Top Drawer Zinc Ltd

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Vented roof design for residential projects
The most common and the oldest type of roof design is a vented roof. Practically all residential roofs were built in this way. Zinc works fine with venting. If in any doubt use this method. The principal of venting is simple; A 50 mm air gap is either left at the top of the joists or rafters. This air gap must be left completely free for air to circulate from the eaves where the gutter is located to the wall where the flashing is located. For this air gap to work a full width slot at the eaves must be created of 10 mm to 15 mm. This is usually made just above the fascia. The gap must be covered with a fine mesh to prevent insects from entering. A full width slot at the top of the roof must be created during construction. The easiest way to do this is to leave the roof deck 25 mm short of the building.
Variations and recommendations on roof build up for Vented roofs
The best method of vented roof build up is;
The joists or rafters are fitted with a minimum spacing of 400 mm centres. A breather membrane is then fitted on top of the rafters to separate the air gap from the roof structure where all the insulation is fitted. The membrane also creates a waterproof barrier which can prove useful during the build. The clever part is to fix 50 mm battens on top of the rafters to create the 50 mm air gap. When fitting these battens you can create air paths around Velux windows or similar roof lights. You can create air paths thru' or around hips. You can even use the added depth of roof structure to create stepped valleys should they be required as part of the roof structure.
If there were no air gap or no circulation slots
VENTED ROOF DESIGN
If the roof was built without a clear 50mm air gap and no air slots the moisture would build up as there would be nowhere for it to escape. The moisture would attack any metal roof from the inside making it corrode. The wooden roof structure would resist the moisture attack in the short term as the timber will be preserved. In the long term and after the builders warranty has elapsed the timber roof structure will rot away.
Warm roof design for residensial projects
Warm roofs have more components and have more to go wrong. The specification for warm roofs for zinc or any other metal roofing is higher than other roofing materials. It is best to seek professional advice or stick with the vented roof. Warm roofs are more expensive and best left alone unless there are definitive reasons for using a warm roof system. If warm roofs are installed correctly they are fine!