Biobâtir · model data sheet

The Biobâtir model

Closed trusses, set on a continuous plate

The long walls with a continuous plate go up first, then closed trusses are set on them, and the roof bears on purlins. Roof thrust closes inside a continuous tie beam, in bearing, and every joint that stays critical has a second path, all in timber. You set the dimensions and the pitch in the generator; the truss adapts to the span — plain up to 16 ft, queen posts up to 30 ft, king post beyond.

Span: 16–28 ft (drawing: 20 ft)
Pitch: adjustable (drawing: 10/12)
Spacing: trusses at 8–10 ft
Timber: green, portable sawmill
Snow: your location's; Structure subject to approval by a structural engineer

The truss in section

timber in view
timber in section
hardwood keys and wedges
hidden or optional
forces

Thrust never leaves the triangle. The principal rafters bring compression down to the tie beam, which takes it back as tension in one continuous piece. The post only takes compression. Purlins carry the roof from truss to truss: no rafter pushes on the plate. At the heel, keyed straps (6) clamp rafter and tie beam. The summer beam (11) and its post are only needed if the loft carries a floor. Schematic drawing, no construction dimensions.

The wall-top joint

Where post, plate, tie beam and rafter foot meet. The same joint, at the same scale.

What the model does not do single link

The tie beam hangs from a one-peg half dovetail, 16 in below the plate: the post acts as a lever right where it is most cut away. On a 12 × 16 ft frame, the joint works at 69% of its limit under 2 kPa of snow and exceeds it by 38% under 4.5 kPa. Nothing backs it up, and it stays in tension the whole time the green timber dries.

What the model does two paths

The tie beam sits on the plate, which sits on the post: everything goes down in bearing. The wedged key stops it sliding and is re-tightened as the timber shrinks. Thrust enters the tie beam through the double birdsmouth; if the relish shears, the keyed straps take over. As an option, one bolt perpendicular to the rafter replaces the straps.

Joint by joint

Mark	Joint	Joinery	Why
1	Post foot	Short 2 in tenon housed in the sill, not pegged. Sill anchored to the foundation.	Works in compression, resists sliding, traps no water.
2	Post	Straight, no jowl (8×8), as long as the plate passes in cross-grain compression; otherwise a wider plate.	The head gets only two cuts: the plate tenon, and the brace mortises lower down.
3	Post → plate	4 in top tenon, pegged; the continuous plate sits on the post.	Load goes down in bearing; the peg only serves during raising.
4	Plate	Continuous. Bladed, keyed scarf at the inflection point, next to a brace, never over a post.	No more short tenon pulled out by wind between two trusses.
5	Tie beam → plate	Tie beam set on top, over the post. A hardwood key with folding wedges, driven in from the side after setting, in a groove half in the plate and half in the tie beam; vertical peg against uplift.	No dovetail: the key works in bearing, and its wedges are re-driven as green timber shrinks.
6	Heel (rafter → tie beam)	Double birdsmouth into the continuous tie beam; hardwood keyed straps, one on each face, clamping rafter and tie beam. Option: one bolt perpendicular to the rafter, nut under the tie beam.	Thrust enters the tie beam in bearing. If the relish shears, the straps take over: a second path in timber, which can be tightened.
7	Thrust between trusses	None: 6×7 purlins at 48 in on the principal rafters, housed and wedged.	The plate no longer takes thrust; it only bends vertically.
8	Ridge	Ridge purlin between the rafters, in a bridle or tenoned into the king post; ridge braces king post ↔ ridge both ways.	Lengthwise stability comes from members, not from the decking.
9	King post → tie beam	Through tenon with two keys, taper no steeper than 1:12.	Under roof load alone the king post foot is in compression; it only pulls under a floor: the joint can be re-wedged.
10	Roof slopes	Decking, then sheathing nailed over the decking; no cross bracing in the slope.	The sheathing is the main lengthwise path; the ridge braces (8) are the second, in timber, with no steel.
11	Summer beam	Carried by a post under the tie beam; never a deep mortise in a tie beam under tension.	The member that ties is kept apart from the member that carries.
12	Bracing	Main path: braced walls (sheathing, or reinforced plastered straw from sill to plate) and a roof diaphragm. Second path: braces in opposed pairs, at least 36 in long, below the head joints.	The frame leans on the walls; the braces keep a compression path in each direction.

Along the building

Section on the ridge line, long wall seen behind. The plate runs unbroken over the three bays; its scarf falls near the inflection point, next to a brace, never over a post. At the top, the ridge purlin is held by ridge braces on both sides of each king post.

One roof slope in plan, in three layers. Purlins span from truss to truss. The slope has no cross bracing: decking then sheathing close it, and the ridge braces at every king post, seen in the long section, hold the ridge line. Lengthwise stability never rests on the boards alone.

Why there is no single link

Load path	What carries it	If it gives way
Gravity	timber-on-timber bearing: birdsmouth, key, top tenons	slow, visible crushing
Thrust	continuous tie beam + double birdsmouth heel	the keyed straps take over; wedges re-tighten
Rafter thrust	removed by the purlins	—
Lengthwise	roof sheathing, ridge braces at every king post	two paths
Lateral	braced walls, diaphragm, opposed braces	three paths
Green timber shrinkage	wedged keys, birdsmouths, re-wedgeable straps	is adjusted

Relish length behind the birdsmouth

20 ft truss, 10/12 pitch, 7 in wide tie beam, 120 psi allowable shear.

Spacing	S = 2 kPa	3 kPa	4,5 kPa
8 ft	6 in	9 in	12 in
10 ft	8 in	11 in	16 in
12 ft	10 in	13 in	19 in

Closely spaced trusses keep the heel within the proportions of an ordinary tie beam: all these lengths stay under the 20 in limit a 10 in deep tie beam allows.

Raising sequence

Anchored sills
Long walls — assembled flat on the deck (posts, scarfed continuous plate, braces, girts), tilted up, plumbed and braced. A 24 ft wall in 8×8 weighs on the order of 1,200 to 1,500 lb in green timber.
Gable ends — and their girts.
Whole trusses — assembled flat on the ground, lifted with a gin pole or small crane (≈ 1,000 to 1,300 lb for 20 ft), set on the plates over the posts; the wedged keys are driven and the pegs set once the truss is in place.
Roof — ridge purlin and ridge braces, purlins, decking, sheathing.
Re-tightening — the wedges of the keys and straps are driven again after one heating season.

What it costs

No knee wall: the loft starts above the tie beam, and one tie beam per truss crosses the loft.

Trusses of half a ton and more: a gin pole or small crane, with the permits and safety that come with lifting.

More joinery: straps, keys and wedges take shop hours a bolt would save; the bolt remains available as an option.

Orders of magnitude: basic statics and unfactored allowable stresses. The design is redone to the local standard by an engineer, whose seal is mandatory.

Set the dimensions in the generator