How the two differ in kind
A pitched roof does not only press down. Gravity on a sloped rafter resolves into two components, and the horizontal one arrives at the bottom of the rafter as outward thrust against the top of the wall — harder the flatter the pitch gets. In an ordinary roof nothing at the top resists that. A ridge board is a plank, asked by code only to be at least as deep as the rafter's plumb cut so the cuts have something to bear against, and it carries almost no load. What actually holds the roof together sits at the bottom: the ceiling joists, working in tension as the third side of a triangle, provided they run parallel to the rafters and are fastened to the rafter feet rather than merely sharing a room with them.
Take that tension member away for a vault and the triangle is gone, so the load needs a different route down. There are only two. Carry it up — make the ridge a real beam, deep enough that each rafter bears on it the way a joist bears on a girder, so the reaction at the top turns vertical and the whole thing goes down two posts at the beam's ends. Or tie it across — keep the cheap ridge board and put the tension back somewhere else, at or near the wall plate where it is most effective, or raised into the roof within the limits the code allows.
Which is right is decided by the ceiling below, and then by what is under the walls. If there is a flat ceiling and the joists run the right way, you already own the entire structure and a beam is money spent on nothing. If the ceiling is opening to the ridge, the beam is rarely the hard part — the posts are. Each one carries roughly a quarter of the whole roof's load into whatever is beneath it, floor after floor, down to a footing. On a single-storey addition with gable walls to land on, that path is short and obvious. Over an open-plan room on a suspended floor it is the entire job, and the honest conclusion is sometimes that the full cathedral costs more than it is worth, and the partial vault bought with raised ties is the version worth having.
The factors that actually differ
| Structural ridge beam | Ridge board with rafter ties | |
|---|---|---|
| What resists the thrust | The beam does — rafters bear on it, the reaction at the ridge is vertical, and nothing pushes outward at the eaves. | The ties, in tension: parallel to the rafters, fastened to their feet with the nailing the schedule specifies, and carrying more force the flatter the pitch. |
| Where the load lands | Two new point loads. The beam takes half the roof, so each post carries roughly a quarter of it and needs a continuous path through every floor to a footing; the eave walls still take the other half as before. | Spread along the two eave walls, which were already designed to carry roof load — no new bearing anywhere. |
| How it goes wrong | The beam deflects and the roof plane follows it: a curve along the ridge, and cracked board in a vault that shows every millimetre. | Thrust wins slowly — the ridge drops, the wall tops lean out, and it reads as settlement: cracks fanning from openings, doors binding at the head. |
| The ceiling you get | Open to the ridge, minus the depth of the beam hanging into it unless the rafters are flush-framed into its face. | Flat at tie level. Raised ties within the code's limits buy a partial vault; a full cathedral is not on this menu. |
| The member, and getting it up there | Engineered — LVL, glulam or steel, sized by deflection, heavy enough to want a crane or to be built up in plies in place. | A nominal plank, carried up a ladder by one person and nailed between the rafter cuts. |
| Design route | Specific and engineered: a beam sized for your span and loading, its posts, its bearings, usually with a stamped calculation for building control. | Prescriptive: rafter span tables and a heel-joint nailing schedule straight out of the code, with no engineer in the loop. |
| Where the money goes | Concentrated in material and design — one member, its posts, and whatever carries them; a design fee a small roof amortises badly, and stiffness demand climbing with the cube of the span. | Diffuse: ordinary lumber and carpentry hours, scaling with roof area and joist count, with no fixed cost to spread. |
| Insulation and services | All of it in the rafter depth — insulation plus a vent channel above it, or an unvented assembly built to the rules for one. No duct route, and every downlight is a hole in the air barrier. | An attic above the ceiling: insulation to whatever depth you care to blow, and a free run for ducts, cable and pipe. |
| Changing your mind later | Reversible downward for very little — a flat ceiling can be hung under a vault whenever you like. | The expensive direction. Vaulting later means propping a loaded roof, retrofitting the beam through a finished ceiling, and finding a post path nobody planned for. |
Which one, and when
Choose structural ridge beam when…
- The ceiling is opening to the ridge and staying open — a full cathedral has no prescriptive alternative.
- There is somewhere for the posts to land: gable walls, a bearing line, a path to a footing that exists on the drawing rather than in hope.
- The ties would have to sit too high to be ties — above the part of the roof where the code still helps.
- The roof is already up and the ceiling is coming out. The beam goes in and takes load first; the joists come out second.
- The volume is the point of the build — a conversion, a great room, a gable window that only reads with the height behind it.
Choose ridge board with rafter ties when…
- There is a flat ceiling under this roof and the joists run parallel to the rafters. You already own the structure; the only question is the fastening at the feet.
- Nothing below can take a post: open plan, a room over a room, or a suspended floor that would need its own beam to pick up the point load.
- The prescriptive route is worth real time — span tables and a schedule, no engineer, nothing waiting on someone's desk.
- The attic earns its keep: insulation depth, storage, and the service route a vault takes away.
- A partial vault will do, and raised ties within the code's limits buy the height without buying a beam.
Now run your own numbers
This page holds no prices on purpose — a national average is wrong for almost every real project. Quantify both options with your dimensions and your local quotes.
Frequently asked questions
- Can I just take the ceiling joists out to open up the room?
- Not before something else is doing their job. Those joists are the tension side of the roof's triangle; cut them and the rafters spread, the ridge drops and the top of each wall leans outward. It rarely fails dramatically — it creeps, and the symptoms read as ordinary house settlement: cracks fanning from the corners of door and window openings, doors binding at the head, and a ridge line that has quietly developed a curve when you sight along it from the garden. The sequence matters as much as the decision. The beam is installed and taking load first, the joists come out second. If joists are already gone, the roof needs supporting before anything else is cut.
- Aren't collar ties and rafter ties the same thing?
- No, and the two words get swapped constantly. A rafter tie sits at or near the top of the wall and works in tension against the outward thrust — that is the member stopping the walls spreading, and in most houses it is simply the ceiling joist. A collar tie sits high, in the upper third, and does a different job: holding the rafter pair together against wind uplift and separation at the ridge. Adding collar ties to a roof whose rafter ties you removed does not put the triangle back. The higher a member sits, the less leverage it has against the thrust and the more force it must carry to do the same work, which is exactly why there is a limit on how high a tie can be raised.
- My ceiling joists run the wrong way — do they count as ties?
- Only joists running parallel to the rafters, in the same plane, connected to the rafter feet, are tying anything. Joists running perpendicular are ceiling framing: as far as the roof's thrust is concerned they hold up board and nothing else. Roofs framed that way need rafter ties installed separately at the spacing the code calls for, or a structural ridge instead. It is worth going up and looking rather than assuming, because a house can have a perfectly conventional flat ceiling and no rafter ties at all — in which case something else is resisting the thrust, or something has already started moving.
- How high can rafter ties be raised, and what do I actually get?
- Raising the tie trades headroom for force. The tension it carries climbs steeply as it moves up the rafter, and the heel connection then has to be designed for that amplified load rather than nailed to the ordinary schedule — which is why codes generally permit raised ties only within the lower part of the roof's rise, with an adjustment factor applied at the connection. What it buys is a partial vault: a sloped ceiling that flattens out below the ridge, not an open one. Scissor trusses are the factory version of the same compromise. If a true cathedral is the requirement, this route runs out and the beam is the answer. Check your own code's table before designing around it, because both the limit and the factor are jurisdiction-specific.
- Which one costs more?
- They are made of different money, which is why no figure appears here. The beam route is concentrated: one engineered member sized for your span, the posts under it, whatever carries those posts, and a design fee that is largely fixed whether the roof is small or large — so a modest addition amortises it badly. Its material cost also grows faster than intuition suggests, because a ridge with a finished ceiling under it is sized by deflection rather than strength, and the stiffness demanded climbs with the cube of the span. The tie route is diffuse: ordinary lumber, ordinary hours, no design fee, scaling gently with roof area and joist count. But the comparison that decides most real cases is not beam-now against ties-now. It is beam-now against beam-later, and later includes propping a loaded roof, opening a finished ceiling and buying a post path through rooms built without one.
- If the ridge is a big beam, is the thrust really gone?
- Only if the connection makes it so. A structural ridge works because each rafter bears on the beam the way a joist bears on a girder — seated over the top of it, or hung from its face in a hanger rated for the load — so the reaction at the top of the rafter is vertical and the walls receive nothing but downward load. A deep beam that rafters are merely nailed to the sides of is an expensive ridge board, and the roof will still push. Uplift is a separate question with separate hardware: the rafters need holding down to the beam and to the wall plates, and that path has to be continuous whichever route you take.