Mastic Universal J-Block Siding Mounting Block, 5" x 6" and 9" x 16", 100+ Colors
Mastic universal J-Blocks are two-piece vinyl mounting blocks that create a smooth, stable surface for mounting lights, outlets, faucets, doorbells, house numbers and mailboxes on siding. They adjust from 1/2 in. to 1-1/4 in. to fit a range of sidings, including insulated vinyl. Choose from standard 5" x 6" or oversized 9" x 16" blocks in 100+ colors.
Key Features:
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Two-piece adjustable design: The two-piece block adjusts from 1/2 in. to 1-1/4 in. for installation over a variety of siding thicknesses.
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Fits insulated siding: Adjusts to mount over insulated vinyl siding as well as standard profiles.
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Color matched: Available in more than 100 colors to match or coordinate with leading siding manufacturers' colors.
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Stable mounting surface: Creates a smooth, stable surface for installing exterior accessories.
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Large-item ready: Ideal for installing large items such as lighting fixtures, house numbers and mailboxes.
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Multiple sizes: Standard 5" x 6" blocks plus oversized 9" x 16" blocks for bigger fixtures.
Specifications Table:
| Specification |
Details |
| Brand |
Mastic |
| Sell unit |
1 piece or carton depending on color/size |
| Product line |
Mastic (Ply Gem) universal mounting blocks |
| Available colors |
121 color/size variants listed, including AW White, Almond, Victorian Gray, Misty Shadow, Deep Granite, Brandy Wood, Natural Slate, Scottish Thistle, Portsmouth Blue and more |
| Available sizes |
5" x 6" and 9" x 16" |
| Dimensions |
5" x 6" (standard) or 9" x 16" (oversized/XL) |
| Material |
Vinyl |
| Adjustment range |
1/2 in. to 1-1/4 in. two-piece design |
| Intended use |
Mounting lights, doorbells, electrical outlets, water faucets, house numbers and mailboxes on exterior siding |
Fit and Compatibility:
Designed to coordinate with Mastic and other leading vinyl siding colors, including Mastic Structure insulated siding. Sold as individual blocks; fixtures, lamps and mounting fasteners are not included.
Frequently Asked Questions (FAQ):
Q: What comes in the package?
A: One J-Block mounting block in the color and size you select; the fixture or accessory to be mounted is sold separately.
Q: Which size block should I choose?
A: Use the 5" x 6" block for outlets, faucets and small lights, and the 9" x 16" oversized block for large fixtures, mailboxes or house-number groups.
Q: Will it work with insulated vinyl siding?
A: Yes, the two-piece design adjusts from 1/2 in. to 1-1/4 in. so it installs over a variety of sidings, including insulated vinyl.
Q: How do I match my siding color?
A: Mastic blocks are available in more than 100 colors; pick the numbered Mastic color that matches your siding (for example AW White, 53 Almond or 36 Victorian Gray).
Q: How is it installed?
A: Separate the two halves, trim the opening if needed, place around the fixture opening, and fasten both halves to the sheathing following the included instructions.
Q: Do colors match exactly across brands?
A: They are made to match or coordinate with leading siding manufacturers' colors, but verify against a physical sample before ordering large quantities.
Expert Verdict — Reviewed by the SuperArbor Pro Desk
Order J-Blocks at the same time as the siding so the color dyelot matches; the 5" x 6" size covers the vast majority of exterior outlets and standard wall lanterns. For coach lights, mailboxes or grouped house numbers step up to the 9" x 16" XL block. Install blocks after the weather-resistive barrier but before setting final siding courses, and seal the perimeter per the fixture manufacturer's instructions.
Have exterior electrical outlet and light fixture connections made or inspected by a qualified electrician and seal penetrations to keep water out of the wall cavity.
Choose your Mastic J-Block color and size today for a clean, weather-tight mounting surface on any siding.
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