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Slate Roof Installation – Underlayment

In this 8-part series we will show you some basic principles and good practices for installing a slate roof. In this segment let’s start with underlayments.

Slate Roof Underlayment: The Basics

It is not uncommon to find older buildings with slate roofs that continue to perform very well with no underlayments at all. Today underlayment is required by the International Building Code, as well as most local codes. The IBC requires a minimum of type I 15 lb felt and Ice and water shield on all eaves in regions where the average daily temperature during the month of January is 25 deg or less. The eave ice and water shield should extend 3’ past the exterior wall. We also suggest that Ice and water shield is used at details including valleys, rakes, gussets, low sloped areas, and built in gutters to name a few.

Traditionally 30lb asphalt felts have been used and perform very well. Modern Synthetic underlayments can offer advantages of a longer exposure time before being covered and can provide better grip for walking. With a wide range of underlayments available offered be sure to do some research to be sure you are getting the best underlayment for your project. On a standard sloped slate roof 3”of lap and 6” of side lap is suggested, and of course always install all underlayments with the laps shedding water. Low sloped roofs will have different underlayment requirements which can be found in the NSA slate manual. I hope this has helped you understand the basics of underlayments when installing a slate roof.

For part 2 of our series, on Eave, Gable & Cant Details, click here.

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The standard sizes for roofing slate start at 12″ in length and increase by 2-inch increments up to 24″ in length. Read More
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On average, North American produced S-1 rated roofing slate will cost $400 – $600 USD per square (100 square feet) or $4.00 to $6. Read More

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