Russell Thomson, Regional Sales Manager at Pasquill, the UK supplier of trussed rafters, argues that replacing attic trusses with open web solutions can mean benefits for builders
For some time now, builders have looked towards ‘Room-in-Roof’ or attic trussed rafters in order to provide additional living areas in roof spaces, thereby helping to optimise land usage.
However, even more benefits can be gained by using metal web to form the bottom chord of the attic truss.
In order to explain what metal web attic trusses are it is necessary to understand what standard trusses and attic trusses are. A standard trussed rafter roof incorporates triangulated timber sections positioned to form roof structures.
In the case of attic trusses the individual truss components are designed to allow ‘Room-in-Roof’ solutions using larger timber sections due to a lack of full triangulation.
When builders submit designs for attic trusses to Pasquill, we sometimes recognise them as an opportunity for conversion into a metal web attic truss solution.
This is particularly the case when heavy deflections, lots of load-bearing walls and MVHR systems (mechanical ventilation with heat recovery systems) are required by the customer. A metal web attic truss solution is ideal for tackling these challenges.
With standard attic trusses, ducting, pipework and cabling have to be run through attic void locations, which usually means longer, more complex routing. However, with metal web attic trusses, pipework and cabling, including the heavier MVHR elements, can be routed at any points throughout the metal web joists.
Metal web attic solutions can incorporate much greater spans than traditional versions.
Exact span maximums vary depending on factors such as pitches and loading. They therefore facilitate better design freedom, and are ideal for large room-in- roof applications. In addition to their strength and depth, they are in fact lightweight.
As metal web attic trusses are manufactured off-site and craned into position, they offer distinct health and safety advantages.
Here at Pasquill, for metal web attic trusses we ensure that there are no manual off-loading issues to contend with, because instead of installers having to access the bed of the delivery truck, straps around the metal web attics enable a crane to offload safely.
Contractors should also ask their supplier for a pre-delivery load plan to ensure that delivery vehicles are offloaded in the most time-efficient way.
We believe that the use of metal web attic trusses is set to rise, primarily in the housebuilding market where optimising land use is such a key part of maximising profits. We are also seeing their increased use in commercial projects too.
A great example of the use of this solution is at an impressive 290m2 five bedroom property currently under construction at Washingborough in Lincolnshire. In this project Pasquill created a bespoke design on behalf of Elite Timber Homes.
Here, we converted an original design using traditional attic trusses and incorporated posi-joists. This meant we were able to offer a combination of the strength of a steel web with the light weight of timber flanges to produce a ‘posi-attic’.
Manufactured off-site, the solution was delivered to site ready to be craned into position, supported by technical design drawings detailing exactly where each truss was to be positioned.
The feedback from the customer was that it had been “much easier on site, both from an installation point of view and from the perspective of positioning pipework and services,” than if they had gone with a traditional timber attic truss configuration.
A fascinating example of unusually large-scale posi-attics in a commercial setting was our manufacture and supply for a historic church in Nairn near Inverness.
The project was part of a conversion of a listed building into commercial premises. With a clear span of over 14 metres – supported only at the external walls – and a height of 7 metres the posi-attics used at the church were amongst the largest ever manufactured by Pasquill.