Thirty small pieces, not one big one
Run an ordinary house and the opening inventory comes to something like forty individual pieces of mesh across five or six types: soffit vents, gable louvers, foundation vents, an under-deck skirt, and the odd appliance termination. The total area is trivial — a few square feet. The work is not, because it is forty separate cut-and-fix operations at forty separate ladder positions, and the mesh only comes in a roll of one width.
That roll width is what the takeoff on this page is really for. A gable louver cut piece at 21 inches across will not lie two abreast on a 24 inch roll, so it takes a whole run to itself and three inches of every foot is dead. A soffit vent piece at 11 inches goes two abreast on the same roll and uses half the length. Change the roll width to 36 and both improve. The calculation nests each opening type on its own, laying pieces with their short dimension across the roll, which is what anyone actually does with a roll of mesh and a pair of snips. It does not nest different opening types together, so the roll count is slightly conservative and the offcut is real rather than theoretical.
The trade nobody checks
Here is the part that matters more than the material count. A vent has a net free area — the actual open cross-section air moves through, which is always less than the size of the hole. Attic and crawl space ventilation is sized against that number. Put a mesh over the vent and you have multiplied it by the open area of the mesh, and the finer the mesh, the more you take away.
So a house that was adequately ventilated on Friday can be under-ventilated on Sunday, without anything visible changing, and the consequences of that show up as moisture in a roof deck or a crawl space over years rather than days. Nobody connects the two. The honest sequence is: total your existing net free area with the attic vent audit, apply the open-area percentage of the mesh you are considering, and see where that leaves you against the ratio you are working to. If the answer is short, the fix is more vent area rather than less mesh, and that is a roofing conversation.
| What you know | Where it comes from | What this page does with it |
|---|---|---|
| Opening sizes and counts | You, with a tape | Cut sizes, area, pieces, roll runs |
| Mesh open area percentage | The product data sheet | Free area remaining, and area given up |
| Mesh opening size that is acceptable | Your fire authority or building department | Nothing — this page publishes no mesh figure |
| Existing net free area of each vent | The vent product data | Nothing here — that is the vent audit page |
The openings people forget
Soffits and gable ends get remembered. What gets missed is everything at ground level and everything horizontal: foundation vents, the gap under a deck, the space behind a skirt, the open ends of a corrugated roof panel, and the gap where a garage door meets a slab. An under-deck skirt in the default list is a single line item and it is by far the largest area on it, which is worth noticing before you buy a roll sized for soffit vents.
Appliance terminations are on the list as a warning rather than as work. A dryer vent, a combustion air inlet and a bath fan outlet all move air for a reason, and putting a fine mesh across any of them has consequences that are nothing to do with the hazard you were addressing. Lint blocks screens. Combustion appliances starved of air produce carbon monoxide. Those go to the appliance manufacturer and a licensed contractor, and the dryer vent length calculator covers the airflow side of the duct itself.
What the page refuses to tell you
It does not say what mesh size is acceptable, what material it should be, which openings need screening, how a seam should be made, or whether the result complies with anything. Those are set by whoever adopted the rule where you live, and in wildfire country the answer often comes from more than one place at once — a state law, a local ordinance, and an insurance condition that is stricter than either. Ask the fire authority and the building department, and get the insurance version from your insurer in writing, because an underwriting requirement is not published in a code book. The ground around the building is handled separately by the defensible space zone calculator, and general background sits in the home fire safety guide.
Questions people ask
What size mesh should I use for ember screening?
This page will not tell you and it does not have a number hidden anywhere in it. Acceptable mesh material and opening size are set by whoever adopted the rule for your area, which may be a state law, a county or fire district ordinance, a building department, or a condition in your insurance policy — and more than one of those can apply at once with different answers. Ask the fire authority and the building department, and ask your insurer in writing. Once you know which product you are using, its data sheet gives the open area percentage, and that is the figure this page asks for.
Why does the calculator care about the open area of the mesh?
Because screening a vent reduces the air that passes through it, and attic and crawl space ventilation was sized on the assumption that it was not there. If a mesh is 60 percent open, then covering an opening leaves 60 percent of its free area, and the ventilation total for that assembly has dropped accordingly. That does not produce a dramatic failure — it produces moisture in a roof deck or under a floor over a period of years, which is exactly the kind of problem nobody traces back to a weekend of work. Run the numbers through the attic vent net free area audit before and after.
How is the roll count worked out?
Each opening type is nested on its own. The cut piece is laid with its shorter dimension across the roll, the calculator works out how many fit abreast, and the pieces are laid out in runs down the roll. Roll length used is the sum of those runs, plus your waste allowance, divided by the roll length. It does not nest different opening types together, so if you are careful with the offcuts you will use slightly less than it says. If a piece is wider than the roll it is counted as seamed strips and flagged, because a seam is a detail with requirements this page has no business inventing.
Should I screen the dryer vent and the combustion air inlet?
Take that one to the appliance manufacturer and a licensed contractor, not to a takeoff. Those openings exist to move air for a specific reason and restricting them has consequences unrelated to the hazard you were addressing: lint accumulates against a screen in a dryer duct, and a combustion appliance that cannot get air produces carbon monoxide. They are on the default list to make sure you count them and then think about them separately, not to suggest that the same treatment applies.
What does the extra material past the opening do?
It is the margin that gives you something to fix through. The cut piece runs past the hole by that amount on every side, so a 16 by 8 inch opening with a 1.5 inch margin becomes a 19 by 11 inch piece, which is what drives the area, the roll usage and the perimeter fixing count. It is a fabrication allowance you choose, not a specification. If the product you are using or the authority you are working to specifies how the edge is captured, that governs, and this field just has to match whatever that turns out to be.