The current economic climate that has seen large numbers of retrenchments and resizing or closing of businesses has created a demand for recycled old buildings in declining established residential suburbs to be converted into workshops, offices and showrooms. The most favoured being those adjacent to local suburban business areas located along main arterial roads or rapid transport systems.
In their simplest form these premises merely require converting rooms in a house to offices with a reception area and adding a few steel framed carports. However, once it becomes necessary to increase the floor area the choice is between going up or out. The most common choice is to build an extension out into the garden or service area behind the building with a structure covered by an almost flat mono pitch metal clad roof, the apex of which is tucked under the eaves of the original pitched roof. Invariably the new roof is tucked behind parapets for aesthetic reasons and some form of rooflight is included to increase the amount of natural light within.
This is one instance where it is essential to employ the services of an experienced and conscientious professional architect rather than a person who simply draws plans. There is a large number of ‘architects’ who choose to circumvent the requirements of mandatory regulations, disregard the merits of specifying voluntary national standards, use inappropriate cut and paste contractual notes and/ or specifications (or neither), plus leave the resolution of functional details and choice of materials to builders and other tradesmen. All to the detriment of the owner. Heaven forbid you have one of these as principal agent.
When it comes to small builds there are very few competent builders available and even fewer skilled roofing contractors. This is the realm of chancers, fly-by-nights, and believers that any roof can be effectively sealed with paint-on membrane or expanding foam.
When it comes to the design of flat (less than 5° slope) metal clad roofs there are a number of mandatory and practical requirements;
1) SANS 10400-L: 2011 limits the slope to a minimum of 11° for conventional pierce-fixed profiles and 3° for concealed-fix profiles.
2) Sheets to be in a single length from apex to eaves i.e. no end laps.
3) Side laps of pierced-fix profiles to be sealed with reinforced butyl based sealer strips.
4) Junctions between roof cladding and walls to be comprised of two parts i.e. standard head or sidewall flashing covering the greater of 230mm or two ribs and a counter flashing fixed into the wall. Under no circumstances are the two components to be mechanically fixed to each other.
5) Gutters to preferably be fitted to the outside of walls.
6) Gutters to be sized to facilitate the easy clearing of leaves and debris.
7) Gutter to be fitted with hail guards in areas prone to large falls of small hail.
8) Rooflights to be of the out-of-plane variety. It is impossible to achieve a long term durable seal between translucent (plastic) and metal roof cladding on a flat roof.
9) Small penetrations through the roof cladding to be sealed with proprietary flexible flashings. Large (greater than 300mm) penetrations to be fitted with two-part flashings plus back flashing to apex.
For details of hail guards and flashings please refer to the SAMCRA website.