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04 803 34 79

roof flashings

 Crown Roofing
Roofing and roof repairs for Wellington and the surrounding region.

Call 04 803 34 79


Roof Flashings 

Roof Flashings. Roof flashings are a common cause of leaks in roofs. If you would like a free roof inspection to check the condition of your roofing flashings, please call the number above. Lead flashing has been used for many hundreds of years and is still in common use today on concrete and clay tile roofs. It is a very durable material and properly installed can last in excess of 70 years. It can be used on valleys, chimneys, apron flashings and flat roofing. Lead rolls for roofing comes in varying sizes depending on the situation. As a rule of thumb, larger areas require thicker grades/codes of lead sheet. Under no circumstances should any length of lead sheet exceed 1.5 meters in length. Thermal movement is one of the main causes of roof flashing failure. Lead has significant thermal expansion properties, causing it to expand and contract considerably. This movement will cause the lead to split or crack, if over sized sheets are installed. This is a particular problem in Wellington, where strong sunlight can be replaced with heavy cloud cover in a matter of seconds. Thermal movement is also of prime importance during installation. Over fixed lead flashings prevent movement and will cause failure of the roofing system. It is now common practice to use an approved lead flashing silicone to fix the sheets into chased brickwork. The sheet is installed in the normal way, held by small lead wedges. A neat silicone fillet is then applied in place of the traditional mortar. This provides a watertight seal and allows a degree of thermal expansion. On valley installations a thermal sheath should separate the valley structure from the lead. The use of bitumen impregnated roofing paper is not recommended. Over time the bitumen melts and bonds the lead to the valley board restricting movement. Again, use the minimum amount of fixings to hold the lead secure and always keep the sheet size under 1.5 meters. It is important to follow the guidelines laid out by Branz when fitting roof flashings.

Flashing are pieces of sheet metal or other waterproof material fitted to prevent the passage of water into a building from an angle or joint. They protect the roofing from water ingress. Flashing usually operates on the principle that, for water to penetrate a joint, it must work itself upward against the force of gravity or in the case of wind swept water, it would have to follow a difficult path during which the driving force will be removed. In other words the flashing makes the roof watertight not waterproof.  Flashings may be exposed or concealed for protection. Exposed flashing is usually of a sheet metal, such as copper, aluminium, painted galvanized steel, stainless steel, zinc alloy, and lead.
Metal flashing may be provided with expansion joints on long runs to prevent deformation of the metal sheets due to expansion and contraction. The selected metal should not discolour or be stained by adjacent materials or react chemically with them. Galvanic corrosion is an issue with roofing when different metals are adjacent to each other. Roof Flashings hidden within a construction assembly may be of sheet metal or a water proofing membrane such as bituminous membrane or modern plastic sheet material, depending on the climate and the building design. Examples of this are secret gulley and valleys/ Aluminium and lead react chemically with cement mortar. Some roofing flashing materials can deteriorate with exposure to uv light and need to be protected from the sun. Roof Flashing can assume different forms. Around chimney openings the flashing is often made of lead. This may consist of a step flashing and a cover flashing.
Lead flashings can last in excess of 70 years if fitted correctly. Copper may also be used to form flashings around chimneys and can often be spotted by their green patina. Modern flashings come in the form of synthetic materials. Rubber, Plastic and product impregnated with bitumen are a few examples of modern roofing flashings.