifting, positioning, and securing the RSJ in place. However, more complex installations may take longer and may require multiple days to complete.

It's essential to factor in additional time for any necessary adjustments or finishing touches to ensure the RSJ is properly installed and meets safety standards.

Type of opening	Average time to fit
External single doorway	1-1.5 days
External window opening	1.5-2 days
Bifold doors	2 days
House extension	2-3 days
Loft conversion	3 days

Can I fit an RSJ myself?

It’s strongly recommended that you don’t try to fit an RSJ yourself. RSJs provide essential support to your home, and if not installed correctly it could have disastrous consequences. Plus, there are strict building regulations surrounding the strength of RSJs based on the weight they need to bear, so you should hire a structural engineer and a specialist builder to ensure you get this right.

It’s easy to get quotes for RSJ installation with HaMuch. Simply post your job and we’ll send it to builders in your area who are happy to help.

Estimates for common extension builder jobs;

Rates for other trades