RESIDENTIAL DESIGN SERIES.

Attics 2

 

THE ATTIC ROOM Some Structural Know How for Dummies

Creating an attic room in your roof space is the type of renovation project that most owner builders can attempt successfully.

Before you proceed you must do your design and documentation submission, with council.

Clients do occasionally still ring, assuming, that because the work is internal or out of sight that a DA is not required.

My theory is that if there is any structural work  (BCA building code of Australia) or aesthetic changes to the appearance then you require the DA. An attic involves, structural floor, fire wall, structural beams that support both the rafters and the ground floor ceiling and the new floor joists, stairs and sky-windows, the famous Velux! Every cavity should be insulated.

To begin you will need a good sturdy stepladder and a tape measure. If you feel uncomfortable going up the stepladder then ring a professional. The difference between storage and habitable space is about. 50cm. If you have 2.5 or greater from top of ceiling joists to top of ridge then you have habitable space. Anything less and you are at the mercy of the particular interpretation of the BCA.

If your home has party walls with your neighbour or neighbours you will need a letter from the neighours allowing you “to use the wall for vertical and lateral support”. This letter will be required either when you submit the DA or after the DA has been approved. If in the worst case scenario your neighbour refuses you use of their half of the .230 wall then you will have to independently support both the fire wall and the structural floor. This is an expensive and intrusive exercise. You will also have to redraw your plans.

As a Roman senator once said to me,  “Prepare for the worst and hope for the best”. He was referring to the barbarians.

The new structural floor will run parallel to the ceiling joists (in most cases). Your ground floor ceilings will remain intact and the ceiling joists can be tied off to the new structural floor.

To create space in your attic, the hangers that support the ceiling joists as they span the room below, the purlins and collar ties that support and tie the roof together will all be removed.

To stop the walls from being pushed over by the weight of the roof or the roof or ceiling sagging these structural issues must be addressed in the design of your attic space.

Most designers run a universal beam at right angles to the floor and ceiling

joists. You can also build a plywood, webbed beam . This is both a beam and your room defining wall.

The beam is located at the edge of your room and is buried in the cavity. You may require a beam at either end of the attic. The new joists either sit on the flange of the beam or are tie to the beam. The beam is supported on the party wall and all loads are spread evenly.

You also support the rafters from this beam and the walls that define either end of your attic sit on the beam.

There are allowable spans for floor joists, ceiling joists and rafters and the someone who fully comprehends the BCA should prepare your specification and drawings.

The other major structural detail of the attic is the firewall that sits on the party wall. This must have a FRL (fire rating level) of 60/60/60. Most councils insist on either masonry or hebel brick.

Next month I shall give you more detail on finishes and building procedures.

Good building and design!

Note: The views expressed in this article are of a broadly based nature and are in no way to be taken as the basis of an individual design. Should a reader wish to pursue the ideas expressed then at all times we recommend they consult with the appropriate professionals.