A shed owner wanted horizontal lap siding, painted to match a HardieBoard house, instead of the vertical T1-11 panels the shed designer used to assume. The designer now lays lap siding out course by course, in fiber cement (HardiePlank-type) or engineered wood (LP SmartSide-type). It prices the result and draws it in 3D and on the elevations.
What the makers' guides ask for. Both guides require a weather-resistive barrier behind the siding. Neither product braces a wall: IRC Table R602.10.4 lists wood structural panels as a bracing method, not lap siding. A T1-11 panel is its own sheathing; lap siding needs OSB (or let-in bracing) behind it. The designer puts 7/16" OSB and house wrap under every lap wall. The other figures come straight from the guides:
- James Hardie (HS11119): an 8-1/4" plank shows 7" to the weather, with at least 1-1/4" of overlap. A 1-1/4" starter strip goes at the bottom. Nails are blind, 3/4" to 1" down from the top. Planks butt in moderate contact over joint flashing. James Hardie's installer advice keeps butt joints on studs, two stud bays apart from course to course at 16" o.c.
- LP (LPZB0515): the plank is 7.844" wide and laps at least 1". The starter is 3/8" x 1-1/2". Butt joints get a 3/16" gap and are staggered from course to course.
- Both: the siding starts at least 6" above the ground, and the primer must be painted within 180 days.
Courses, not square feet. Each wall is laid out from the starter up at the product's exposure. Openings cut the courses, and the gables cut them to the rake. A run longer than a plank gets a butt joint on a stud centre, clear of the joints below. The designer's connection check confirms both on every joint: wood behind each plank end, and the stagger. It passes on all 224 sizes and spacings in the test sweep. On the default 8x12 no course needs a joint at all, because a 12 ft plank spans each long wall between the corner boards.
What it costs, at Home Depot prices from search-result copies seen on 2026-10-30, with the siding painted white:
| 8x12, painted | Siding and what goes under it | Whole shed |
|---|---|---|
| T1-11 panels | $542 | $2,968 |
| LP SmartSide 3/8" panels | $508 | $2,934 |
| Fiber cement lap | $781 | $3,261 |
| Engineered wood lap | $803 | $3,283 |
Lap adds $293 (fiber cement) to $314 (engineered wood) to the whole shed. That is not because the planks are dear: 47 fiber cement planks come to $499, against $515 for ten T1-11 panels. The difference is the wall under the planks:
- 11 sheets of OSB ($128);
- two more boxes of 8d nails to fasten the OSB ($54);
- two rolls of house wrap, tape and cap nails ($115).
Correction, later on 2026-10-30: the first version of this post said lap was $207 to $248 more. Those figures counted only the siding section of the list. They missed the extra boxes of nails for the OSB, which made the true difference $261 to $302 at the time. The figures above also include a later change: the courses are now evened out, at 6-1/2" on this shed instead of 7". That closes a 1/2" strip the top course used to leave under the eave, and costs three more fiber cement planks.
On a 10x8 the fiber cement wastes 16% of what it buys: 12 ft planks cut for 10 ft walls. Engineered wood comes in 12 and 16 ft, and the cut plan wastes about 3%. Untick 16 ft boards and the designer uses only 12 ft planks, which are easier to haul.
What would bring it down. James Hardie allows plank direct to studs at up to 24" o.c., and LP's 3/8" lap at up to 16". That would drop the $128 of OSB, but the walls would then need another way to resist racking, such as let-in diagonal braces. The designer does not model that yet. The wrap is required either way.
Prices are from search-result copies of store pages, some of them months old. Every line links its listing, and you can edit every price. The designer is a free planning tool: it is not engineered or checked against any code.