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Choosing the correct supporting beams is usually the most important structural question that needs to be answered when planning a new loft project.
Many newer homes don’t have internal load bearing walls and sometimes those older properties that were built with strong internal walls still don’t have sufficient strength to support the extra structural stress created by a loft conversion.
In most loft conversions steel beams are used to take the extra load and they are typically installed running from side to side of a property, i.e., from one party wall to another.
The positioning of the beams is of course vital and this is where an architect or structural engineer really earns their fees by making sure they draw plans that provide maximum load bearing support from the new beams.
The architect will most likely decide to site any new loft conversion beams as close to the edge of the rooms as possible.
They will be placed above the old ceiling joists with a gap of around a centimetre between the tops of the old joists and the new beams.
The most common type of steel beam is an RSJ or Rolled Steel Joist.
The thing that distinguishes RSJs is that they are always taller than they are wide with tapered flanges.
It is those tapered flanges that differentiate an RSJ from the similar universal beam (UB).
Rolled Steel Joists and Universal Beams are most commonly used when converting the loft though a universal column, which has a shorter depth than a UB, is generally used in shorter lengths when space is tight.
The final steel beam that is sometimes used in loft projects is the PFC or Parallel Flange Channel but these are usually used as replacements for roof purlins.
Steel doesn’t have to be used and more and more builders and architects are looking to use beams made from timber.
There are some advantages in using timber loft conversion beams, not least the fact that they can span longer lengths whilst unsupported and they are light, but steel is relatively cheap and is still used extensively.
Each builder or architect will have their personal preferences but there are basically two types of beam to choose from.
Steel loft beams tend to be the most commonly used as they provide incredible strength for their small size.
Certainly, in a project where space is at a premium these kind of beams are the easiest to use.
Their only disadvantage is the sheer weight of the beams which can make manoeuvring them very difficult in the confined roof space.
The alternative to steel is to use timber beams for loft conversions though they can often prove to be impractical.
Both types have their good and bad points and, indeed, many loft projects will employ both steel and timber beams in their final construction.
Steel Loft Beams
The most commonly used beams for loft conversions by UK builders are the UB or universal beam.
This type of steel loft beam is popular because they are very easy for timber joists to fit into because of their flat flanges that are parallel between top and bottom.
The RSI or rolled steel joists are similar to universal beams in that they are always taller than they are wide. But they differ from the UB in that their flanges are tapered which makes them less easy to use with timber joists.
The UB and RSI are the most commonly used steel loft beams but when space is a particular issue a UC or universal column can be used.
They have the same flat flanges as universal beams but they are usually used when the steel loft beam has to be incorporated within the new floor structure.
Timber Loft Beams
Although timber beams for loft conversions have traditionally been rather limited in their use because of their short span (typically only around 12 feet without additional support), factory produced ‘I’ beams are now being used in loft projects and modern house building.
Designed exactly like steel beams the ‘I’ beams have added strength as they have a central core made from composite wood and won’t shrink over time.
The drawback with these beams for loft conversions, which can be as strong and as wide as their steel counterparts, is that they are much more expensive than steel.
Apart from the relatively new ‘I’ beams there are two other timber loft beams sometimes used in loft projects.
Laminated beams are factory made from laminated and glued timber or from laminated veneer lumber (LVL). These beams are very strong and, importantly, can be cut to size without losing their strength.
The final kind of timber loft beam sometimes used in loft projects are ‘sandwich’ beams. These beams are usually made on site and feature two strips of timber with a steel plate sandwiched between them.
Because of their lack of real structural strength the main use for these beams for loft conversions are as floor trimmers.
For more info see the related pages below
All About The Structural Calculations You Need
The Importance Of Existing Internal Walls