Deck skirt calculator
A skirt closes the gap under a deck. It is also the one part of the build that is measured at the highest point of the ground, not the average — which is why skirt estimates are almost always short.
Skirt dimensions
How this is calculated
- Area = perimeter × max height. Using the maximum, not the average, is deliberate: skirt panels are rectangles cut to the tallest point and then trimmed, and the offcut is waste.
- Lattice or panel count = ⌈area ÷ 32⌉ for 4 × 8 sheets.
- Board count = ⌈area × 144 ÷ (5.5 × board length in inches)⌉, with boards run vertically.
- Vent area = deck area ÷ 150, per IRC R408.1 for a crawl space with a vapor barrier. Without a vapor retarder the ratio tightens to 1:1500 net.
Worked example
72 ft of skirting, 30″ at the high corner, 8″ of slope, lattice:
- Area = 72 × 2.5 ft = 180 sq ft
- Lattice = ⌈180 ÷ 32⌉ = 6 panels
- Average height is 30 − 4 = 26″, but 6 panels at 32 sq ft each is 192 sq ft — 12 sq ft of waste, which is the honest number
- Vent area for a 240 sq ft deck = 1.6 sq ft, roughly two 12 × 10 vents
Ventilation is not optional
A skirt turns an open deck into a crawl space, and a crawl space with no airflow grows mould on the joists within two seasons. The code requirement exists for that reason, and it applies even when the skirt is decorative lattice with holes in it — the openings in a standard lattice panel are not counted as vent area unless the panel is rated as such.
Practically: install removable panels or access doors rather than sealing the perimeter, and keep the ground under the deck graded away from the house. A skirt that traps water against the foundation is worse than no skirt.
What the skirt board count leaves out
- It does not account for framing. Posts, rim joists and stairs interrupt the skirt; add 5–10% for the cuts around them.
- It does not check whether a skirt is permitted. Some jurisdictions treat a skirted deck as enclosed space and change the footing or ventilation requirements.
- It assumes vertical installation. Horizontal boards change the waste pattern and the fastening layout.
- It ignores the stair opening, which usually needs its own skirt and is measured separately.
Questions people actually ask
How do I calculate how much lattice I need for under a deck?
Multiply the perimeter you are closing by the height at the highest point, then divide by 32 for 4 × 8 panels. Using the high point instead of the average covers the fact that every panel is cut to the tallest dimension.
Does a deck skirt need vents?
Yes, if it encloses the space under the deck. The usual requirement is one square foot of vent per 150 square feet of crawl space area, and decorative lattice holes do not count toward it.
Can I close under a deck that is only 18 inches high?
You can, but access and airflow become the problem. Below about 24 inches most people use a fixed lattice panel with a removable section rather than a permanent enclosure.
What is the cheapest way to skirt a deck?
Standard 4 × 8 pressure-treated lattice, screwed to a treated frame, is the lowest cost per square foot. Composite panel systems cost two to three times as much and are chosen for appearance rather than price.