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Floors, walls and roofs

The JJI-Joist system is extensively used for domestic floors in the UK but its flexibility allows it to be used for domestic roofs and walls and larger commercial applications, and it has been combined with a number of other products to develop a range of bespoke systems to help solve a variety of common construction problems. To see examples of suitable applications of JJI-Joists and JJ-Beams in a broad scope of project types, view our case studies.


Historically, the vast majority of JJI-Joists have been used in domestic floors for houses and flats and, despite a significant increase in roof and wall application, this remains the case. 

The system is independent of the building construction method and can be used with traditional masonry construction or timber frame with equal ease, as well as being flexible enough to adapt to emerging construction methods like insulated concrete formwork, SIP panels and cross laminated timber (CLT).

Standard details exist to address common framing arrangements where airtightness, thermal and acoustic considerations must be met. In particular, there are several floor make-ups specifically aimed at meeting the stringent acoustic performance levels required for both intermediate and separating floors.  The pre-cut joists are quick to assemble onsite and the long lengths and dimensional stability improve performance and reduce differential movement. The 9mm OSB web material is easily cut and drilled to allow services of all common shapes and sizes to be routed within the floor depth. Where particularly large service holes are required, custom reinforcing metalwork is available in a range to match the joist sizes.

Our UK Technical Manual provides a comprehensive guide to the correct use of the JJI-Joists system and you can read it as a page turner here. Additionally if you register on the website you can download it as a pdf and hard copies are available from our Technical Support team. Additional advice can be obtained from our trained distributors or direct from the Timber Systems Division Technical Support team.


JJI-Joists are ideally suited for use as wall studs where their availability in depths up to 450mm allows designers to insulate external walls to unprecedented levels. Even when shallower joists are used the narrow web profile provides a restricted path to heat transfer (reduced repeated thermal bridging) when compared with solid timber.

James Jones & Sons recommend that JJI-Joists are incorporated into prefabricated wall panels in order to take advantage of the improved precision and quality typically available in a factory environment.

The structural design of JJI-Joist studs should be undertaken by a suitably qualified Engineer who should pay particular attention to buckling restraint, axial load distribution between inner and outer flanges and member to member fixings.

Download JJI-Studs Flyer (PDF 461 KB)

Roofs and lofts

JJI-Joists are ideal for use in large, flat roofs where their excellent spanning capability can be used to good effect. Given that most of the heat loss from a building is through the roof, JJI-Joists once again provide the opportunity to insulate this area of the building to unprecedented levels with ease. Roof cassettes up to 12m long can take weeks off the installation time for large, flat roofs.

Roof cassettes can also be produced as an alternative to attic trusses and other time consuming framing arrangements used to construct room-in-roof top floors on houses. This system is independent of the building construction method and can be used with traditional masonry construction or timber frame with equal ease.

The JJ-IntelliRoof™ brings the proven benefits of JJI-Joists to both the floor and roof elements of room-in-roof construction. The system consists of lightweight prefabricated cassettes to form the floor, followed by standard timber frame spandrel panels up to ceiling level. A top hat section is then assembled at ground level and lifted into place on top of the spandrels, resulting in a substructure ready to receive the roof cassettes. The roof cassettes are then lifted into place and fixed to the substructure, using proprietary fixings.

 Combining JJI-Joists with the Simpson JES product provides an innovative alternative to traditional loft conversion methods allowing the structural loft floor to be installed between existing trusses with the temporary removal of only two or three courses of roof tiles to provide access.

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