Evaporative Cooler Roof Jack Installation: Swamp Cooler Curb, Flashing, and Leak Prevention

By The Roofing Brief Team. Last reviewed: October 2026.

Swamp cooler roof jack installation means cutting an opening sized to the jack (an 18 by 18 inch jack for 3,000 to 4,500 CFM down-draft coolers, a 20 by 20 inch jack for 5,500 to 7,000 CFM units), sliding the jack’s flange up under the shingle courses, fastening and sealing it, and setting the cooler on a separate leg kit so the jack carries air, not weight. Dial Manufacturing’s Multi-Pitch roof jack fits flat roofs up to a 6/12 pitch, with a 17 3/4 inch discharge opening on the 3,000 to 4,500 CFM size and 19 3/4 inches on the 5,500 to 7,000 CFM size, and Dial warns that its jacks “are intended for use as air distribution plenums only and not as supports for coolers.” The flashing is where leaks start, and the support is where sagging framing starts; the step flashing rules that apply at a chimney apply here too.

The short answer: what an evap cooler roof jack actually is

“Roof jack” in residential plumbing means a small flashing collar for a vent stack, covered in our roof vent pipe boot guide. On a swamp cooler it means a sheet-metal plenum with a roof flange: the bottom flange sits on and under the roof covering around the opening, the top is leveled to receive the cooler’s discharge, and a slide-in or barometric damper closes it off in winter. Dial’s Multi-Pitch jacks adjust from flat to 6/12 and its Universal Down Draft Cooler Installation Kit (PN 7826) covers flat to 4/12, so on steeper roofs a sheet metal shop builds a custom curb. IRC Section M1413.1 requires evaporative coolers to be installed per the manufacturer’s instructions, on a level platform, and with potable water backflow protection under Section P2902, per the IRC M1413 text on UpCodes.

Roof jack vs roof curb: terminology

Sellers use the terms interchangeably, but the parts behave differently. A manufactured roof jack (Dial 78317 or 78318, Hessaire 12735 or 12736) is a non-structural air plenum with a flange, and the cooler stands on a leg kit beside it. A site-built roof curb is a framed box, usually 2×6 or 2×8 lumber skinned in galvanized sheet metal, that can carry the cooler and is flashed like a skylight curb. The rest of this page uses “jack” for the manufactured plenum, “curb” for a site-built box, and “flashing” for the weatherproofing at the roof line.

Tools and materials

Item	Spec	Source for the spec
Roof jack, 3,000 to 4,500 CFM coolers	Dial 78317 Multi-Pitch, 17 3/4 in discharge, flat to 6/12	Dial product page
Roof jack, 5,500 to 7,000 CFM coolers	Dial 78318 Multi-Pitch, 19 3/4 in discharge, flat to 6/12	Dial product page
OR universal kit	Dial PN 7826: jack, barometric damper, slide-in damper, 12 self-tapping screws, apron, collar, 18 in x 7 ft flex duct; flat to 4/12	Dial PN 7826 instructions
Leg kit	Dial round-corner kit: two 30 in legs, two 20 in legs, flat to 6/12; Phoenix kit: two 29 in uprights, two 19 in short legs, four 4 in feet, sixteen 1/4-14 x 3/4 in self-tapping screws	Dial listing; Phoenix kit 1-999-2323
Step flashing	Not less than 4 in high and 4 in wide	IRC R905.2.8.3
Roof cement	ASTM D4586 asphalt roof cement	ASTM standard
Ice and water shield	ASTM D1970, 36 in wide	Maker data sheets
Fasteners for the leg feet	Matched to your roof type (not supplied in the Phoenix kit)	Phoenix kit 1-999-2323
Jig saw or reciprocating saw, drill with 1/4 in nut driver, level, caulk gun, tin snips	For the deck cut, screws and leveling	Dial and Phoenix instructions

Buy the jack that matches the cooler’s discharge before you cut anything. A Home Depot shopper Q&A on the Dial 78317 page notes that an 18 by 18 jack will not fit a smaller duct without an adapter, and that the smaller duct restricts airflow; a site-built curb only makes sense when the cooler’s discharge or the roof pitch falls outside the manufactured sizes.

Which roof jack fits your roof and cooler? Decision table

If your situation is	Use	Why
Down-draft cooler, 3,000 to 4,500 CFM, roof flat to 6/12	18 in jack (Dial 78317, 17 3/4 in discharge)	Dial’s published size and pitch range
Down-draft cooler, 5,500 to 7,000 CFM, roof flat to 6/12	20 in jack (Dial 78318, 19 3/4 in discharge)	Dial’s published size and pitch range
Hessaire DX4800 or DX6800	Hessaire PN 12735 (18 x 18) or PN 12736 (20 x 20) with leg kit PN 12728	Hessaire installation instructions
Roof flat to 4/12 and you want one box with flex duct	Dial Universal kit PN 7826, 18 in round hole	Kit is rated flat to 4/12 and includes 7 ft of 18 in flex duct
Roof steeper than 6/12	Custom curb from a sheet metal shop	Outside the Dial jack range
The 18 in or 20 in cut would sever a rafter or joist	Move the opening, or resupport with members spanning to the nearest uncut joists	Dial PN 7826 instructions
Penetration more than 30 in wide across the slope	Add a cricket or saddle on the up-slope side	IRC R903.2.2
Side-discharge cooler	Leg kit or wall bracket, no roof jack	No roof penetration needed

Step 1: locate the opening

Site the cooler over an opening that satisfies three constraints:

• Between framing members. Dial’s PN 7826 instructions say to place the 18 inch hole “while not cutting through any roof joists.” With rafters at 16 inches on center the clear bay is about 14 1/2 inches, so an 18 or 20 inch opening usually means cutting one rafter and heading it off (step 3).
• Above the duct run. Dial recommends a location near an existing air conditioning or heating duct, or near a central hallway, and its kit ships 7 feet of flex duct. Runs longer than 7 feet need more duct, and Dial cautions that over-stretching flex duct damages it.
• Close to a bearing wall. A 135 lb operating cooler (Hessaire DX4800, full) standing on four feet loads the roof deck at four points, so keep it near the wall plate rather than mid-span.

From inside the attic, measure off a fixed reference (the ridge, a gable end wall or a plumbing stack) to locate the rafters and mark the centerline of the opening on the underside of the sheathing. Drive a 16d nail up through the sheathing at each of the four corners of an 18 by 18 or 20 by 20 inch square; on the roof, those four nail tips mark the cut lines.

Step 2: cut the roof opening

From the roof, snap chalk lines between the four nail tips, drill a pilot hole and cut with a jig saw or reciprocating saw, as Dial’s PN 7826 instructions describe, through the shingles, underlayment and sheathing. Set the blade depth so it clears the sheathing without scoring the rafters.

Opening sizes published by the jack and cooler makers:

• 3,000 to 4,500 CFM down-draft coolers: Dial 78317 jack with a 17 3/4 inch discharge opening (sold as an 18 by 18 inch jack)
• 5,500 to 7,000 CFM down-draft coolers: Dial 78318 jack with a 19 3/4 inch discharge opening
• Hessaire DX4800: 18 by 18 inch square opening; DX6800: 20 by 20 inch square opening
• Dial Universal kit PN 7826: 18 inch diameter round hole, using the supplied collar or apron as the template

Match the opening to the jack’s bottom opening, not to the cooler cabinet. The cabinet is far larger (a DX4800 measures 32 by 31 inches in plan, per Hessaire’s dimension table), and the flange that seals the roof only covers a few inches beyond the cut.

Step 3: reinforce the framing

Where an 18 or 20 inch opening cuts a rafter, Dial’s PN 7826 instructions recommend that “the roof be resupported” by adding members above and below the cut, perpendicular to it, spanning to the two closest uncut joists. Skipping that header leaves the cut rafter hanging from the sheathing, which shows up later as a dip in the roof plane or cracks in the ceiling below; our guide to water stains on a ceiling covers telling a structural crack from a leak.

Header detail

From the attic, set a header across the up-slope and down-slope sides of the opening, matching the depth of the rafters (2×6 on 2×6 rafters, for example) and doubled, spanning between the two uncut rafters. Hang each header with a joist hanger sized to the lumber, such as the Simpson Strong-Tie LU26 for 2×6 members, and hang the cut rafter ends from the headers the same way. Trusses are a different case: truss chords cannot be cut without an engineer’s repair detail from the truss maker.

Side blocking

Add blocking between the headers along the two remaining sides, flush with the underside of the sheathing, so every edge of the 18 or 20 inch opening has wood under it for the jack flange fasteners. Dial’s kit does not include roof fasteners, so buy nails or screws long enough to pass through the sheathing into the blocking.

Cooler weight check

Cooler	Dry weight	Operating weight	Roof jack the maker specifies	Source
Hessaire DX4800 (4,800 CFM)	67 lb	135 lb (8 gallon reservoir)	18 x 18 in, PN 12735	Lowe’s spec listing; Hessaire instructions
Hessaire DX6800 (6,800 CFM)	Check the listing	Check the listing	20 x 20 in, PN 12736	Hessaire instructions

The water arithmetic is the part buyers miss: water weighs about 8.34 lb per gallon, so the DX4800’s 8 gallon reservoir adds roughly 67 lb, doubling the dry weight. Larger 6,500 CFM and up cabinets hold more water, so read the operating weight on the cooler’s own spec sheet before you size the headers, and if the cooler is larger than a 4,800 CFM unit or the roof is framed with 2×4 rafters or trusses, have a framing carpenter or engineer check the bay.

Step 4: install ice and water shield

Before the jack goes down, lift the shingles around the opening and apply ice and water shield to the bare deck, lapping it at least 12 inches beyond the jack flange on every side. ASTM D1970 self-adhering membrane seals around the nails that pass through the flange, which is the property that makes it the secondary barrier if the surface sealant fails.

On shingle roofs, re-lay the side and up-slope shingles over the membrane once the jack is set so they lap the flange and the 12 inch membrane border. On metal roofs, the membrane goes under the panels in the same position the original underlayment occupied, and the jack or curb needs a flashing detail matched to the panel rib (step 6).

Step 5: set the curb

For a manufactured jack, follow Dial’s sequence: slide the bottom flange up under the shingles from the down-slope side until the jack fully covers the hole, then nail or screw the flange to the roof. Level the top half on its rear hinge and lock each side with four self-tapping screws, because “both the Roof Jack Assembly and the Evaporative Cooler must be level,” per Dial’s PN 7826 instructions.

For a site-built curb, set it on the membrane and bed the base in a continuous bead of ASTM D4586 roof cement or butyl tape, then fasten through the base into the headers and blocking with galvanized lag screws, snugged without squeezing the sealant bead out of the joint.

Step 6: install base flashing

Water reaches the deck at three edges: the down-slope apron, the sides and the up-slope head. Each needs its own detail, and IRC R903.2.1 requires flashing around every roof opening.

Down-slope base flashing

The down-slope apron laps over the shingles below the opening; on Dial jacks it is part of the flange, and on a site-built curb it is an L-shaped piece of corrosion-resistant metal not less than 0.019 inch thick (No. 26 galvanized sheet, per IRC R903.2.1) turned up the curb face at least 4 inches and out over the shingles below. Do not lift the shingles under the apron; water must run off the apron onto the top of the course below.

Step flashing on the sides

The sides of a site-built curb get one step flashing piece per shingle course, each not less than 4 inches high and 4 inches wide per IRC R905.2.8.3, set so the piece above laps the piece below and the shingle above covers each step. The technique matches step flashing at a wall.

Up-slope detail (the leak point)

The up-slope edge takes all the water running down the roof above the cooler. Two acceptable methods:

• Cricket or saddle flashing. A small peaked diverter behind the curb that splits water around both sides. IRC R903.2.2 requires one on the ridge side of any chimney or penetration more than 30 inches wide measured perpendicular to the slope, and allows sheet metal or the roof covering material.
• Counter-flashing. An L-shaped piece fastened to the curb wall at least 4 inches up, matching the R905.2.8.3 height, above the shingle course, with the next course tucked under it, so water from above lands on the L and is turned around the sides.

An 18 or 20 inch jack falls under the 30 inch R903.2.2 trigger, so a cricket is optional there; on wide site-built curbs, or where a valley dumps onto the cooler, adding one keeps water from ponding against the up-slope face in a monsoon downpour.

Step 7: seal everything

Seal every flashing joint with ASTM D4586 roof cement or a sealant the flashing maker approves. Dial’s PN 7826 instructions say to “liberally apply roofing sealant to all bottom interfaces” of the jack and the shingles, to cover the exposed flange nails, and to caulk the top flange before the cooler goes on. Specifically:

• Where step flashing pieces lap each other (bedded in cement)
• Where step flashing meets the down-slope apron at the corners
• Along the top edge of every flashing leg on the curb wall
• Over every exposed nail or screw head in the flange
• The joint between the cricket and the curb back wall

Match the sealant to the substrate. ASTM D4586 asphalt roof cement bonds to bitumen and shingles; on galvanized or painted metal, check the sealant’s label for metal adhesion. Avoid silicone at the roof line, because asphalt cement and most roof sealants will not bond to cured silicone, which turns the next repair into a strip-and-clean job.

Step 8: mount the cooler

Set the cooler on the jack with its discharge opening aligned to the jack opening, then build the leg kit around it. In the Phoenix kit, with two 29 inch uprights and two 19 inch short legs (instruction sheet 1-999-2323), the four feet attach loosely to the uprights, the uprights fasten to the cooler with four self-tapping screws per leg “well above the sump water level,” and the feet are then fastened to the roof with fasteners suited to the roof type, which the kit does not supply. Seal every foot fastener the same way as the jack flange.

Vibration isolation

Rubber or neoprene pads under the leg feet and a flexible jack-to-duct connection keep blower vibration out of the rafters; check which isolators the kit includes.

Step 9: connect the supply duct

The supply duct runs from the jack collar to the supply plenum, a central hallway ceiling or an existing duct. Two details:

Duct-to-curb seal

In Dial’s kit, the apron and collar mount to the attic side of the 18 inch opening, and the flex duct slides over the collar by 2 to 4 inches, taped and, as Dial suggests, secured with self-tapping screws and washers over the duct wire. Any gap here dumps cooled air into the attic instead of the house.

Plenum or ceiling drop

For a hallway ceiling outlet, Dial’s instructions cut a second 18 inch hole between ceiling joists and build a “joist box” of blocking, caulked airtight, to carry the ceiling collar. Keep the flex duct as straight as possible with no sharp bends. In winter, close the jack’s slide-in damper (Dial instructs removing the top front cover plate to insert it) so warm house air does not escape up the duct; see our attic ventilation guide for the related winter sealing detail.

Step 10: water supply, drain, and overflow

A roof cooler needs a water supply line to its float valve (Hessaire’s DX series uses a standard 1/4 inch float fitting and a 60 psi maximum water pressure), a bleed-off drain line routed to a downspout or roof drain, and an overflow. Each one that passes through the roof or a wall is a separate leak point.

Water supply

Run the supply line through its own sealed penetration, not through the jack. A small roof penetration boot or a sealed sidewall penetration works. IRC M1413.1 and Section P2902 require backflow protection on the potable supply, and a shutoff valve in the attic or at the hose bib lets you drain the line for winter.

Drain and overflow

The bleed-off drain carries mineral-laden reservoir water. Route it to a downspout, gutter or roof drain, never onto the roof field.

Common installation mistakes

1. No structural reinforcement

A rafter cut for an 18 or 20 inch opening and left unheaded, or a cooler sitting on the jack instead of on legs. Dial states its jacks are “not as supports for coolers,” and its PN 7826 instructions say “an additional cooler support system, such as a Cooler Leg Kit, is required.” Retrofitting headers later means working from the attic around the duct.

2. No saddle on a wide curb

A site-built curb more than 30 inches wide across the slope with no cricket breaks IRC R903.2.2, and water ponds against the up-slope face in heavy rain. The resulting stain shows up on the ceiling along the up-slope edge and is often blamed on a different leak.

3. Step flashing with the wrong overlap direction

Step flashing pieces installed with the down-slope piece lapping over the up-slope piece instead of under it. Water runs into the lap and reaches the deck, the same failure as backwards roof flashing at a wall, and R905.2.8.3’s 4 by 4 inch minimum does not help if the laps run backwards.

4. Silicone caulk used as primary flashing seal

Silicone at the roof line blocks the next repair, because ASTM D4586 roof cement and most roof sealants will not bond to it. Use roof cement or a sealant the flashing or shingle maker lists.

5. Drain water on the roof field

A bleed-off line dumping onto shingles leaves white scale, holds water and stains painted metal. Route it to a downspout or roof drain.

6. No winter sealing

A cooler left with its duct open all winter pulls conditioned air up and out through the jack. Close the slide-in damper that ships in Dial’s PN 7826 kit (or the barometric damper on Multi-Pitch jacks) and cover the cooler.

7. Cooler tilted off level

Dial says both the jack and the cooler “must be level to obtain maximum cooler performance,” and IRC M1413.1 requires a level platform. A tilted reservoir starves one side of the pads. Level the jack top on its hinge and adjust the leg kit before you fasten the feet.

Roof-mounted vs side-discharge evap coolers

Coolers come in two configurations: down-discharge, where the cooler sits on the roof and blows straight down through the jack, and side-discharge, where the cooler sits on legs or a wall bracket and blows horizontally into a duct or window. Hessaire’s DX4800 and DX6800 ship in down-discharge position and convert to side discharge by moving the motor, venturi and cover panel. Roof-mounted down-discharge units are common in Arizona, Nevada, New Mexico and west Texas because the duct run is short and the cooler stays out of sight. If you are hiring for a rooftop cooler install in New Mexico, we cover roofers in Albuquerque, Santa Fe and Las Cruces.

Side-discharge skips the roof penetration entirely, so there is no jack to flash and no rafter to cut; Hessaire’s side-discharge setup uses the same PN 12728 leg kit. The tradeoff is a longer duct and a wall or window opening that IRC M1413.1 requires to be flashed per R703.4.

Climate considerations

Climate	What to plan for	Where the rule comes from
Arizona and Nevada desert	Hard, mineral-heavy water means more bleed-off; route the drain to a downspout, never onto the roof	Cooler maker’s bleed-off instructions
New Mexico and Colorado high desert	Snow load on the cooler and legs; check the ground snow load in your jurisdiction’s IRC Table R301.2 before sizing headers	IRC Table R301.2
West Texas	High wind; fasten all four leg feet into blocking, not sheathing alone	Leg kit instructions; IRC R301.2 wind speed
Freeze climates	Drain the supply line, close the damper and cover the cooler each fall	Dial PN 7826 winterizing note

Cost to install

Item or scope	Listed price	Source and date
Dial 78317 Multi-Pitch roof jack, 3,000 to 4,500 CFM	$97.48	Home Depot listing, October 2026
Dial 78318 Multi-Pitch roof jack, 5,500 to 7,000 CFM	$99.59	Littlewood listing, October 2026
Dial cooler leg kit	$44.98 (7808) to $50.94 (7805)	Home Depot and Lowe’s listings, October 2026
Ice and water shield, 200 sq ft roll	$80.52 (GAF WeatherWatch)	Lowe’s listing, October 2026
Hessaire DX4800 down-draft cooler	$569.99	Zoro listing, October 2026
Professional install, unit and labor	$1,200 to $3,600 on average	HomeGuide, 2026
Professional install, full range	$400 to $5,800, average $2,543	HomeAdvisor

For a DIY down-draft install with a Hessaire DX4800, the listed parts add up to $569.99 + $97.48 + $50.94 + $80.52 = $798.93 before sealant, fasteners, flex duct and electrical. A contractor price inside HomeGuide’s $1,200 to $3,600 band also covers the rafter header, flashing, duct, water line and wiring, which span roofing, carpentry, HVAC, plumbing and electrical work; ask for each of those as a line item.

When to call a pro

• A new roof opening on a roof steeper than 6/12, outside the Dial Multi-Pitch range
• Roof has any active leak history near the planned cooler location (diagnose first, see how to fix a roof leak)
• The opening cuts a truss chord, or more than one rafter
• Metal roof install (the curb flashing required by IRC R903.2.1 has to match the panel ribs; the panel-type and compatible-metal questions are the same ones covered in our guide to chimney flashing for a metal roof)
• Coolers larger than about 4,800 CFM, where the 135 lb operating weight of a Hessaire DX4800 is the low end

Frequently asked questions

What size roof opening do I need for an evaporative cooler?

Cut the opening to the roof jack, not the cooler. Dial’s Multi-Pitch jacks have a 17 3/4 inch discharge opening for 3,000 to 4,500 CFM coolers and 19 3/4 inches for 5,500 to 7,000 CFM. Hessaire calls for an 18 by 18 inch opening on its DX4800 and 20 by 20 inches on its DX6800. Dial’s universal kit uses an 18 inch round hole.

Do I need a roof curb or can I just bolt the cooler to the shingles?

Neither alone. The cooler sits over a roof jack that carries air, and on a leg kit that carries its weight. Dial says its roof jacks are intended as air distribution plenums only, not as cooler supports, and its installation kit instructions require a support system such as a cooler leg kit. Bolting a cooler straight to shingles leaves no flashed opening for the duct, which IRC R903.2.1 requires around roof openings.

Can I install an evaporative cooler on a metal roof?

Yes, but the flashing has to match the panel profile. Ask the panel maker for its curb or penetration detail for your rib spacing, or have a sheet metal shop build a curb that ties into the ribs with butyl tape and closures. Shingle-style step flashing (4 by 4 inch minimum under IRC R905.2.8.3) does not work across corrugated or ribbed panels.

How do I winterize an evap cooler on the roof?

Drain the reservoir (8 gallons on a Hessaire DX4800) and shut off and drain the supply line, close the roof jack damper (Dial’s universal kit ships a slide-in damper inserted behind the top front cover plate), and put a cover over the cooler. Closing the damper stops conditioned air from escaping up the duct all winter.

What happens if the curb leaks?

Water enters the attic through the deck opening, runs along framing, and shows up at the ceiling either directly under the cooler or at a low point downstream. The fix depends on where the leak originates. Most curb leaks are at the up-slope flashing or where step flashing transitions to base flashing. The repair is to lift the affected shingle courses, replace the failed flashing piece with step flashing of at least 4 by 4 inches (IRC R905.2.8.3), and re-bed in roof cement.

Bottom line

A swamp cooler roof jack is an air plenum, so a sound install comes down to four checks: the jack matches the cooler’s discharge (17 3/4 or 19 3/4 inches on Dial jacks), any cut rafter is headed off to the nearest uncut members, the cooler stands on a leg kit, and the flashing laps shed water at every edge with step pieces of at least 4 by 4 inches.

For a DIY job, buy the jack and leg kit the cooler maker names (Hessaire PN 12735 or 12736 with PN 12728, or the Dial equivalents) rather than building a curb. If you are hiring out, ask the contractor to name the jack and leg kit part numbers in writing, confirm a rafter header is in the scope, and look at the up-slope flashing before the cooler goes on. For the smaller-scale version of the same flashing logic, see our bathroom vent through roof guide.