A cathedral greenhouse is a glass house with a second, raised roof section on top, like a lantern. That gives it extra height and light. Janssens makes two very large glass houses we sell: the Janssens Cathedral greenhouse and the Gigant. Pick the Cathedral for a showpiece garden room. Pick the Gigant for the most floor space for your money.
We are a small, woman-owned business in Georgia, and these two models get some of our most detailed questions. This guide puts the real specs side by side.
✅ Quick answer
- The Cathedral is one size: 19'10" x 15', about 298 sq ft, with a lantern roof that peaks at 12'2".
- The Gigant comes in several sizes, from about 260 sq ft up to 600 sq ft, with "High" versions for extra headroom.
- Both use 4mm tempered glass on an aluminum frame and are made in Belgium.
- The Cathedral has a published rating of 25 psf ground snow and 105 mph wind on the specified foundation. The Gigant is custom, so its ratings are set by an engineer for your site.
- Both need a concrete foundation, and we strongly suggest a professional crew for either build.
What is a cathedral greenhouse?
Most glass greenhouses have one roof line. A cathedral greenhouse adds a second, smaller roof that rises from the middle of the main roof. Think of a church with a raised center nave, and you get the idea.
On the Janssens Cathedral (sold in Europe as the "Kathedral"), the main roof has a first gutter at 6'8" and a first ridge at 9'1". Then a second level rises to a second gutter at 10'9" and a top ridge at 12'2". That raised section measures about 5'4" x 10'2".
- More light: the raised section adds glass high up, where sun reaches deep into the room.
- More headroom: tall plants, a small citrus tree or a chandelier all fit under the center.
- A grand look: from the garden, it reads as an orangerie or conservatory, not a shed.
What comes with the Janssens Cathedral greenhouse?
The Cathedral is sold in our store as the Janssens Cathedral Victorian Orangerie, 15ft x 20ft. It comes in one size and two frame colors: black (RAL 9005) or bronze (RAL 7006). Here is what the kit includes:
- Glazing: 4mm tempered safety glass, TÜV and GS tested.
- Doors: one set of double hinged doors with closers, plus one sliding door.
- Ventilation: 4 roof windows, 1 louver window and 2 top-hung side windows that swing up.
- Strength: pre-welded horizontal frame sections and 2 heavy-duty aluminum supports running front to back.
- Floor space: about 298 sq ft inside a 19'10" x 15' footprint.
Many of our customers use it as a garden room first and a greenhouse second. Some put a dining table under the lantern roof. Others use it as a hot tub cover or sunroom.
What is the Janssens Gigant greenhouse?
The Janssens Gigant greenhouse is the largest Victorian-style glass house in the Janssens line. It keeps the classic look, with a 22° roof pitch and ridge decorations. But it is built on a much bigger scale, with heavy portal frames for strength.
Where the Cathedral is one fixed design, the Gigant is a family of sizes. It also comes in "High Gigant" versions with taller walls and a higher peak. Colors are black, green or white.
What every Gigant includes
- Frame: 17-gauge tubular aluminum with stainless steel hardware.
- Glazing: 4mm tempered glass, in full-size single panes held by heavy rubber seals.
- Hybrid option: a 10mm polycarbonate roof with 4mm glass walls. It cuts summer heat and glare.
- Roof windows: half open automatically at about 71°F with no electricity; the rest open by hand with spindle openers.
- Extras: gutters with downspouts and a 4" high foundation frame.
Which Gigant sizes are there?
Here are the Janssens Gigant greenhouse sizes in our store, from the manufacturer's specs. The "High" models trade a higher price for more headroom.
| Model | Outside size | Floor area | Peak height | Roof windows / doors |
|---|---|---|---|---|
| Gigant 13x20 | 12'7" x 19'11" | ~260 sq ft | 9'4" | 6 / 1 double door |
| Gigant 15x20 | 15' x 19'11" | ~300 sq ft | 9'10" | 6 / 1 double door |
| High Gigant 15x20 | 15' x 19'11" | ~300 sq ft | 11'5" | 6 / 1 double door |
| Gigant 20x30 | 19'11" x 29'6" | ~600 sq ft | 10'10" | 8 / 2 double doors |
| High Gigant 20x30 | 19'11" x 29'6" | ~600 sq ft | 13'9" | 8 / 2 double doors |
Specs from the manufacturer, as listed on our product page in October 2026. A High Gigant 15x15 in black glass is also offered. Call us for its details.
The 13x20 and 15x20 models also have louvered windows for low airflow, and the 20x30 models have 6 side windows. The 20x30 sizes use 3 portal frames, or 4 on the High version, to carry the longer span.
Cathedral vs Gigant: how do they compare side by side?
This table compares the Cathedral with the Gigant sizes buyers most often weigh it against. The Royal Victorian VI46 is added for reference.
| Model | Floor area | Peak height | Colors | Price guide* |
|---|---|---|---|---|
| Cathedral 15x20 | ~298 sq ft | 12'2" | Black, bronze | ~$41,398 |
| Gigant 15x20 | ~300 sq ft | 9'10" | Black, green, white | ~$35,500 |
| High Gigant 15x20 | ~300 sq ft | 11'5" | Black, green, white | ~$41,999 |
| Gigant 20x30 | ~600 sq ft | 10'10" | Black, green, white | ~$51,500 |
| Royal Victorian VI46 | ~250 sq ft | 9'2" | Black, green | ~$16,599 |
*Prices checked in October 2026, all-glass versions. Prices can change and sales may apply, so click the model name to see today's price.
The Cathedral and the Gigant 15x20 have almost the same floor. What you pay extra for in the Cathedral is the lantern roof, the extra height and the second door. The Gigant 20x30 doubles the floor for about $10,000 more than the Cathedral when we wrote this.
📌 About prices: manufacturers update their prices during the year, and we often run sales and promotions. The price on the product page is always the current one. Prices in this article are a guide from when we wrote it (October 2026), not a quote or guarantee. Found a lower price for the same model at an authorized retailer? We offer a Price Match Guarantee - please check the rules first. Call us and we will confirm today's price and any current deals.
Which large glass greenhouse gives more value per square foot?
With greenhouses this size, cost per square foot is a fair way to compare. It shows what you pay for each bit of usable floor.
📐 Cost per square foot
Cost per sq ft = Kit price ÷ Floor area (sq ft)
Kit price = today's price on the product page. Floor area = the square feet listed in the specs.
🧮 Worked example (October 2026 prices):
- Cathedral: $41,398 ÷ 298 = about $139 per sq ft
- Gigant 15x20: $35,500 ÷ 300 = about $118 per sq ft
- Gigant 20x30: $51,500 ÷ 600 = about $86 per sq ft
The bigger Gigant wins on pure floor space. The Cathedral costs more per foot because you are paying for height and design.
A taller room also holds more air, so it takes more energy to heat in winter. Our guide on the cost to heat a greenhouse in winter walks through those numbers by size.
Can a cathedral greenhouse handle snow and wind?
The Cathedral has a structural certification letter from a licensed engineer. It rates the Cathedral for 25 psf ground snow load and 105 mph wind (3-second gusts, Exposure C). A full structural permit set is also available, which helps a lot at the permit office.
Those ratings assume it is built the way the engineer specified:
- A continuous concrete beam or solid-grouted masonry wall under the whole perimeter, at least 8" wide by 16" tall.
- A 16" x 16" x 16" concrete footing under each portal column.
- A foundation anchor at each vertical wall member, about every 30 inches (750mm).
What about the Gigant?
Because the Gigant is so customizable, we cannot provide a standard structural certificate for it. We can put you in touch with the engineering firm that worked with the manufacturer. Your area may need extra heavy-duty supports, and only an engineer can decide that.
⚠️ Clear the snow: the Janssens glass greenhouse warranty does not cover damage from snow left to pile up on the roof. It also does not cover the glass itself, or glass broken by large hail. If you live in heavy snow country, call us before you order.
For tougher climates, compare our picks in the guide to the most durable greenhouses for wind, snow and harsh winters.
What foundation does a cathedral greenhouse need?
Neither model sits on bare ground. The Gigant comes with a 4" high foundation frame, but you still need concrete under it. On the Gigant, the heavy portal frames are meant to be set into the foundation itself. That makes site prep a job for professionals.
For the Cathedral, it helps to know roughly how much concrete the perimeter beam takes. Here is a simple way to estimate it.
📐 Perimeter beam concrete
Cubic yards = Perimeter (ft) × Width (ft) × Depth (ft) ÷ 27
Perimeter = 2 × (length + width). Convert inches to feet by dividing by 12.
🧮 Worked example (Cathedral): perimeter = 2 × (19.8 + 15) = about 70 ft. The beam is 8" wide (0.67 ft) and 16" deep (1.33 ft). So 70 × 0.67 × 1.33 ÷ 27 = about 2.3 cubic yards, plus the column footings and some extra for waste. This is an estimate only. Your contractor will size the real pour.
Our practical guide to a greenhouse foundation explains concrete beams, block walls and anchoring in more detail. You can also try our greenhouse foundation calculator.
How hard is it to assemble a Gigant or Cathedral?
These are not weekend kits. The Cathedral's two-level roof is more complex than a simple rectangle. The Gigant's roof panes are large and heavy, and may need scaffolding.
- Check permits early. At about 300 sq ft or more, most towns will want a permit. Ask before you order.
- Plan the delivery. Both ship free to the continental US by freight, curbside, usually on an 18-wheeler. Tell us about tight roads or low wires.
- Book the foundation. It must be level, square and cured before the crew arrives.
- Hire a crew. We strongly suggest a contractor or experienced crew. See our Assembly Crews page.
💡 Timing tip: Gigants are special order, with a turnaround of about 14 to 16 weeks when we wrote this, though it can change. Order in late fall or winter to have it built for spring.
Which one should you buy?
Choose the Cathedral if:
- You want a striking orangerie-style room near the house.
- Height and light matter more to you than raw floor space.
- You want published snow and wind ratings and a permit set ready to go.
- You like black or bronze frames.
Choose the Gigant if:
- You need the most growing space, up to about 600 sq ft.
- You want a white or green frame, or a polycarbonate roof for hot, sunny climates.
- You plan a big use like a pool cover, event space or market garden.
- You are happy to work with an engineer on your site's loads.
🌿 Our verdict: if you mainly want to grow, the Gigant gives more floor for the money. If you want a room people stop and stare at, the cathedral greenhouse is in a class of its own. Want a smaller Janssens instead? See the Victorian greenhouse collection.
For a deeper look at the Cathedral alone, read our full Janssens Cathedral orangerie analysis. For more Gigant photos and uses, see Grow Big with the Gigant.
Frequently asked questions
How big is the Janssens cathedral greenhouse?
It measures 19'10" wide by 15' long, with about 298 sq ft of floor. The top ridge of the raised roof is 12'2" high, and the first side gutter is at 6'8". It comes in one size only.
How much does a Janssens Gigant greenhouse cost?
When we wrote this in October 2026, Gigants ran from about $33,999 for the 13x20 to about $68,500 for the High Gigant 20x30 in glass. Prices change and sales happen, so check the product page for today's price.
What is the difference between a Gigant and a High Gigant?
They share the same footprint, but the High Gigant has taller walls and a higher peak. For example, the 20x30 peaks at 10'10", while the High 20x30 peaks at 13'9". The High versions cost more.
Do I need a permit for a cathedral greenhouse?
Most likely, given its size. Check with your local building department before you order. The Cathedral comes with an engineer's certification letter and a structural permit set, which makes the process easier.
Can I build a Gigant greenhouse myself?
The manufacturer says it should only be built by a contractor or a highly skilled handyman. The roof glass is large and heavy, and the portal frames are set into the foundation. We can help you find a crew.
Can I use a large glass greenhouse kit as a sunroom?
Yes. Many owners use both models as garden rooms, dining spaces or pool covers. For living use, plan for shade, airflow and heating, since single-pane glass gets hot in summer and cold at night.
Cathedral greenhouse or Gigant? Let's plan it together.
We are a small, woman-owned business, and we answer every question personally. Tell us your space, climate and plans. We will help you choose, confirm today's price and any current sale, and line up shipping and an assembly crew.