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Mounting the A/C and the fan

Jack's question killed a design: if the appliance has to be cut into the roof and sealed to the roof, why is it sitting up on the solar rack? It shouldn't be. The appliances seal at roof level, an internal frame under the roof carries them, and the solar rack is a separate structure that stays clear of them. It starts with the factory structure the frame attaches to — GM's own body-structure figure, not a concept — then four drawings, the assembled section first, because that is the one that answers the sealing question. Drawn by Mickey. Concept geometry only: no member sections, no spans, no hole patterns, no fastener specs.

2separate structures — exterior solar rack, interior reinforcement

0tall weather throats between the roof and the rack

1.6mm — the composite roof panel, which is why the frame exists

0roof bow dimensions published — the manual renders the cage, never dimensions it

What changed, and what it replaces

Supersedes the exterior appliance bayThe orange A/C and fan bays bolted on top of the crossbars in the superseded rack drawings — is no longer the preferred direction. Its job moves inside the van. It is not the same frame relocated underneath; it is a different design problem with different members, different connections and a different load path. Do not end up building both and accidentally sharing one load path between them.

The earlier concept raised the A/C and the fan to rack height. That looked tidy in a drawing, and it quietly created the hardest unsolved problem on the roof: a tall, structural, weather-sealed passage bridging the gap from the roof opening up to a raised appliance, on a unit whose manual never approved that mounting height in the first place.

"Don't rely on unsupported roof sheet" was never the same statement as "everything must sit on the exterior rack." The roof sheet needs support underneath it; it does not need the appliance lifted away from it. Those are two different fixes, and the second one costs a lot more than it buys.

The revised architecture, in one line each

Exterior solar rack. Carries solar. Crossbars get arranged around the A/C and the fan so they never obstruct airflow, the fan's lid travel, or service access. It no longer carries any appliance load.
The appliances. Sit at their roof openings, at roof level, sealed by their own factory gasket / flange systems, exactly like a normal roof install.
Interior reinforcement frame. A new, separate frame under the roof. Carries appliance weight, clamp reactions and screw engagement into verified vehicle structure.
The roof sheet. Participates locally in a supported sandwich and passes compression through. It is never asked to span and carry an appliance on its own.

The structure this actually attaches to

not a concept drawing — GM's own body-structure figure, 25MY Chevrolet BrightDrop 400/600 Body Builder Manual, page 9

GM body structure figure. On the left, a BrightDrop body with the body side outer and roof shaded green and labelled COMPOSITE PANELS, 1.6mm thick, with a translucent roof option CC4. On the right, the same body with its skins removed, showing the load-bearing cage: members shaded red are aluminium, grey are steel. In the cargo section the cage is two heavy longitudinal cant rails running the full cargo length at the top of each side wall, a run of transverse roof bows spanning between them, and vertical side posts aligned with the bow stations carrying down to the belt rail and floor structure. — **The green is the roof. The red is what holds it up.** Body side outer and roof are **composite panels, 1.6 mm thick**. The load-bearing cargo structure is an aluminium cage: two longitudinal **cant rails**, **transverse roof bows** spanning between them, and **vertical side posts** at the bow stations. Figure from GM's Body Builder Manual, reproduced here as the structural reference for the frame below.

Every earlier version of the drawings below ran the load out to pink members captioned "symbolic — not actual BrightDrop factory rails." That caveat existed because we had no documented structure to draw. We do now. Sheets 01 and 02 have been redrawn against this cage and no longer contain invented endpoints; sheets 03 and 04 are unchanged and still carry the old symbolic pink. What the documented cage changes is four things.

The roof skin is not structure, and that is now sourced rather than assumed. A 1.6 mm composite panel is a weather enclosure. The argument for an internal frame no longer rests on a general claim about "thin roof sheet" — it rests on the manufacturer's own material callout.
There is a candidate load path, and it is short. Appliance → cradle → roof bows → cant rails → side posts → lower body and floor structure. The frame is no longer hunting for somewhere to land; the landing points are factory members that already carry the roof. But the number of bows is not settled. Sheets 01 and 02 draw the cradle spanning two adjacent bows to make the path legible — two bows are not proven sufficient, and the A/C's actual footprint and loads may require a wider span, more bows, or a different support arrangement entirely.
The roof-rail airbag envelope is documented, which is not the same as clearance. The roof-rail airbags are 1800 mm wide and 1087 mm tall and are shown in the cab (manual p35). That figure is a deployment envelope drawing — it is not permission to ignore wiring, harnesses or VIN-specific variation anywhere else in the vehicle, and it does not certify any particular cant-rail station as clear. Forward of the bulkhead the caveat plainly stands; aft of it, clearance is still something to establish on the actual van, not read off a figure.
The published no-drill / no-cut zones do not name the cargo roof — and that is not the same as clearing it. Manual p30 calls out no drilling above the HV battery, no cutting orange HV cable, no modification of HVAC plumbing, and cab SRS / airbag hazards. They are not all underbody. An area left unshaded on p30 is not an authorisation to drill it — the page marks known hazards, it does not issue permission. Drilling or through-bolting any factory member on this vehicle remains unapproved until GM documentation or a qualified inspection says otherwise.

Use the numbers, not a generic boxBrightDrop 600 cargo area: 168.9 in internal length, 83.68 in wall-to-wall width, 82 in internal height with the door closed (76 in with it open). Wheelbase 183 in. Anything drawn or fabricated works to these, not to a sketch.

What the manual still does not give usIt renders the cage; it never dimensions it. Roof bow spacing, bow count and bow section are unknown — any spacing shown in a drawing is read off a picture and has to be measured on the van. Whether a bow may be drilled or through-bolted, and at what capacity, is unknown, so clamp-style attachment is the default and anything bolted needs confirmation. Cant rail section and capacity are unknown. Where the bulkhead falls relative to the first bow is unknown.

What page 9 is evidence of, and what it is notIt documents materials and topology — which members exist, roughly where, and what they are made of. It is not a roof-bow load rating, and the drawing alone does not establish exact bow or post alignment, bow count, or any dimension. It is also a 25MY document: applicability to the actual vehicle Jack ends up with has to be checked against that vehicle before anything is fabricated.

Tie-downs are not roof anchorsThe 8 fixed tie-downs on the 600 are rated 1,000 lb each and they are in the floor. They are not an attachment story for anything on the roof.

1 · Start here: the assembled section

this is the drawing that answers "how does it seal if there's a frame in the way"

Schematic cross-section through an appliance mounted at roof level, drawn assembled rather than exploded. Left and right halves show the same stack either side of a dashed centreline marked air passage. From top: the grey appliance body, teal factory gasket and support pads beneath it, a pale contour adapter where required, the thin blue-grey roof panel line, an orange structural backing directly under the opening and under the outboard pads, and a gold load-carrying reinforcement beam below that running out to pink vehicle attachment targets at both ends. A white vertical line marks a factory threaded rod passing through to a silver factory lower clamp bracket beneath the orange backing. Four keyed notes: A weather seal at the exterior mounting surface, B weight and clamp support carried by backing and beams, C outboard bearing pads also supported from underneath, D solar rack not in this stack. — **04 — the seal and the frame do two different jobs.** The gasket keeps water out at the exterior mounting surface. The backing and beams stop that mounting area flexing or crushing. **The opening through the middle stays clear** — the frame surrounds the air passage, it does not cross it. Drawn by Mickey. **Not redrawn:** this is a **generic sealing schematic**, and the pink endpoints in it are still symbolic. For BrightDrop body context see the factory structure and sheets 01 and 02.

outside appliance body · factory gasket and perimeter support pads · contour adapter only if the roof profile needs one

the roof roof panel with the cut opening — weather enclosure and supported skin, not an unsupported appliance platform

inside continuous structural backing at every bearing and clamp surface · reinforcement beams · factory lower clamp bracket · interior air-distribution box or fan garnish

Read top to bottom that is one continuous compressed stack. There is no gap, no riser and no tall throat anywhere in it. That is the entire point of the revision.

2 · Where the internal frame actually sits

GM's 2025 BrightDrop body-structure illustration, red for aluminum and gray for steel, is placed beside a simplified 600-proportion cargo-box cutaway. Both identify transverse roof bows B, longitudinal upper or cant rails C, vertical posts P and lower body/floor F. The cutaway adds an orange internal A/C cradle I spanning between two bows, with an open center and outboard bearing support. The conditional load path is A/C to cradle to bows to cant rails to posts and lower body. Three states are distinguished: GM-documented body topology, proposed added cradle, and connections proposed, not load-rated or approved. Bow spacing, count, section, crown, bulkhead position, drilling permission and bow/cant-rail capacity remain unverified; two bows are not proven sufficient. The 1.6 mm composite roof is not assigned unsupported appliance load capacity. The solar rack is separate. — **01 — the van is no longer missing.** GM's actual body cage sits beside the proposed internal cradle in a full cargo-box cutaway. **Existing structure is documented; the connection to it is proposed, not load-rated or approved.** Bow pitch, sections and actual attachment permission are still unverified. Drawn by Mickey.

The load path, written out

appliance → its own supported mounting surfaces → structural backing → internal frame → verified vehicle structure.

Not appliance → unsupported roof panel. Not appliance → ceiling lining or trim. And not appliance → solar rack, which is the path the old drawing implied.

A backing ring is not a load pathA ring bonded or screwed only to thin roof skin can distribute clamping pressure and still carry none of the appliance's road loads into the vehicle. Those are separate claims. And the existing roof ribs cannot be assumed suitable as the anchor either — that is a documentation-and-inspection question, not something a drawing settles.

The anchor is no longer a blankThis sheet no longer has invented pink endpoints. The cradle lands on the transverse roof bows, cant rails and posts documented on manual page 9, and the GM figure is shown alongside it so the two can be compared directly. What that closes is where the load goes. What it does not close is bow spacing, count, section and crown, and whether a bow may be drilled or through-bolted at all — the manual renders the cage without dimensioning it, so those stay measurement-and-inspection questions, and the connection itself is still unapproved and unrated.

The cradle drawn here exists to make the load path legible. Its span, section and the two-bow arrangement are illustrative — not proof that this geometry is necessary, and not proof it is sufficient. The fan in particular may need much less structure than the A/C if suitable vehicle structure is verified nearby.

3 · The A/C: clamped through a supported sandwich

Exploded A/C assembly above a BrightDrop-style roof bay with two transverse bows, upper side rails and posts continuing below. Numbered layers are the A/C, factory gasket and perimeter support pads, composite roof opening, rigid backing/interface, proposed orange cradle and factory lower bracket with rods. Separation is illustrative; installed mating faces touch. An enlarged inset shows a candidate saddle between the cradle and a factory-bow role, with the roof skin cut away only to expose the joint. The connection is explicitly proposed, not load-rated or approved; clamp access, grip, positive retention, local crushing, fatigue and bow capacity remain unresolved. No factory drilling, bolt pattern or torque is prescribed. Actual pad locations, roof crown, total mounting thickness and duct engagement must be checked. — **02 — the roof-level seal and the internal load path are separate.** The exploded stack shows both; the same sheet enlarges the candidate cradle-to-bow connection. Every air gap drawn between mating layers **closes in assembly**. **This is an unresolved connection study, not a clamp specification and not permission to drill a factory bow.** Drawn by Mickey.

The correction the square-frame idea misses

A ring around the opening is not enough. The A/C's own manual calls for perimeter support blocks outboard of the opening, and those blocks need structural bearing beneath them too. A frame that supports only the hole leaves the unit's outer bearing points sitting over unsupported roof panel — which is the exact failure the whole design is meant to avoid. Extend the members, or add supported landings where the real unit needs them.

Keep the factory attachment systemThe four supplied M8 rods and the factory lower mounting bracket are the clamp assembly. Do not substitute a generic bolt kit, and do not apply rack torques to it. Its own instruction is a low clamp torque; that figure has nothing to do with the reinforcement or rack joints.

Mounting thickness is a real constraint, not a detailThe rods, the duct and the interior air-distribution box all have to fit through the finished stack. Adding backing, adapter and frame increases that thickness. It has to land inside the manufacturer's permitted range — you do not get to change the mounting depth to suit the picture.

4 · The fan: a screw flange, not a clamp bracket

Shaded 3D exploded view of the roof fan mounting stack. From top: the white factory fan assembly with its perimeter screw flange and screws, a teal seal system and conditional roof adapter, the blue-grey roof sheet with its opening, an orange structural flange backing providing flat continuous support and screw engagement, a gold internal load-carrying frame running out to pink vehicle attachment members, and an interior garnish shown as trim. A legend numbers the six layers and notes that the fan may need less structure than the A/C, and to check lid travel, airflow and service access. — **03 — same support principle, different attachment.** The fan hangs off a perimeter screw flange rather than the A/C's lower clamping bracket, so what it needs underneath is **flat continuous backing with real screw engagement**, tied into the reinforcement. **The interior garnish is trim. It carries nothing and clamps nothing.** Drawn by Mickey. **Not redrawn:** this is a **generic fan schematic** with symbolic pink endpoints — **it does not document a BrightDrop fan attachment** and proves nothing about where or whether a fan may be fixed. For body context see sheets 01 and 02.

Sharing a principle is not sharing a specification. The fan and the A/C agree on seal above, support below — they do not necessarily agree on beam sizes, hardware, mounting depth or sealing materials.

Confirm the exact fan model before any of this gets dimensionedEarlier project discussion used a MaxxFan Deluxe 7500K. The selected model's own installation manual governs the opening size, screw pattern, pilot holes, acceptable backing material, sealants and the permitted mounting-thickness range. And the lid needs its full opening envelope kept clear — including from the solar rack above it.

The part key

identification for design and quoting — nothing here is a selected section

ID	Part	Role	Status
S	Exterior solar rack	separate	Carries solar only. Appliance load is removed from its job in this concept — but the rack itself is still not an approved design. See the rack build page.
I1	Internal transverse beams	cut	Bridge to verified vehicle structure; carry the opening frame and the outboard A/C bearing pads. Closed structural tube concept — section, count, span and anchors all unselected.
I2	Trimmers and headers around the opening	fabricate	Keep the air passage clear while carrying the mounting interface into I1. Custom tube / angle frame; every connection still needs design.
I3	Continuous backing and bearing surfaces	fabricate	Support gasket and flange compression, clamp reactions, screw engagement and the perimeter pads. Appliance- and roof-specific. Not a decorative adapter.
I4	Vehicle attachment brackets and targets	fabricate	Transfer load into verified body structure. The pink members that remain in sheets 03 and 04 are symbolic targets — not verified BrightDrop rails, not approved locations, no capacity established. Sheets 01 and 02 now land on documented factory bow, cant-rail and post roles instead, but the connection to them is still proposed, not load-rated and not approved.
OEM	Factory flange / bracket, rods, screws and seals	supplied	Attach and seal per the exact appliance manual. Do not substitute generic hardware and do not apply rack torque values.
J	Frame joints and connection hardware	buy + cut	Positive structural connections, named retention, crush protection where a bolt clamps hollow wall. Engineered per substrate — not a universal rivnut prescription.

Candidate stock families — not chosen sections

I1 / I2 tube: 6061 rectangular tube →
I2 angle: 6061 angle →
I3 / I4 plate: 6061 plate →

What the manufacturer documents actually say

retrieved and read, not remembered

Vehicle — 25MY Chevrolet BrightDrop 400/600 Body Builder Manual

GM's upfitter manual for the platform, read directly rather than summarised. Page 9 gives the body structure: body side outer and roof are composite, 1.6 mm thick, over an aluminium cage of cant rails, transverse roof bows and side posts — the figure is at the top of this page. Page 30 flags the 600's no-drill / no-cut hazards — above the HV battery, orange HV cable, HVAC plumbing and cab SRS; they are not all underbody, and an area it leaves unmarked is not an area it authorises. Page 35 gives the roof-rail airbag deployment envelope, 1800 mm × 1087 mm, in the cab — a hazard drawing, not a clearance certificate for wiring elsewhere. Page 9 also rates the eight fixed floor tie-downs at 1,000 lb each.

The manual renders the cage, it does not dimension itThere is no published bow spacing, bow count, bow section or cant-rail capacity anywhere in it. Everything structural below that level comes off measuring the actual van, and attachment capacity is an inspection question GM's catalogue does not answer.

A/C — Velit 3000R

The planning direction from the main document is the Velit 3000R. Its manual was retrieved from the manufacturer link and read. Four things in it matter here:

The opening is 14¼" × 14¼", not "14 × 14". The marketing shorthand and the manual disagree. Cut to the supplied unit's manual and template, never the shorthand.
Also specified for the existing-opening case: slope below 15°, and 3" of interior trim clearance around the opening edges. That is an interior-fit constraint — not a statement about exterior condenser clearance.
Gasket over the opening plus perimeter support foam blocks. This is the line that forces the reinforcement to reach outboard of the opening ring.
Four M8 rods, factory lower mounting bracket, washers, anti-slip nuts, with a 4.5–5.6 Nm instruction for that clamp assembly. That is not the reinforcement or rack bolt torque and must never be borrowed as one.

Confirm the model and revision you actually receiveDiagram outlines, rod coordinates, the hole pattern and foam-pad positions in Mickey's drawings are symbolic, not copied templates. A custom reinforcement and mounting interface still needs its own design and its own compatibility check against the manufacturer's procedure.

Fan — MaxxFan Deluxe family

Product family: maxxair.com →. Prior project discussion used the 7500K. Nothing about the opening, screw pattern, pilot holes, backing material, sealants or mounting-thickness range is settled until the actually-selected model's installation manual is in hand.

How it gets built — bench first, roof last

Map the appliance underside. Opening, gasket line, support pad positions, screw holes or clamp reactions, and the interior trim footprint.
Establish where the reinforcement can legitimately attach. Vehicle documentation or qualified inspection — not a tape measure, not a photograph.
Mock up the full mounting height with the real duct, air-distribution box or fan mounting thickness before anything is cut.
Only once sections and joints are resolved: cut, deburr, assemble on the bench, check both diagonals for square and the mounting surface for flat.
Fit the backing and every required perimeter bearing landing.
Install the frame in the van with its specified connections, sleeves and retention.
Dry-fit the appliance before cutting the roof. Every bearing point, clamp and screw access, airflow, fan opening envelope, drainage and service access.
Only after that fit is accepted: create the roof opening and the weather connection, install per the appliance's own spec, leak-test, inspect.

Step 7 is the final check before cutting the roof openingTreat it as the last gate, not as a guarantee that everything before it is reversible — installing the frame in step 6 may already involve drilling or bonding that does not come back apart.

Where this lands against the rack work — and which of it is gated behind the measurement pass — is the rack build order. Note the rack's supports are now catalog strut fittings, while this internal frame is still a fabricated structure with an unverified joint to the factory roof bows. Do not cut structural ribs to make the illustrated opening fit.

What this settles, and what it does not

SettledWhere the parts sit relative to each other. That sealing and support are two different jobs done by two different sets of parts. That the appliances belong at roof level. That the solar rack is a separate structure with no appliance load on it. And that the raised-curb, tall-throat arrangement is off the table.

Not established by any drawing on this pageBrightDrop roof material and side-rail geometry · structural capacity and where the reinforcement may legitimately attach · hidden components and airbag clearances behind any fastener location · member sections, wall thicknesses and permissible span · hole patterns, fastener grades, lengths, torque and retention · the exact appliance models, revisions and their own installation requirements · thermal breaks, corrosion isolation and bonding that must not become creeping or compressible structural bearings · accommodation of roof contour and body movement without damaging seals · drainage, service access and the fan's full lid envelope.

Status: a revised concept and the preferred direction to investigate. Not a released structural design, not a cut list, not an approval to open the roof. Those need vehicle documentation or qualified inspection, plus a completed design — the same gate as the rest of the roof work.

All four sheets are in the printable version: the appliance mounting concept (PDF) →