Simply stated, base attachment is the installation of secondary attachment at an angle change to prevent the field membrane from moving. Mule-Hide requires base attachment at angle changes of 2 inches per foot or greater (see Figure 1). Two areas where many contractors become confused are base attachment on fully adhered projects and at raised roof edges.
Many contractors believe that base attachment is not required when the field membrane is fully adhered. The membrane is glued down, so there’s no need to fasten it too, right? That would be like wearing a belt and suspenders at the same time, right?
That’s the wrong way to look at base attachment. True, the membrane is adhered to the roof and it should not move much, if at all. But what happens if it does move? Base attachment provides an important second line of defense to keep the system in place if the adhesive bond becomes weakened or broken (see Photo 1). Base attachment also helps prevent serious structural damage caused by unrestrained membrane, such as edge metal being pulled off or masonry parapet walls toppling over (see Photo 2).
Raised roof edges are when the metal edges, normally installed flush with the surface of the roof, are raised slightly and installed on top of the wood blocking (see Photo 3). They fall somewhere between a standard drip edge and a short parapet wall, and they occur quite often on the rake edges of sloped roofs to create a constant elevation at the perimeter.
When working with a raised roof edge, many contractors misinterpret the base attachment requirement of "any angle change of 2 inches per foot or greater" to mean "any elevation change of more than 2 inches." Figure 2 illustrates the difference and why the best way to address a raised edge is to install a tapered edge that will eliminate the need for base attachment.