Low Pitched Slate Roof with a 4″ Headlap

Posted By On 24 Aug 2018

The Trenton City Museum (formally known as Ellarslie / Henry McCall House) in Trenton NJ was built in 1848 for Henry McCall a military commander at the Battle of New Orleans.  In the 1880’s McCall’s son sold the house to the city of Trenton (Cadwalader Park).  Over the years the mansion changed hands and served as an ice cream stand, a monkey house zoo exhibit and restored in 1970’s as the Trenton City Museum.

Italianate style architecture the mansion has a low pitched roof. A slate roof should not be installed on a roof slope less than 4:12 as per the International Building Code (IBC). Anything below this slope increases the possibility of water migration from snow and ice back up and wind driven rains. By increasing the standard 3” headlap to a 4” headlap on a low slope roof system will protect the roof from water damage.

In North America, the quarries punch or drill slates for a 3” headlap. When provided with a quote, it will be based on the number of pieces per square (100 square feet) based on a 3” headlap. We supplied our North Country Unfading Black roofing slate 20” x 10” x ¼”  for the Trenton City Museum which required a 4” headlap due to the low slope.

A standard 3” headlap for a 20″ x 10″ slate would have 170 pieces per square.  A 4” headlap requires more pieces per square. The multiplier to find this quantity is 170 pieces (pieces per square for 3″ headlap) x 1.06 = 180 pieces required for a 4” headlap

Multiplier for some sizes:

20” x 10” = 170 pieces (3” headlap) per square x 1.06 = 180 pieces (4” headlap)

18” x 10” = 192 pieces (3” headlap) per square x 1.07 = 205 pieces (4” headlap)

16” x 10” = 222 pieces (3” headlap) per square x 1.08 = 240 pieces (4” headlap)

When the headlap changes from 3” to 4” so does the slate’s Exposure.  The formula is the slate’s length less the headlap divided by two.  The exposure for  20” x 10” x ¼” slate with 3” headlap will be 8.5”. When this changes to 4” headlap the exposure will be 8”.

For more information on standard sizes of slate please go to our FAQ page.

Sheena Owen

Sheena Owen began working for North Country Slate on October 12, 1999. As the Sales Coordinator, Sheena’s day to day duties include providing customer service, new business development, inside sales, handling all cross-border customs issues/documentation, arranging shipments, health and safety representation and social media marketing.

Sheena graduated from the University of Guelph in 1992 with a BA in English and Drama and in 1995 graduated with a Diploma in Radio and Television Broadcasting from Centennial College. Sheena’s first job was the Producer’s Assistant for TV shows including Robocop, F/X The Series and Nikita. She went on to becoming an Assistant Manager at Eddie Bauer. In her spare time, Sheena enjoys traveling, gardening, volunteering at her local soccer club and cheering on her kids at their sporting events.

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