Scissor Truss Calculator
Estimate scissor truss span geometry, roof pitch rise, vaulted ceiling rise, chord lengths, truss count, tributary loading, and preliminary layout notes.
▣Real scissor truss presets
▦Truss geometry and load inputs
▰Truss / spec grid
▱Reference tables
| Scissor preset | Span range | Roof / ceiling pitch | Common use |
|---|---|---|---|
| Low garage vault | 20 to 26 ft | 6:12 / 3:12 | Garage, porch, bonus storage-free roof |
| Cabin cathedral | 24 to 32 ft | 8:12 / 4:12 | Cabins and compact great rooms |
| Workshop vault | 28 to 36 ft | 7:12 / 3:12 | Shop clearance with modest roof height |
| Steep chapel roof | 34 to 44 ft | 10:12 / 5:12 | Sanctuary, hall, or tall open room |
| Heavy snow scissor | 24 to 40 ft | 8:12 / 3:12 | High load areas needing deeper heel |
| Pitch pair | Scissor ratio | Peak depth on 30 ft span | Layout note |
|---|---|---|---|
| 6:12 roof / 2:12 ceiling | 3.00 | 5.0 ft | Deep truss body, lower ceiling vault |
| 6:12 roof / 3:12 ceiling | 2.00 | 3.75 ft | Common efficient scissor relationship |
| 8:12 roof / 4:12 ceiling | 2.00 | 5.0 ft | Steeper roof with balanced vault |
| 9:12 roof / 5:12 ceiling | 1.80 | 5.0 ft | Higher ceiling, less truss depth margin |
| 10:12 roof / 6:12 ceiling | 1.67 | 5.0 ft | Looks dramatic but can tighten web design |
| Load item | Typical range | Where it acts | Design caution |
|---|---|---|---|
| Roof dead load | 8 to 20 psf | Top chord roof plane | Roofing type and sheathing matter. |
| Roof live load | 20 psf minimum often used | Projected roof plan | May not control in snow regions. |
| Ground / roof snow | 20 to 100+ psf | Code-defined roof load | Drift, exposure, and slope factors apply. |
| Ceiling dead load | 5 to 15 psf | Bottom chord / vault plane | Drywall, insulation, lights, and MEP loads. |
| Wind uplift | Site specific | Top chord and connections | Requires code wind speed and connectors. |
| Member / layout item | Typical choice | Why it matters | Verification needed |
|---|---|---|---|
| Chord stock | 2x4 to 2x8 | Compression, tension, and joint plate area | Truss engineer sizing and grading |
| Raised heel | 10 to 24 in | Insulation depth and eave energy detail | Bearing, blocking, and uplift path |
| Spacing | 16 or 24 in o.c. | Roof deck span and tributary load | Sheathing span rating and bracing |
| Web count | 3 to 6 panels per half | Panel length, load transfer, and joint layout | Computer truss design output |
| Bearing | Exterior walls or beams | End reactions must land on a full load path | Posts, headers, footings, and connectors |
△Practical tips and engineering safety
In a thirty foot span, the scissor truss make the room seem much larger then it is while using minimal interior column. It creates a vaulted ceiling in house built for prefabrication and efficiency. You don’t notice that the whole thing is held up by precise angles of steel plate when you’re standing in there. The engineering goes away and space gets to come through.
Somebody calculated this stuff out beforehand and then shipped it to the job site all wrapped up in plastic. Enter your pitch preference and span and let it handles the geometry for you. That way you don’t have to guess what angle you need the roof to hit.
How Scissor Trusses Work
People typicaly think “I’d like a high ceiling,” but then they don’t consider walls. How tall do you want your walls? What is the ceiling pitch? Do you want insulation to go up far enough to get enough R-value (for example for window insulation)? Does the heel height work with those goals? The heel is vertical part of the truss base where the rigid foam board must remain its full thickness up to the top plate without being compressed. If the truss cut into the rigid foam board, you immediately lose all the R-value in that spot. A high heel lets you make sure the rigid foam board goes to the top plate and gives you tall walls for insulating window or just making a house look realy tall.
In the calculator, you can specify height of the heel, so it will account for that. It makes a difference since eave insulation and rim boards is getting tighter each year due to energy codes.
These are non-negotiable: scissor trusses requires the roof pitch to always be greater than the ceiling pitch. Why? Because if they weren’t, your ceiling would be sloping while the floor was flat. This is not very structurally efficient for shedding snow or rain. So the angle difference determine how deep the truss will be on top. The deeper it is, the more expensive the materials, but also the more cavernously high the ceilings may feel. Conversely, a shallower design mean a thinner truss body, which leaves less space for internal webbing to transfer loads. You want to find a happy medium there and that’s where this tool can be helpfull because it shows you those trade-offs ahead of time so you don’t make a plan and then regret it later.
The other thing most people don’t think about until their builder talks to them is loads. The weight of snow on the roof dwarfs the weight of the truss in a northern climate, especially when wet. The calculator takes into account tributary loading from the roof you specify (dead and live loads). That’s a starting point, sort of a load honesty check. It will tell you if you’re building a light garage roof or a really heavy snow load system that requires thicker lumber and maybe engineered steel plates. Don’t rely on this for final engineering, but it does give you an idea why code matters locally. Twenty pounds per square foot is nothing in Florida. It may be a fraction of the actual load you’ll see on the structure in Januarys storm in Minnesota.
Then there’s the spacing thing. If you space them at say 24 inches rather than 16 inches on center, you’ll have fewer trusses to do. However, those trusses will now are carrying more weight. They’ll need to support longer spans in the roof, so you may want to use thicker sheathing for the roof deck. So it becomes a trade-off between how much your deck costs compared with cost of members. You can run the numbers and get an idea about how many trusses you’re looking at by entering both building length and spacing into the tool.
If you want to do some quick sanity checking, use the reference tables below. They list typical ceiling and roof combinations. You’ll notice a lot of the same ones there: 4/12 ceilings paired with 8/12 roofs; these are tried and true combinations that have worked in thousands of homes. These are the old standbys; proven combos that’ve performed well in thousands of houses over time. Going outside the norm can make your house look special, but frequently comes at price of custom engineering fees. If keeping costs down is a goal, stick to the presets.
Remember a scissor truss is a system. A good one works in theory, but it also relies heavily on proper bracing during construction. Voiding warranties and cutting out holes for wiring or HVAC ducts post fabrication could of been a recipe for disaster. Make sure to get your dimensions right from the beginning. Let the geometry be your friend rather than your enemy.
The beauty in the vaulted ceiling is that someone started with first calculation, making it look easy.
