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Introduction to heavy trailers

Important vehicle standards for heavy trailers.

This content is for general information only. It does not, and is not intended to, provide legal or technical advice or to represent a legal interpretation of the matters it addresses.

The differences between a light and heavy trailer:


A light trailer has a design gross vehicle weight (DGVW) of 3,500 kg or less.


A heavy trailer has a design gross vehicle weight (DGVW) of more than 3,500 kg.

Heavy trailers

Heavy trailers fall into two weight categories


O3 - Trailers with a maximum mass exceeding 3,500 kg but not exceeding 10,000 kg


O4 - trailers with a maximum mass exceeding 10,000 kg, e.g., heavy trailers including articulated trailers


The difference between a semi-trailer and a trailer


Trailers, also known as drawbar trailers


Semi-trailers, also known as articulated trailers, towed by a truck or agricultural tractor


Heavy trailer walkaround checks

Driving for work involves a risk not only for the driver, but also for fellow workers and members of the public, such as pedestrians and other road users. View our walkaround checks webpage


Frequently asked questions

O3 Trailers: DGVW* = more than 3,500 kg but less than 10,000 kg.

O4 Trailers: DGVW* = more than 10,000 kg.

*Design gross vehicle weight (DGVW) is the weight of the trailer including the maximum load it can carry as per manufacturer’s design specifications.

See our  Weights and Dimensions Leaflet which outlines the maximum weights permitted under Irish law for various tractor and trailer combinations. If your load exceeds this limit you may be able to get an abnormal loads permit. For further information on abnormal loads permits see weights and dimensions webpages.

Check the trailer’s weights and dimensions plate or authorisation plate. If your trailer does not have one, contact the original manufacturer or authorised distributor. If they are no longer in service or the trailer brand is unknown contact an existing trailer manufacturer who may be willing to plate it for you.

If your heavy trailer has a maximum mass greater than 3.5 tonnes (Category O3 and O4*), it needs to be licensed with your local motor tax office.


*As defined in Regulation (EU) 2018/858.

No. Under EU law, mutual recognition of national approval schemes is only permissible between EU Member States.

This will depend on whether it is a European approval or a UK approval.

If the trailer has EU type-approval or an Irish national IVA, it can be licensed in Ireland. Trailers without EU type-approval or without an Irish IVA, cannot be licensed in Ireland. 

If your trailer is category O3 or O4 (greater than 3,500 kg design weight), this will fall within the scope of roadworthiness testing. It is important to note that regardless of whether your trailer needs to be tested or not under road traffic law, the owner and or driver must ensure that it is maintained in a roadworthy condition and is safe at all times when used in a public place.

Trailers of category O3 and O4 (greater than 3,500 kg design weight) need to be fitted with a weights and dimensions plate. Contact the original manufacturer or authorised distributor.

If they are no longer in service contact an existing trailer manufacturer who may be willing to plate it for you.

You will need to contact the Motor Tax Office when licensing an imported trailer.

Since 29 October 2012, depending on the trailer category, new trailers required type-approval and must be accompanied by either a certificate of conformity (CoC) or a national individual vehicle approval (IVA) certificate. This means that the trailer has met a European safety and quality standard.

The table below shows the enforcement dates for different categories of heavy trailers:

Categories concernedEnforcement dates
Incomplete and complete trailers of Categories O3, O4 29 October 2012
Completed trailers of Categories O3, O4 29 October 2013
Special-purpose trailers of Categories O3, O4 29 October 2014


Consult owner’s manual or contact original manufacturer or authorised distributor. Then check the DGVW of the trailer. Always ensure that this does not exceed the towing capacity of your vehicle.

No. Trailers with air operated brakes and a DGVW exceeding 3,500 kgs do not require the use of safety chains.

For multiple trailers of any kind attached to a vehicle a duplicate of the registration plate shall be exhibited on the back of the rearmost trailer. At all times the towing vehicle and trailer number plates should match.

Under UN-ECE Regulation 48 reversing lights are mandatory on O2, O3 and O4 category trailers (trailers with a maximum mass over 750 kg) but are optional on O1 category trailers (trailers with a maximum mass less than or equal to 750 kg).

Reversing lights are compulsory on all new trailers manufactured since 29 October 2012.

Side lamps Marker lamps
Side lamps are located at the fronts of a trailer, display a white light to the front and a red light to the rear. For testing of side lamps, see HCV Testers' Manual Marker lamps are fitted to the side of a trailer and are amber in colour and must be less than or equal to 7 Watts. For testing of marker lights see HCV Testers' Manual

 Yes. In order to be able to carry the maximum axle weight brakes are required on all axles.

A large tractor is permitted to tow two trailers and exceed 18.75 m in overall length but NOT in any town with a population exceeding 10,000 people. The limit for this combination is 22 m. For vehicle combinations that carry abnormal or indivisible loads see our weights and dimensions webpage

Yes. A certificate of roadworthiness has a detachable disc that you are legally obligated to display on your vehicle either on the windscreen or in the case of a goods trailer can be displayed on the chassis or as close as possible to the registration plate where it can be easily inspected.

In Ireland a drive axle tyre on a trailer must adhere to certain rules;

  • each tyre to ensure that the tyre load index rating for a single fitment tyre is not less than 50% (or for a twin fitment tyre is not less than 25%) of the design axle weight.
  • all tyres on the same axle must be of the same structure and nominal size.

Further details regarding tyres can be found in HCV Testers' Manual.

Under European and National Type Approval Requirements a set of safety and quality standards for newly manufactured vehicles must be met before they can be sold or registered throughout Europe and that if rear ground clearance of a vehicle exceeds 550mm then some form of underrun needs to be added. See section 6 of the HCV Tester's Manual detailing underrun limits.

Related pages

Commercial Vehicle Roadworthiness Test (CVRT)

About Commercial Vehicle Roadworthiness Testing (CVRT) in Ireland. Why it matters, where CVR tests are carried out, where to book and manage your CVR test.

Commercial Vehicle Operator Risk Indicator (CVORI)  

Commercial Vehicle Operator Risk Rating Indicator (CVORI). How and why CVORI works and how to access your CVORI rating with your CVRT online account.

Weights and dimensions

Maximum permitted weights and dimensions for commercial and agricultural vehicles and trailers on Irish roads.

Vehicle components

Information and regulations for important vehicle components including lights, seat belts, glass and tyres.


Road traffic legislation for vehicles in use on Irish roads. Type approval legislation for new vehicles. Irish and EU safety and environmental standards.