Horizontal Lap Siding in HardiePlank: Profiles and Reveals

The Problem It Solves

Lap siding displays every layout decision in long straight lines. A first course a few millimetres out of level becomes a visible taper at the soffit. An exposure that drifts between elevations makes two walls look like different products, and courses that ignore window heads and sills leave thin slivers of plank that look patched. James Hardie’s own instructions warn that irregularities in framing and sheathing can mirror through the finished application. Most lap-wall problems start before the first nail.

Joints come next. A plank is 3,660 mm long, so wide elevations need butt joints, each one a place water can reach the wall. On a direct-applied wall in Calgary, the Canadian product evaluation calls for factory-finished ends meeting over a joint flashing 150 mm wide that laps 25 mm onto the course below, with the joint lined up on a stud. What often goes in instead is two field-cut ends filled with caulk. Caulk ages, unsealed cut edges take on water, and joints stacked one above another read as a seam.

The third problem sits behind the planks, where a common shortcut is new synthetic housewrap with planks nailed straight over it. CCMC evaluation 12678-R states that direct-applied cementitious claddings were not evaluated in contact with polymer-based membranes, and that asphalt-impregnated, paper-based membranes must be used. Both routes work, building paper with planks applied directly or housewrap with a furred cavity, but half of one with half of the other matches neither evaluation.

How It Works

HZ5 is James Hardie’s product line for cold, freeze-thaw climates, part of what the company calls its HardieZone system of matching products to climate, with a surface James Hardie says it engineered for paint adhesion and moisture resistance. Its Canadian instructions state the plank is non-combustible when tested to CAN/ULC-S114, with a flame spread rating of 0 and smoke developed of 0, and James Hardie Canada notes that fire resistance does not extend to applied paints or coatings. On hail, the City of Calgary’s climate-ready guide calls fibre cement one of the most resilient siding types for wildfire, hail and high wind. No siding carries a hail rating, though, and very large hail can still crack a plank.

The lap does the water management. Each course overlaps the one below by at least 31 mm, and blind nailing places the fasteners in that overlap, where the next course covers them. Face nailing is reserved for high-wind code requirements and heads sit snug, not driven into the board. Where the lap wall meets a band, a roof line or a change of material, Z-flashing sheds water outward with a 6 mm gap above it that stays open, and kickout flashing at roof-to-wall intersections turns water into the gutter rather than down the planks.

What Decides the Specification

Plank height and exposure are the main design choice. HardiePlank HZ5 is made in heights from 133 to 305 mm, and the exposure you see is the plank height less the lap, which cannot drop below 31 mm. A narrow exposure gives a traditional wall; a wide one reads calmer and more modern but depends more on flat framing and careful joint placement. The wall itself sets limits too: the planks fasten to studs at no more than 610 mm centres or directly to sheathing of at least 11.1 mm OSB, and very long walls need thermal and moisture movement considered.

The finish decides several details downstream. ColorPlus is James Hardie’s factory-applied, oven-baked finish, available subject to dealer availability. James Hardie does not recommend caulked joints on it, and handling changes: nail heads are not caulked, the protective laminate comes off after each course, and large damaged areas are replaced rather than painted. Primed plank leaves colour to the paint you choose but must be painted within 180 days with 100% acrylic, never stain, oil-based paint or powder coating. James Hardie offers a limited warranty on its siding and on ColorPlus finishes; see James Hardie Canada’s current warranty for terms.

Around Calgary

Across southwest communities such as Bridlewood and our own Evergreen, largely built through the 2000s, and southeast lake communities like Auburn Bay, where a house has builder vinyl with a fibre cement or stone accent on the front elevation, re-siding the remaining walls in lap plank starts with measuring the reveal on that front accent so new courses carry around the corners at the same line. Hail matters in the south too: the July 15, 2023 storm damaged siding and roofs in Calgary’s south centre neighbourhoods. In Okotoks, which exempts minor exterior repairs, a full re-side is confirmed with the Town’s permit services first.