1 metre long plastic duct designed to extend the periscope.
The periscope (PER) is able to extend to allow air to be deflected down across 5 courses of brick work. If the periscope requires extending 6 or more course then this duct is required. It will require sawing or cutting and normally, a 1 metre length of PERVEXT will be sufficient to extend several periscopes. Ideally, an electric radial saw set on high speed will make light work of cutting to length and provide a neat finish