Two heights, and you buy for the taller one
Grade under a deck is never level, and the height from the bottom of the frame to the ground can easily vary by six or eight inches across one side. That matters twice. The average height gives you the area and therefore the quantity, and the greatest height gives you the width or length of material you have to buy, because every piece has to be cut from stock that reaches the tallest point.
The calculator asks for both for that reason. Board stock is priced on the length you cut from, not the length you use, so a run that averages 30 inches but peaks at 38 is a run of 38 inch cuts with the bottoms scribed. Lattice in 4 by 8 sheets is more forgiving up to 48 inches and completely unforgiving past it.
Then take the gap off. Skirting run into the soil wicks moisture up the end grain, holds it against the bottom of the frame, and gives insects a bridge from ground to structure. Two inches of daylight at the bottom is a common allowance and it costs you nothing but a scribe line. It is deducted from the covered height here, which is why the covered figure is smaller than the height you entered.
Ventilation is the whole reason to leave gaps
An open deck underside dries out after rain. A closed one does not. The ground under a deck stays damp, the deck above it stops that moisture escaping upward, and if the skirting is solid the air has nowhere to go. The result is framing that stays wet, fasteners that corrode, and an environment that suits fungus, insects and animals looking for somewhere sheltered.
How much open area is enough is a code question and it varies, so this page takes the ratio as an input. The common form is one square foot of net free opening per some number of square feet of enclosed ground, and the number in that ratio is what you get from the building department. What the calculator does is tell you the open area your chosen skirting actually delivers, so you can compare.
| Skirting | Open fraction | On 100 sq ft of skirting |
|---|---|---|
| 5-1/2 in boards, 1-1/2 in gaps | 21% | 21 sq ft |
| 3-1/2 in boards, 1-1/2 in gaps | 30% | 30 sq ft |
| Lattice, typical open pattern | 35 to 50% | 35 to 50 sq ft |
| Lattice, privacy pattern | 10 to 20% | 10 to 20 sq ft |
| Solid panel | 0% | Nothing |
Note the spread within lattice. Products sold under the same name range from genuinely open to nearly solid, and the privacy versions exist precisely to block airflow and sightlines. Measure a square of the actual product rather than assuming, or take the manufacturer figure and put it in the field.
A screen behind the skirting changes the arithmetic. Insect mesh stapled to the back of open skirting keeps things out and reduces the free area by whatever the mesh blocks, which for fine mesh can be half. If you are screening, allow for it in the open percentage rather than pretending the gaps are still fully open.
Where the access panel goes
Everybody puts the access panel in the least visible corner and everybody regrets it. The panel should be where the things you will need to reach are. That means the hose bib, any drain cleanout, the low point where water collects, and the shutoff for anything that runs out to the deck. If the deck covers a basement window well or a dryer vent, the panel goes there.
Size it for a body, not for an arm. Under about eighteen inches of clear height you are not crawling under anything regardless of how wide the opening is, and the panel is really an inspection hatch. Above that, thirty inches wide is about the minimum that a person plus a light plus a tool can pass through without a struggle.
Frame the opening properly and fasten the panel with screws. A panel nailed in place is a panel that gets destroyed the first time it comes off. Make sure it swings or lifts clear of the ground in front of it, which on a sloping site means checking the low side rather than assuming.
What skirting does not do
It is cladding. It carries nothing, braces nothing, and does not make a deck stiffer, so nothing about the skirting compensates for a frame problem. It also does not keep water away from the structure. If water pools under the deck, skirting makes that worse by slowing evaporation, and the fix is drainage rather than a nicer panel. The hardscape slope and drainage calculator covers getting water away from the area in the first place.
The rest of the deck build is elsewhere: the deck framing calculator for what the skirting is hanging off, the deck post height calculator for the same sloping grade problem seen from the post side, the deck railing and baluster calculator for the guard above, and the deck sealing calculator for the finish, which should include the back face of the skirting boards before they go up. If the skirting is really a privacy screen, the privacy fence board calculator handles board and gap layouts at fence heights.
Questions people ask
How much ventilation does an enclosed deck underside need?
That is a code question and the answer depends on the code your jurisdiction adopted, on how the enclosure is classified, and sometimes on whether the ground is covered with a vapour retarder. The common form is a ratio of net free opening to enclosed ground area, and the ratio field on this page exists so you can enter whatever the building department tells you rather than trusting a default. What the calculator supplies is the other half of the comparison: the open area your particular skirting actually delivers, which for solid panels is zero and for board skirting is simply the gap divided by the board-plus-gap pitch.
Board skirting or lattice?
Board skirting gives you direct control over the open fraction, since it is just the gap divided by the pitch, and it can be made from the same material as the deck so it matches. Lattice is faster to install over a large run and comes in a wider range of open fractions, but the products vary enormously and a privacy lattice can be nearly solid. Lattice in sheets also has to be framed, since an unsupported edge waves about and eventually breaks. Cost tends to favour lattice on tall skirting and boards on short skirting, but that depends on your prices.
Should the skirting touch the ground?
No. Anything wooden in contact with soil wicks moisture, and skirting is thin, so it goes first and then hands the moisture to whatever it is fastened to. Keeping a gap of an inch or two also lets you scribe the bottom edge to follow the grade rather than fighting it, and it gives water somewhere to run out. On a site where the gap will simply fill with leaves and mulch, the answer is not to close the gap but to keep the material away from the mulch, which usually means holding the skirting up and letting the shadow do the visual work.
Do I need to seal the back of the skirting boards?
If you are finishing them at all, yes, and before they go up. The back face of a skirting board faces a damp enclosed space and gets no sun, which makes it the face most likely to absorb moisture and the one you can never reach afterwards. Finishing one side only sets up an imbalance that cups the board. The same argument applies to the end cuts, particularly the bottom end, which is the closest point to the ground and the most exposed end grain in the assembly.
How do I skirt a deck where the ground slopes a lot?
Scribe rather than step, if you can. Run the top of the skirting level with the deck frame, cut every piece to reach the tallest point, and trim the bottoms individually to follow the ground with a consistent gap. That is slower than a straight cut but it is the only version that looks deliberate. Buy material for the tallest point, which is why the calculator asks for it separately from the average. On a very steep run, dividing it into two or three level sections with a deliberate step is the honest alternative, but a stepped skirt needs the steps to look intentional and evenly spaced.