Squashed unsafe chimney rain cap (C) InspectAPedia TWChimney Rain Cap Clearance & Opening Dimensions
Chimney rain cap vent area requirements & codes

  • CHIMNEY RAIN CAP VENT OPENING SIZE - CONTENTS: what is the clearance distance between chimney rain cap top and flue opening top? What net free venting area should be provided by a listed or site-built chimney rain cap?
  • POST a QUESTION or READ FAQs about types of chimney rain caps & chimney caps / chimney crowns & their inspection, installation, troubleshooting & repair or replacement.
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Chimney rain caps dimensions, clearances, or free venting area:

This article describes the distance between the under-side of the cap of a chimney rain cap and the top of the chimney flue. We discuss the effective venting area provided by different rain cap designs and mesh or spark arrestor choices and we explain the possible safety hazards of using an un-listed chimney cap that does not provide adequate flue venting opening area.

The home made chimney rain cap and cover shown at the top of this page may not provide adequate flue exhaust area and if not, would be unsafe.

In this article series we illustrate common chimney cap & crown types, choices, & defects, and we cite pertinent chimney top cap / crown building defects, hazards, codes & standards for fire and other safety concerns.

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Chimney Cap Clearance From Flue: Chimney Cap / Screen Vent Opening Dimensions

Squashed unsafe chimney rain cap (C) InspectAPedia TW Chimney rain cap (C) Daniel Friedman

[Click to enlarge any image]

In the photo at above left are two rain caps: a rectangular one over a rectangular clay flue tile and a round rain cap over a round metal flue venting an oil fired heating appliance.

Watch out: At above right the photo shows a flat stone rain cap atop a stone or stone-veneer masonry chimney. If the installer left too little space between the under-side of this flat stone, or if the stone does not project sufficiently past the chimney top the chimney may be damaged by rain or wind-blown rain, makinmg it unsafe. Also, the chimney rain cap at above right has no spark arresting screen, making it non-compliant where such spark arrestors are required (E.g. California in the U.S.).

Rain caps atop chimneys are defined and explained at CHIMNEY RAIN CAP / RAIN COVER INSPECTION. This article, beginning below, discusses a concern of inadequate venting through a chimney whose rain cap does not provide enough vent exit area or space.

Reader Question: red rust and yellow substance at chimney top

We replaced our chimney crown a year ago and had the chimney company seal it with crown seal. There is a fireplace flue (unused) and a oil furnace flue. There is a red rust color coming from the area of the oil flue and cap as well as what a pears to be a small circle of yellow substance surrounding the SS flue pipe.

The chimney sealant has now peeled leaving a real mess. Photos are available if you need them If you have any ideas on what is causing this that would be great. - T.W. 18 Jul 2015

Reply: blocked flue? Corrosive & yellow staining effects high sulphur in heating fuel?

Moderator said:

Watch out: Urgent: That cap doesn't look right. Is the flue blocked?

If the flue is obstructed there are possible fatal CO(carbon monoxide) hazards. Be sure you have working CO detectors or shut off the equipment.

(Oct 14, 2015) Paul said:
The yellow substance mentioned in the reader question may well be sulphur deposits. Two possible causes suggest themselves: 1)The heating oil supplier is providing a sub-standard product; or/and 2) The heating oil holding tank has collected sulphur over time. The sulphur sinks to the bottom and accumulates with time, getting stirred up with each new delivery. The tank may need to be drained and cleaned.

Sulpher is corrosive. It would account for both the rusting and the yellowing.

Reader follow-up:

We have working CO detectors. The oil furnace company is here tomorrow and I will have them check the flue. The flue was cleaned last July by the chimney people. What do you mean by the cap doesn’t look right? Also, what is the yellow substance near the pipe? - T.H. 19 Jul 2015


I could not see that the chimney cap had adequate vent opening. Let me know what your oil heat tech thinks about the chimney top deposit. I'd be looking for a boiler leak or an odd combustion situation. We might see rust stains but not expect it at a stainless steel chimney and cap.


The right-hand round metal chimney flue in the photo at above left is covered by what appears to be a squashed-down metal chimney cap. As you'll read in the email exchanges below, the homeowner recognized that there was trouble from the red stains on the chimney concrete crown or top seal and and from the yellowed melting, bubbling sealant around the penetration of the concrete top seal by the round metal flue.

We think that there was probably inadquate exhaust clearance between the chimney rain cap and the top of the flue opening (blue arrow) that may have had the effects of directing flue gases and heat down onto the chimney top seal, thus causing both the melting bubbling sealant (yellow material & arrow) and the red stains (flue gas products or rust from the flue interior - red arrows).

Reader follow-up:

The oil service company just left. The furnace is in good condition and clean. He removed the flue pipe from the boiler into the chimney SS flue and it was clean. He looked inside the furnace for any water dripping
or buildup and saw nothing. These numbers don’t mean much to me but they may to you:

Efficiency: 82.5
Stack Temperature: Gross 490 Net 430
CO2 Reading: 11%
Smoke test: 0

Breach Draft: -.05

Over fire draft: -.01


The heating system numbers measured at the oil burner and flue look fine except I like to see a small trace of smoke. If the temperatures were high (say 600 or more) I'd think the system was running too hot, sending too much heat up the chimney, and maybe generating orange crud - but you're ok on that score.

Either we're looking at a component of flue gas that is condensing out and staining or we're looking at actual metal rust.

Or there is some third material that I couldn't see that explains the staining.

Are you burning straight No. 2 heating oil?

Reader follow-up:

We are burning regular #2 heating oil. I have a call in to our chimney company. The last time I spoke with them they suggested replacing the existing 304 flue cap with a 316 SS flue extension and a 316 SS Cap made with an insulated inverted cone to reduce staining similar to the photo at below left.

Metal chimney cap "Enviro_Stealth_ from Homesaver chimney rain cap -

Reply: lack of specifications of chimney rain cover net vent free area in product specs and codes

Above at left we see a double-insulated Class "A" chimney top rain lid or "rain cap" Enviro Stealth rain cover provided by Sleepy Hollow Chimney Supply, Ltd., a Brentwood New York company [Website:] . This chimney top rain cover uses a 2x12" mesh (or in California a 2x2" mesh) to keep animals out of the flue while providing a minimal restriction of the exiting flow of flue gases through the cap assembly. A cap for a 6-inch round flue retails for about $200. USD.

A similar product shown at above right is a stainless steel chimney cap from HomeSaver useing a 5/8" expanded metal lath type screen that meets fire safety spark-arresting specifications required by California and other areas and available from for about $150. USD.

Interestingly, and perhaps because variation in actual chimney cap designs such as the inverted cone in the above left chimney cap (orange arrow) makes difficult the comparison of the vertical clearance distance "h" (green arrow) or more accurately, a specification of the effective free venting area afforded by the rain cover, this dimension is rarely given in the product specifications for rain caps. Some building codes also omit this dimension. Yet the actual effective vent opening is a critical dimension for all rain caps and other chimney coverings.

5-inch Chimney Cap to Flue Clearance Space Recommendation

As you'll read below, in our opinion we think that 5" is a reasonable minimum dimension. Ideally every chimney cover or rain cap would include a specification of the net effective free opening for venting that the cap affords. 5-inches of opening space allows an extra safety margin for adequate venting that may be reduced by the mesh or screening, by the effects of the shape of the chimney cap, by variations in weather conditions, variations in stack temperature and other conditions.

As you'll read in a reader question and answer below where we do some calculations, a smaller cap to flue distance may still work and may still be safe provided there is objective data, not just arm-waving opinion to support that view.

Reader Question: minimum distance from chimney rain cap to top of flue

Brick chimney rain cap and cap - San Miguel de Allende, Mexico (C) Daniel Friedman(Nov 8, 2012) Devon Bowman said:

What is the minimum distance a Rain cap can be from the flu?



The critical dimension is not only the distance from the under-side of a rain cap to the upper edge of the chimney flue. We must also consider the total opening space around the rain cap. For example, the masonry rain cap shown in my photo at left is rather enclosed and may be inadequate, depending on how this chimney is used.

Similarly for metal rain caps or rain covers the presence of and type of spark arrestor mesh or screening may be a factor in chimney performance, particularly if small-sized openings in the screen become clogged with soot or creosote as can occur on wood fired heating appliance flues.

Minimum chimney cap vertical cap bottom edge to flue upper edge clearance distance as described below is described by the cap manufacturers we surveyed as 5". Less may restrict chimney draft.

CHECK with your local state or provincial fire and chimney code and your building department as different specifications may apply where you live. Some states are silent on this detail: For example in New York State see Title 27 / Subchapter 15 Chimneys and Gas vents you'll find "[1501.10] 27-865 Chimney caps is silent on dimensions.

I think the reason this particular clearance distance has not come up is that factory rain caps for metal flues are pre-fabricated - one would not modify that device and it would provide its own clearance by its design. This is true for UL-Listed pre-fab chimney caps that fit into masonry flues as well.

The typical manufacturer's / distributor's chimney cap measurement instructions for ordering address the flue inner dimensions into which the cap mounting sleeve must fit (for non-extended flues) or the flue's outer dimensions (for extended flues) over which the cap sleeve fits.

Typically manufacturers who discuss the vent opening refer to the actual effective opening - which is more important than the actual distance about which you asked, as the chimney cap screen height.

Watch out: The required screen height is really the actual opening availalable for venting, and this height may need to be further adjusted depending on the screen mesh opening size as that in turn also obstructs venting both on its own and by its tendency to collect soot and debris (smaller mesh means faster clogging and more obstruction).

Watch out: a chimney cap is usually described just by the dimensions that fit the flue size - the actual screen height or venting area is often not given.

Again, look for AT LEAST 5" of vertical screen opening height. This is not quite what you asked but it's the best reply.

OPINION: I would like to see the greater of 5" unobstructed vertical screen opening height or for larger diameter chimney flues, a vertical opening height that equals the largest flue top horizontal opening (for rectangujlar flues) or its diameter (for round flues). And that opening may need further adjustment for smaller mesh screen openings that obstruct draft.

Watch out: we also don't want an excessive vertical cleareance distance between the top of the flue opening and the lowest portion of the chimney cap cover, since if the opening were too all the rain cap no longer does a good job keeping rain out of the flue.

Safety warning: use only a listed chimney cap on your chimney flues.

Question: Chimney draft opening, chimney cap spacing vs. adequately-hot chimney flue

2016/09/19 Ronald Snith said:

I read your thoughts on the 5 inch clearance for the chimney caps . My question is seeing a wood stove flue needs to be hot as possible to prevent creosote build up does lowering the cap raise flue temperature.But still maintain enough draft ? Thanks For Your Help


I think that if you lowered a cap enough to restrict venting you'd raise flue temperature but I would not recommend that approach without one of us finding expert authority and/or research on the question.

If I need to restrict draft I think it's safe to do it at the intake end of the system where we can both keep an eye on what's happening at the fire and also adjust as needed without climbing up onto the rooftop in the middle of snowy icy winter when our woodstove is hard at work.

Consider that inappropriate or too much chimney draft constriction is unsafe on several counts including increasing the risk of a creosote fire.

Watch out: as we explain at CHIMNEY SHROUDS, use of an improperly designed or wrong-sized chimney cap or shroud can cause a dangerous chimney fire. Chimney caps for downdraft and rain protection must be installed according to either their listings (Such as U.L. in the U.S. or CSA in Canada) and must also be installed following the chimney cap manufacturer's instructions or the applicable building code where the building is located.

Chimney Cap Design & Sizing Research

Question: is 2-3" enough space between the chimney cap and the flue opening? How to Calculate Chimney Top Vent Area Size.

Chimney cap with small clearance to flue opening (C) CM2016/09/20 C.M. said:

I have recently had my chimney company install a liner for my oil burning appliance.
The top of the top of the flue liner is about 2-3 inches from the chimney cap. Is this enough clearance?

Reply: gut feel, opinion, vs. calculation vs. objective measurement data


Thank you for asking. My gut feel is that 2" is probably not enough, but my gut may be wrong. Let’s try to get past general opinion (that 2” between cap underside and flue vent top edge) would be insufficient - which is my general view.

You checked that that fireplace top mounted damper still works OK right? And when it's open, the two flue openings are not at the identical height, right?

Looks like 2 or 3 generations of repairs to the masonry chimney. Be sure your chimney sweep confirms that the fireplace flue is also intact and safe to use.

Here’s some objective data about chimney opening size controlled by the space between the cap underside and the top of the flue opening.

Let’s calculate the actual chimney vent opening size by computing its area.

Chimney vent area with no chimney cap

The area of a 6-inch round flue is A=πr2=π·32≈28.27433 or about 28 square inches.

Watch out: we are not recommending leaving off a chimney cap, as you can read in this article series.

Chimney vent area with a chimney cap with 2" between Cap & Flue Opening

Now let’s cap the flue and raise the cap 2-inches above the very top edge of the flue.
The area of an entire cylinder that’s 6” round and 2” tall is

A=2 π r h + 2 π r2

A = area available for venting out flue gases via the chimney top

π or pi is 3.1416

r = the radius of the circle or 1/2 the diameter or in our case 3”

h = the height of the cylinder wall or in our case 2” - the open space between chimney cap underside and flue vent top edge.

For a 6-inch diameter round flue with a 2-inch space between the chimney top and the flue opening, the available exit vent area is

A=2 π r h + 2 π r2 = 2·π·3·2 + 2·π·32 ≈ 94.24778 or about 94 square inches minus the areas of the top and bottom of the cylinder. That's because this 94" is the area of the entire 2-inch tall cylinde, including its sides, top, and bottom. For chimney venting we want to know just the area of the side of the cylinder of space given by the chimney cap:

From the 94 we subtract back out the area of the flat circle so that we can see just the area provided by the 2” tall x 6” diameter opening space under the chimney cap.

That space, the size of the opening between the cap and the flue top will be 94.2 minus 2 x the flat area of the 6-inch circle (the top and bottom of a cylinder)

That’ll be

94.2 - (2 x 28.2) = about 37 square inches. That's how much vent opening is given by a 2-inch opening between the chimney cap underside and the top of the chimney flue.

[Really? Yeah I know, we could have simplified our starting formula but I included the whole cylinder formula to help avoid other readers from falling into the trap of too-hasty use of online calculator web pages. - Ed. ]

The 2-inch space between chimney cap and flue top is giving us more space (37”) than was given by a 6-inch flue that just opened to the air with no cap at all (28”). The chimney cap is giving us 1.3 times (37 / 28 ) the flue vent cross section in opening size.

So your installer is probably right that based just on vent opening area size, your chimney cap is probably OK. That’s before considering other cap benefits such as preventing downdrafts and it’s before considering whether the chimney size and height are giving adequate draft to the particular heating system being vented in the first place.

Watch out: A vent opening outlet size is important in its control over chimney draft, but other factors such the obstruction or draft reduction due to the screening on a chimney cap, spark arrestors, and variations in fuel, temperature outdoors, temperature inside the flue, wind, chimney height, use of a barometric damper, and use of a draft inducing fan would also be important.

Make an objective, actual measurement of chimney draft

A smarter objective measurement would be done by a trained, qualified heating service technician who measures the actual draft at your heating system both over the fire and in the breech (that's above the heater, below any barometric damper or draft hood) when the heater is up to operating temperature, and under various weather conditions. (Hot weather, downdraft, and other conditions change the draft).

You don't say what fuel is being burned to be vented by your chimney. Earlier in this article we give as an example that for a typical residential oil fired boiler or furnace, we want the draft to be

Breach Draft: -.05
Over fire draft: -.01 to 0.2


Since inadequate combustion air can also show up as poor draft, also see COMBUSTION AIR REQUIREMENTS

Watch out: as we explain at CHIMNEY SHROUDS, use of an improperly designed or wrong-sized chimney cap or shroud can cause a dangerous chimney fire and a blocked chimney flue or inadequate draft can also risk fatal carbon monoxide poisoning of building occupants.


As a separate functional and safety check, to evaluate the size of the newly-lined chimney flue itself, you can also compare the input BTUH of what's being vented into the chimney along with the fuel type (perhaps natural or propane gas?) and look up the required chimney size accordingly. See FLUE SIZE SPECIFICATIONS


Continue reading at CHIMNEY POTS & DECORATIVE TOPS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.



Or see these

Chimney Top Cap, Crown, Rain Cap Article Series Contents

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CHIMNEY RAIN CAP VENT OPENING SIZE at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


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