A Queen Post Roof Truss is a triangulated timber or steel roof frame that uses two vertical posts (the queen posts) standing on a horizontal tie beam, connected at the top by a horizontal straining beam under the principal rafters. It solves the problem of spanning roofs wider than a single King Post can handle without sagging the tie beam. The two posts split the rafter load into shorter unsupported lengths and transfer it down as tension into the tie beam. This lets you clear-span 8 to 12 m for halls, barns, and chapels using sensible timber sections.
Queen Post Roof Truss Interactive Calculator
Vary roof load, span, rise, and heel capacity to see tie-beam tension and truss force response.
Equation Used
The queen post truss tie beam is sized from the horizontal thrust estimate T = W * L / (2 * h). Increasing span or roof load raises tie tension, while increasing rise reduces it. The heel safety value compares the selected heel connection capacity with the calculated tie-beam tension.
- Ideal pin-jointed queen post truss geometry.
- W is the total vertical roof load carried by one truss.
- Tie beam resists the horizontal thrust as axial tension.
- Heel safety factor compares heel capacity to calculated tie tension.
Inside the Queen Post Roof Truss
The Queen Post Roof Truss works by turning a wide roof into a set of stable triangles. Two principal rafters lean inward and meet a horizontal straining beam at the top. Below that, two vertical queen posts hang from the rafters and rest on the tie beam, which spans the full width of the building. The straining beam takes compression between the post heads, the rafters take compression down to the heel joints, and the tie beam takes pure tension across the bottom. That tension is the whole reason the truss does not push the walls outward — without an intact tie beam, you would see the eaves spread and the ridge drop within a season.
The queen posts themselves carry very little vertical load in a properly built truss. Their job is to shorten the rafter span and to hang the tie beam up so it does not sag under its own weight and any ceiling load below. If the posts are mis-sized or the joints are loose, you get classic failure symptoms — a dished tie beam, cracked plaster on the ceiling below, and rafters that bow inward between the heel and the queen post. Heel joints are the highest-stress detail on the whole truss. A traditional mortise-and-tenon heel with a single peg will fail in shear under heavy snow load if the relish (the wood behind the tenon) is less than about 1.5 times the tenon depth.
Get the geometry right and the truss is essentially self-bracing. Pitch typically sits between 30° and 45°. Below 30° the rafter compression rises sharply and the tie beam tension can exceed 40 kN on a 10 m span, which pushes you into steel strapping at the heel. Above 45° you waste timber and lose attic headroom for no structural gain. The straining beam length usually runs at one-third of the total span, which keeps the post-to-rafter angle in the 60° range where the joints behave predictably.
Key Components
- Tie Beam: The horizontal bottom chord that ties the two heel joints together and resists the outward thrust of the rafters. It works in tension, typically 15-40 kN for residential spans, and must be a single continuous member or a properly scarfed joint — never a butt joint. Section sizes of 200 × 250 mm are common for 10 m spans in Douglas fir.
- Principal Rafters: The two sloped compression members running from the heel joint up to the straining beam. They carry the roof dead load plus snow and wind, and must be sized for combined axial compression and bending between the queen post and the heel. A 150 × 225 mm section in Grade SS softwood handles most 10 m residential cases with purlins at mid-span.
- Queen Posts: Two vertical members standing on the tie beam and meeting the rafters where the straining beam lands. They work mostly in tension when supporting a ceiling load below, hanging the tie beam up. Section sizes of 100 × 150 mm are usual. The post-to-rafter joint is normally a mortise and tenon with a steel strap or coach bolt for redundancy.
- Straining Beam: The horizontal compression member between the tops of the two queen posts. It stops the rafters from pushing the post heads inward. Length is typically one-third of the total span. Section size matches or slightly exceeds the queen posts — 100 × 200 mm is typical.
- Heel Joint: Where the rafter meets the tie beam at the wall plate. This is the highest-stressed connection on the truss and resists the full horizontal thrust component of the rafter. Traditional joinery uses a bird's mouth and tenon with a single oak peg; modern practice adds a steel heel strap rated for at least 25 kN.
- Straining Sill (optional): A short horizontal block on top of the tie beam between the bases of the queen posts, used when ceiling loads are heavy. It transfers any compressive load from the post bases into the tie beam without crushing the tie beam fibres across the grain.
Who Uses the Queen Post Roof Truss
Queen Post trusses show up wherever you need a clear span between roughly 8 and 12 m without intermediate columns and without going to engineered steel. They are the workhorse of pre-industrial heavy roof framing and they are still specified today in heritage restoration, agricultural buildings, and high-end residential builds where exposed timber is part of the aesthetic. You see them named in heritage condition reports for everything from rural church naves to 19th-century railway station roofs.
- Heritage Restoration: Re-roofing the nave of a 1860s Anglican church such as St. James in Stratford Ontario, where the original Douglas fir Queen Post trusses span 11 m and need sistered tie beams after a century of plaster ceiling load.
- Agricultural Buildings: Framing a 10 m clear-span hay barn at a Mennonite farm in Lancaster County Pennsylvania, using site-cut white oak with traditional pegged mortise-and-tenon heel joints.
- Residential Construction: Open-plan great-room roofs on custom timber-frame homes built by companies like Riverbend Timber Framing in Michigan, where the exposed Queen Post truss is both the structure and the architectural feature.
- Industrial and Mill Buildings: Original 19th-century textile mills in Lowell Massachusetts used Queen Post trusses on cast-iron column rows to span weaving floors of 12 m without intermediate posts.
- Educational and Civic: Village hall and town-hall roofs across rural England, where a Queen Post truss in pitch pine spans the assembly room. The Grade II listed Suffolk village halls catalogued by Historic England commonly use this configuration.
- Equestrian Facilities: Indoor riding arenas of 10-12 m width, where glulam Queen Post trusses replace the original sawn-timber sections to handle snow loads of 1.8 kPa in places like Calgary Alberta.
The Formula Behind the Queen Post Roof Truss
The single most useful calculation on a Queen Post truss is the tie beam tension, because that is what governs the heel joint design and the tie beam section. Tension scales with span, total load, and inversely with roof pitch. At the low end of typical pitch — 25° — the tension can be brutal because the horizontal component of the rafter thrust dominates. At the nominal sweet spot of 35-40° the geometry balances and tension stays manageable. Push the pitch above 50° and tension drops, but you start losing useful attic volume and the rafters get long. The formula below gives you the tie beam tension for a symmetric truss under uniformly distributed roof load.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| Ttie | Tension in the tie beam at midspan | kN | lbf |
| w | Uniformly distributed roof load per metre of truss spacing (dead + snow) | kN/m | lbf/ft |
| L | Clear span between heel joints | m | ft |
| h | Vertical rise from tie beam to ridge (or to underside of straining beam for queen post case) | m | ft |
Worked Example: Queen Post Roof Truss in a heritage barn re-roofing project
A timber framer in Norfolk County Ontario is replacing the rotted Queen Post trusses in a 1890s tobacco-drying barn. The clear span between top plates is 10.0 m, truss spacing is 3.0 m on centre, and the local snow load is 2.4 kPa with a roof dead load of 0.6 kPa. The original pitch was 35° giving a rise of 3.5 m to the ridge. The framer needs the tie beam tension to size the new white oak tie beam and to specify the heel strap.
Given
- L = 10.0 m
- Truss spacing = 3.0 m
- Snow load = 2.4 kPa
- Dead load = 0.6 kPa
- h (nominal, 35° pitch) = 3.5 m
Solution
Step 1 — combine the area loads and turn them into a line load along the truss. Total area load is 2.4 + 0.6 = 3.0 kPa. Multiply by truss spacing of 3.0 m:
Step 2 — compute the nominal tie beam tension at the design pitch of 35° (h = 3.5 m):
That is the headline number for the heel strap and the tie beam. Now sweep the pitch range. At the low end of practical Queen Post pitch — 25°, h ≈ 2.33 m:
That is a 50% jump in tie beam tension just from flattening the roof 10°. At 48 kN you are off the standard residential heel-strap chart and into custom-engineered steel plates with through-bolts. The tie beam itself needs to grow from 200 × 250 mm to roughly 225 × 300 mm in white oak to keep the working stress under the allowable tension parallel to grain.
Step 3 — at the high end of the practical range, 50° pitch, h ≈ 5.96 m:
Tension drops to under 19 kN — easy territory for a standard galvanized heel strap and a 175 × 225 mm tie beam. But you have just added 2.5 m of rise, which means longer rafters, more roofing material, and significant extra wind exposure. The 35° nominal sits in the sweet spot where tie beam tension is moderate and material use is efficient.
Result
Nominal tie beam tension is 32. 1 kN at 35° pitch, which sizes a 200 × 250 mm white oak tie beam with a Simpson HHDQ-style heel strap or equivalent rated above 35 kN. In practical terms, 32 kN is roughly the weight of a small pickup truck pulling sideways on the heel joint — not trivial, but well within ordinary heavy-timber detailing. Compared to the 48 kN you get at 25° pitch and the 19 kN at 50°, the 35° design uses about 30% less steel at the heel than a flatter roof and avoids the long-rafter penalty of a steeper one. If you measure heel-joint movement greater than 3 mm under full snow load, suspect one of three failure modes: tie beam scarf joint slipping because the table-and-key was cut with too much shrinkage gap, heel-strap bolts loosening as the green timber dries below 18% moisture content, or end-grain crushing at the rafter tenon where bearing area falls below the 12 MPa allowable for white oak.
Choosing the Queen Post Roof Truss: Pros and Cons
Queen Post is one of three classical heavy-timber truss families a builder will compare when sizing a clear-span roof. King Post is simpler and cheaper but limited in span. Howe and Pratt trusses go wider but use more members and are usually steel-jointed. Pick the one whose span and aesthetic match your project — there is no universal best.
| Property | Queen Post Truss | King Post Truss | Howe Truss |
|---|---|---|---|
| Practical clear span | 8-12 m | 5-8 m | 10-30 m |
| Number of primary members | 6 (2 rafters, 2 posts, tie beam, straining beam) | 4 (2 rafters, 1 post, tie beam) | 10+ (multiple verticals and diagonals) |
| Tie beam tension at 10 m, 9 kN/m, 35° | ≈ 32 kN | Not applicable above 8 m span | ≈ 32 kN distributed across multiple panel points |
| Typical timber volume per truss (10 m span) | 0.8-1.0 m³ | 0.5-0.6 m³ (capped at shorter spans) | 1.2-1.6 m³ |
| Joint complexity | Moderate — 6 mortise-and-tenon plus 2 heel joints | Low — 4 joints, simplest of the three | High — every panel point needs steel plates or split-ring connectors |
| Cost per m² of roof covered (rough) | Mid | Low | High |
| Best application fit | Mid-span barns, halls, residential great rooms | Small chapels, garages, porches, secondary outbuildings | Industrial buildings, bridges, long-span warehouses |
| Aesthetic suitability for exposed framing | Excellent — clean symmetrical form | Excellent — iconic single-post silhouette | Poor — busy, industrial appearance |
Frequently Asked Questions About Queen Post Roof Truss
That sag almost always means the queen posts are not actually carrying the tie beam up the way they should. Check the post-to-tie-beam joint first. If the original joiner used a stub tenon with no draw-bolt or strap, the posts can lift slightly under rafter load while the tie beam droops independently under its own weight plus any ceiling load below.
Second cause: a ceiling has been added or upgraded since the truss was built. Lath-and-plaster ceilings run about 0.5 kPa, but a modern double-layer drywall ceiling with insulation can hit 0.8 kPa, which the original posts and joints were never sized for. The fix is usually a steel rod hanger from each queen post down through the tie beam, tightened to take the deflection out.
You can, but you have just turned the truss into something that behaves more like a pair of leaning posts than a true Queen Post truss. Without the straining beam in compression between the post heads, there is nothing stopping the rafter thrust from pushing the post tops inward. Over time the rafters bow, the posts tilt toward the ridge, and the whole geometry racks.
If the architect insists on a clean look without a visible straining beam, the standard fix is a hidden steel tie rod between the post heads, sized for roughly the same compression the straining beam would have carried — typically 8-15 kN for residential spans. Do not omit it entirely.
At exactly 8 m you are at the crossover. The deciding factor is rarely the structure — both will work — it is the ceiling load and the headroom you want underneath. A King Post puts a single member down the middle of the room, which can interfere with a central light fixture or a ridge-aligned partition. A Queen Post leaves the centre clear and frames a flat coffered area between the two posts, which architects often prefer for great-room ceilings.
The structural tiebreaker: if the ceiling load below is heavy (heavy plaster, hung mechanical, storage attic), Queen Post wins because the load is shared between two hangers instead of one. If the roof is bare with no ceiling load, King Post wins on cost — one fewer post, two fewer joints, and a smaller straining beam to omit.
The formula assumes all of the horizontal thrust passes through the heel strap. In practice, a well-fitted bird's-mouth seat on the wall plate carries a significant portion of the thrust through bearing and friction at the wall plate before any load reaches the strap. On a tight new joint with green oak, the strap can read 30-50% of the calculated tension because the wood-to-wood contact is doing the rest of the work.
This is not a problem unless the wood-to-wood path fails. As the timber dries and shrinks across the grain, the bird's-mouth seat opens up, and the strap progressively takes more load. Re-check the strap reading 12 and 24 months after construction — if it has climbed toward the calculated value, that is normal seasoning behaviour, not a failure.
Green or partially-seasoned timber shrinks across the grain as it dries. A 250 mm deep tie beam can lose 4-6 mm of depth as it goes from 25% moisture content down to an equilibrium 12-15%. Every joint in the truss adjusts to that shrinkage, and the adjustment happens in tiny stick-slip increments — that is what you are hearing.
The creaking is harmless provided the joints were drawbored or strapped. What you are listening for is one single sharp crack rather than a chorus of small ones — that suggests a tenon has split along the grain at a peg hole, usually because the peg was too tight or driven into end-grain with insufficient relish. If the creaking is still going strong after two heating seasons, get someone up there with a moisture meter and an inspection mirror.
Yes, and it is increasingly common in restoration work where the original posts have rotted but the rafters and tie beam are sound. The detail to watch is the connection at the rafter-to-post head. Steel posts are dimensionally stable, but the timber rafter will continue to shrink and move seasonally around the steel bracket, opening gaps that water can find.
Use a slotted bracket that allows 3-5 mm of vertical movement, and detail the top so water cannot pool on the steel. Also size the steel post for the same tension load as the timber it replaces — typically a 75 × 75 × 6 mm SHS handles residential Queen Post duty with plenty of margin, and the slenderness is irrelevant because the post is in tension, not compression, under most load cases.
References & Further Reading
- Wikipedia contributors. Queen post. Wikipedia
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