How to Measure for a Chase Cover

The Watertight Lid

Why a new cover so often shows up not fitting, the one idea that makes measuring it simple, and the few numbers that truly matter.

The cover shows up — and it doesn’t fit

You measure the chimney, order the cover, wait two weeks, climb back up — and it won’t sit right. Proud on one side, rocking on a corner, the holes an inch off the pipes. Back it goes.

It is almost always the same mistake: measuring the flue, or the inside of the chase, or the old cover — anything but the surface the new one actually has to fit.

A chase cover is the metal lid on top of the chase, the boxy column that hides the pipes. Its job is to keep rain out, and when one fails it is because water found somewhere to sit. Measure a few things right and you get both at once: a cover that fits, and one that lasts. One idea makes it simple.

It’s a lid, not a plug

Picture a shoebox. The lid does not squeeze down inside — it drops over the top. A chase cover is the same. That is how the trade describes it, too.

A lid is sized to the outside of the box it covers.

The pipes do not set the size — they just poke up through holes in the top.

That picture is the whole job. The lid is as big as the top of the chase. Its sides hang down — the skirt. It has a hole for each pipe. And its top is bent so rain rolls off. From here, every mistake is just measuring the wrong part of it.

A chase cover is a lid over a box: sized to the outside of the top, with sides that hang down as a skirt, a hole and raised rim for each pipe, and a creased top that sends rain to every edge.

Measure the top, from the outside

The size that matters is the outside of the chase top, corner to corner — where the lid lands.

Don’t trust the shape. Chases lean out of square with age, and opposite sides rarely match. Measure all four edges; when two disagree, use the bigger one — the lid has to clear the widest point. Work to the nearest eighth of an inch.

Not the inside. Not the frame. The outside of the top.

Mark where the pipes come through

Each pipe pokes up through the lid, so each needs a hole — right size, right spot. Measure straight across the outside of each pipe (and don’t assume two match). Then pin its location from all four edges, not two. That is the step people rush, and the one that lands the hole where the pipe really is. More pipes, more sets of measurements.

You don’t pad anything yourself: the hole is cut a little bigger so the pipe slides through, and the cover is made to that size for you.

Length and width give you the lid. Each pipe then needs its width across the outside and its distance to all four edges — that set of four drops the hole exactly where the pipe is. The hole itself is cut slightly larger for clearance.

The skirt and the crease

Two parts do the weather-fighting. The skirt is the lip down the sides, hugging the chase like a shoebox rim — about three inches, enough to cover any bare wood and to fasten into. Its bottom edge folds outward into a drip edge that flicks runoff off the wall.

The crease is the part people skip, and it is why covers die. A flat lid puddles in the middle, and standing water rusts it through. A crease — a low ridge or dome — tips water to the edges so it never gets to sit. If you remember one thing, remember this.

A flat cover lets a puddle sit in the middle — the start of rust and leaks. A creased cover lifts the centre so water runs off both sides and the top stays dry.

The two caps at each pipe

Where a pipe comes through, the hole gets a short raised collar — a rolled rim, a couple of inches tall — that lifts the opening above the surface so water can’t run straight in. A storm collar then clamps the pipe and flares over the collar like a small umbrella, throwing runoff outward. Together they seal the one spot you cut open. Both are cheap parts; just make sure they’re on the quote.

At each pipe, two small parts finish the seal: a storm collar that clamps the pipe and sheds water outward, and a raised collar that lifts the hole’s rim above anything running across the top.

The four ways people get it wrong

Same mistake, four disguises — measuring something that isn’t the chase top.

The old cover. A lid someone else cut, maybe wrong to begin with, now bent and rusty. Measure the chase.
Inside the chase. The lid sits over the box, never down in it.
Just the pipe. That tells you where to cut a hole, not how big the cover is.
Assuming it’s square. It isn’t, quite. Take all four sides and use the longest.

And if the chase is a genuine mess — hard to reach, badly out of square, a tangle of pipes — hand it to a pro. At that point it is about conditions on the roof, not the tape measure.

Take this up the ladder

All four outside edges of the top, to the eighth of an inch — the longest wins for length and width.
Each pipe: its width across the outside, and its distance to all four edges.
Skirt depth (three inches is standard), with a crease and a collar at each hole.
Photos — straight down from above, and from each side.
Order it in 304 stainless, not galvanized — that’s the difference between one trip up here and another in five years.

Got the numbers? Punch them straight into the Kaminos Chase Cover Configurator — top size, skirt, hole layout, metal, and finish — and it builds the cover as you go. Order it there, or take the spec to whoever’s fabricating. Either way, no back-and-forth.

Inspired by the hearth, made for the modern home.