Spark Arrestors and Ember Safety Above a Lemont Roof
By Dan Joel, Elite Sweepers Inc.
Most conversations about chimney caps focus on keeping things out: rain, animals, leaves. The screen does something else as well, and on a wood-burning system it is arguably the more important job. It stops what is coming up from getting out.
What a spark arrestor is
The short answer: the mesh screen around the sides of a chimney cap, sized to let smoke through while catching embers.
On most caps the spark arrestor is not a separate device. It is the screening built into the cap itself, and its dimensions matter: the openings need to be small enough to catch burning fragments but large enough not to restrict the flue or clog with creosote. That balance is why a properly specified cap matters more than a homemade screen.
Why embers leave a chimney at all
The short answer: a strong draft carries small burning fragments up with the smoke.
The same rising column that makes the chimney work will lift light material along with it. Certain conditions make it more likely: burning softwood or wood with a lot of bark, a very hot fire, dry windy weather that strengthens the draft, and burning paper or cardboard, which produces large light flaming fragments that rise easily. That last one is why burning wrapping paper and boxes in a fireplace is a bad idea regardless of anything else.
Where the embers land
The short answer: your roof, your gutters, your neighbors’ roofs, and whatever is dry nearby.
An ember leaving an uncapped flue lands somewhere. Gutters full of dry leaves are the classic problem, since they are directly beneath the flue and full of exactly the right fuel. Dry roofing material, deck surfaces, and dry vegetation in an autumn stretch without rain are all realistic landing places. It is a low-probability event on any single evening and a meaningful one over years of burning, which is precisely the kind of risk a cheap component is worth having.
How screens fail
The short answer: they corrode, they tear, or they clog.
- Corrosion. Screening is exposed to weather and to acidic flue gases from both sides. Galvanized mesh rusts through well before the cap body does.
- Tearing. Branches, ice and cleaning equipment all damage mesh, leaving a gap that looks like nothing from the ground.
- Clogging with creosote. On a heavily used flue, creosote builds up on the screen itself and restricts it. That affects draft and is a sign the flue needs sweeping.
All three mean a cap that looks fine from the yard may not be functioning. This is why the cap is examined during every inspection rather than confirmed with binoculars.
What actually reduces ember risk
The short answer: an intact screen, dry seasoned hardwood, sensible fires, and clean gutters.
The cap is the last line rather than the whole answer. Burning dry seasoned hardwood produces far fewer flying fragments than softwood, bark-heavy wood or anything with paper in it. Keeping fires at a reasonable size rather than building the largest one the firebox will hold reduces the draft velocity carrying material up. And keeping gutters clear removes the most likely fuel directly under the flue. Our DIY chimney maintenance guide covers wood selection and burning practice.
Getting the cap checked
Screening is evaluated as part of every chimney inspection, and our chimney caps and crowns page covers what we install. A creosote-clogged screen usually means the flue itself is due, which our chimney sweeping page covers.
For local scheduling, see our chimney service in Lemont, with costs on our pricing page. Elite Sweepers Inc is insured and bonded and works seven days a week, 7 AM to 7 PM. Call (708) 432-3977.
Spark Arrestor Questions
What is a spark arrestor?
It is the mesh screening on a chimney cap, sized to let smoke pass while catching burning fragments carried up by the draft. On most caps it is built into the cap rather than being a separate component.
Do I need a spark arrestor if I have a chimney cap?
A cap with intact screening already provides it. What matters is that the mesh is present and undamaged, because a cap whose screen has corroded or torn still looks correct from the ground while no longer catching embers.
Can embers really start a fire from a chimney?
It is a low-probability event on any given evening but a real one over years of burning. Dry leaves in gutters directly beneath the flue are the most common landing place, along with dry roofing and vegetation.
Why should I not burn paper or cardboard in a fireplace?
Paper and cardboard produce large, light, flaming fragments that rise easily on the draft and can leave the flue. They also burn very hot and fast, which is harder on the system than a wood fire.
Why is my chimney screen clogged?
Creosote building up on the mesh, which happens on heavily used flues. It restricts the opening and affects draft, and it is a clear indication the flue itself is due for sweeping.
What size mesh should a spark arrestor have?
Small enough to catch embers but large enough not to restrict the flue or clog quickly with creosote. That balance is why a properly specified cap works better than improvised screening added by hand.
Does a spark arrestor affect draft?
A correctly specified one has minimal effect. One that is clogged with creosote, damaged, or made from mesh finer than intended can restrict the flue enough to cause draft problems and smoking.
How often should the cap screening be checked?
Annually, as part of the inspection. Screens corrode and tear from weather, branches and ice, and the damage is not visible from the ground even with binoculars.
Published September 30, 2026
