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- Low-Slope Houses
No attic to hide in.
What a shallow roof space rules out, why the machine ended up on the roof, what moving it actually involves, and where the heat in these houses really comes from.
Joist depth, and then the sky.
On a steep-roofed house the attic is a room. There is volume, there is standing height in the middle, and everything mechanical the building needs can be put in it.
On a low-slope house the space between your ceiling and the underside of the roof deck may be no deeper than the ceiling joists themselves. That is a cavity, not a room. It holds wiring and a light fitting. It does not hold a person, a machine, a duct trunk, or much insulation.
Everything on this page follows from that one measurement — the clear depth at the tightest point, not at the hatch, which is why it is the third thing we check on every survey and why we write it down.
What a shallow space rules out
- An air handler up there. There is neither room to fit it nor room to service it afterwards.
- A full-size duct trunk. Branch runs squashed flat between joists move a fraction of the air they are rated for.
- Deep insulation. You can only fill the depth you have, unless the insulation goes above the deck — which is a roofing job, not ours.
- Ventilation you can rely on. A cavity that shallow moves very little air, so the deck sheds its heat downward instead of out.
So where does the equipment go?
Three places, and which of them is available is a property of your house rather than a preference: on the roof in a packaged unit, in the side yard with an indoor half somewhere — a closet, a garage, a purpose-built chase — or nowhere at all, in which case ductless heads replace the idea of a central machine entirely.
We establish which of the three you have before we discuss a single piece of equipment.
Why it is up there, and what moving it costs.
Rooftop package units on single-family houses are ordinary in the South Bay and unusual almost everywhere else. Here is the honest account of them.
- Why it exists
- It solves the "where does it go" problem by leavingThe compressor, the coil, the blower and the heat all live in one cabinet outside. One supply duct and one return drop through the roof into the ceiling space. No attic needed, no closet lost, nothing in the side yard.
- What it costs you
- Full sun on six surfaces, all day, in marine airA machine on a roof ages faster than the same machine in a shaded side yard. Cabinets, fan motors and electricals go first. It is not a fault in the design — it is the price of the position.
- The curb
- A frame that penetrates the roof, and the real liabilityThe unit sits on it, gasketed. A failing curb seal does not announce itself as an HVAC fault; it announces itself as a ceiling stain in the first wet winter, by which point water has been getting in for a year.
- Replacing it
- A lift, an exposed curb and often an adapterThe old unit has to come off before anybody can see the curb, so nobody can quote that part in advance honestly. New units rarely match an old footprint, so an adapter curb is common. Both go on the quote as named line items rather than as a surprise.
- Moving it down
- Possible, and rarely worth it on its ownConverting to a split system means finding an indoor location that does not exist yet, running new line sets, and leaving a roof penetration that has to be closed by a roofer. It is a real option during a remodel and an expensive one outside of it. We will price it and usually advise against it.
Four questions to ask anybody quoting a rooftop replacement
Including us. One: is the lift in this number? Two: what happens if the curb underneath is bad — who decides and who pays? Three: does the new unit fit the existing curb, and if not, is the adapter priced? Four: who is responsible for the roof surface around it afterwards? A quote that answers all four is a quote you can compare.
Duct has to go somewhere, and "somewhere" costs ceiling height.
If the space above the ceiling will not take it, the only honest options are to build a route inside the room, or to stop using duct.
A furred soffit is a boxed-in run along a hallway or the edge of a room, framed and finished so it reads as architecture rather than as a repair. It works, it is the standard answer in this housing stock, and it takes ceiling height along its whole length. We give you that number on the survey — how deep the soffit has to be and where it runs — before anybody agrees to anything.
A dropped chase does the same thing vertically where a run has to cross between areas. Same trade: it solves the route and it is visible.
Ductless removes the question. No trunk, no soffit, no lost height — a slim head on the wall of each room instead, and a line route that runs either inside a wall or outside in a cover.
None of the three is the right answer for every house. What is always wrong is squashing a duct flat into a space that will not take it and calling the job done, because the airflow it delivers has nothing to do with the number on the equipment.
The open-beam house
Some of these houses have no ceiling cavity whatsoever — the finished surface is the underside of the roof deck, with the beams exposed. There is nothing to run duct through and nothing to insulate within. It is a beautiful ceiling and it is a genuinely hard HVAC problem, and anybody who tells you otherwise has not worked on one.
The ceiling is a heat source, and a bigger machine does not fix a ceiling.
This is the conversation people least want to have and the one that explains the most.
In a house with a deep attic, the roof gets hot and then there is a large ventilated buffer and a thick layer of insulation between that heat and your ceiling. By the time it reaches the room it is much diminished.
On a low-slope house the deck is a few inches above the plasterboard, with whatever insulation fits in the joist depth and very little air movement. From mid-afternoon the ceiling itself is warm, and it radiates into the room from above — which is exactly the direction your body notices most.
Cold air from a register does not cancel that. You can feel the air conditioning working and still feel the room as uncomfortable, because two different things are happening at once. This is the single most common complaint we are called about, and it is the one where the equipment is least often at fault.
What actually helps, in order
- Filling the depth you have, evenly, right out to the edges where it is always thin.
- Sealing the ceiling penetrations — duct boots, light housings, wiring — so the cavity is not connected to the rooms.
- The roof surface itself, at the point it is being replaced anyway. What goes on top of the deck matters more here than in almost any other housing type — and it is a roofer's decision, not ours.
- Shading the west glass, which is usually the other half of an afternoon complaint. Also not ours, and worth knowing.
- Then, and only then, the equipment and the duct — which is our part, and which we would rather size after the other four than before them.
What we will not tell you
How much any of that will save you. No percentages, no payback periods. It depends on your roof, your rate plan and the summer you get. What we will tell you is which of the five above is doing the most damage in your particular house, which is a more useful thing and one we can actually see.
Asked once people have read this far.
My house has a hatch. Does that mean I have an attic?
Not necessarily. A hatch often opens into the deepest part of a cavity that shallows out rapidly a couple of feet in — which is precisely why we measure at the tightest point rather than at the opening. Plenty of houses here have a hatch you can get your head and shoulders into and nothing beyond it.
Is a rooftop unit worse than a split system?
Not worse — differently exposed. It ages faster because it sits in full sun on every side, and it is harder to get to. In exchange it takes nothing from inside the house, makes no noise in a side yard, and on a house with no attic it may be the only central option available. If your house has an indoor location, a split system is usually the quieter and longer-lived choice.
Can I put insulation above the roof deck instead?
It is the right answer for this housing type and it is a roofing project rather than an HVAC one — it happens when the roof is being replaced, by a roofer, and it changes the assembly. We will tell you it exists and we will not quote it. If you are re-roofing anyway, that is the moment to ask your roofer about it, and it is worth asking.
Would a whole-house fan help?
Less than in most climates, for two reasons specific to here. It needs the attic to be able to vent what it pushes into it, and a shallow cavity vents poorly. And it works by pulling in outside air, which on a marine-layer evening is genuinely cool and on a smoky day is not. We will assess the cavity before quoting one rather than after, and on a fair number of these houses the answer is no.
The last contractor said I need a bigger unit. Do I?
Possibly, and it is worth checking properly before you buy one. In these houses the two things that most often masquerade as "too small" are a squashed or leaking duct run and a ceiling gaining heat faster than the system can remove it. A larger machine makes the first worse by pushing more air into a restriction, and does nothing about the second. We will measure before we agree with them.
Get the space above your ceiling measured.
It takes ten minutes, it is the number every other decision depends on, and you get it in writing whether or not you use us.