Timber Framing

Part of Permanent Shelter

A timber roof frame is the skeleton that holds your roof covering in place. Without it, thatch, tiles, and bark are just piles of material on the ground. Timber framing is the most widely used roof structure in human history because forests exist on every inhabited continent and wood is the only structural material you can cut, shape, and raise with hand tools in a single day. Master these techniques and you can roof any building from a one-room cabin to a great hall.

Selecting Roof Timbers

Not every tree makes good framing timber. Poor selection leads to cracked beams, sagging roofs, and structural failure within a few years.

Best Species for Framing

Species	Strength	Rot Resistance	Workability	Notes
Oak	Excellent	Very good	Moderate	The gold standard — heavy but nearly indestructible
Pine / Fir	Good	Moderate	Easy	Lighter, straight-grained, easy to work with stone tools
Cedar	Moderate	Excellent	Easy	Naturally rot-resistant; ideal in wet climates
Ash	Excellent	Poor	Moderate	Very strong but rots quickly without protection
Poplar	Fair	Poor	Very easy	Fast-growing, adequate for temporary structures

Selection Criteria

1. Straightness: Sight down the length. A curve greater than 5 cm over 3 meters will cause problems. Reject obviously curved trunks.
2. Diameter: Ridge beams need 15–20 cm diameter. Rafters need 10–15 cm. Purlins need 5–8 cm.
3. Soundness: Knock on the trunk with a rock. A solid trunk rings. A rotten core gives a dull thud. Reject any trunk with soft spots, fungal growths, or extensive insect holes.
4. Length: Cut timbers 30–50 cm longer than needed. You will trim ends during fitting.

Green vs. Seasoned Timber

Freshly cut (“green”) wood is far easier to cut, shape, and join — the moisture makes it softer. However, green timber shrinks 5–10% as it dries, loosening joints. Two strategies: (1) Cut and dry timber for 3–6 months before building, stacked off the ground with spacers between logs for airflow. (2) Build green but make joints slightly oversized and drive in extra pegs after the wood dries and shrinks. Strategy 2 is faster; strategy 1 is more reliable.

Preparing Timber

1. Fell the tree: Cut a notch on the side you want it to fall (one-third through the trunk), then cut from the opposite side slightly above the notch. Stand clear.
2. Limb: Remove all branches flush with the trunk.
3. Debark: Strip all bark immediately using the back of an axe or a drawknife. Bark traps moisture and harbors wood-boring insects. A debarked log dries three times faster than a barked one.
4. Square the ends: Cut both ends flat and perpendicular to the log’s length. This ensures clean joint surfaces.

Ridge Beam Sizing

The ridge beam is the spine of your roof. Every rafter pair pushes against it. If it fails, the entire roof collapses.

Building Width	Minimum Ridge Beam Diameter	Minimum Support Post Spacing
Up to 3 m	15 cm	Every 3 m (end walls may suffice)
3–5 m	18 cm	Every 2.5 m
5–7 m	20 cm	Every 2 m
Over 7 m	25 cm+ or doubled beams	Every 1.5–2 m

Unsupported Spans

A ridge beam spanning more than 4 meters without intermediate support will eventually sag, even with 20 cm timber. For buildings wider than 4 meters, add center posts or an interior wall that reaches up to support the ridge beam. Never trust a single long beam to support itself.

Splicing ridge beams: If no single timber spans the full building length, splice two beams together at a support post. Use a half-lap joint (cut half the thickness from the end of each beam so they overlap) and secure with two wooden pegs driven through both pieces. The splice must sit directly over a support post — never splice between supports.

Rafter Spacing and Angle

Rafters are the sloping members that carry the roof load from the ridge beam down to the wall plate.

Spacing

Roof Covering	Recommended Rafter Spacing
Thatch (light)	75–90 cm
Bark or wood shingles	60–75 cm
Clay tiles (heavy)	45–60 cm
Sod/turf (very heavy)	40–50 cm

Closer spacing means more rafters, more work, and more timber — but a stronger roof. When in doubt, space closer. It is far easier to add an extra rafter during construction than to reinforce a sagging roof later.

Calculating Rafter Length

You need to know the building width and the desired roof pitch. For a gable roof:

1. Half-span: Divide the building width by 2. For a 4 m wide building, half-span is 2 m.
2. Rise: Multiply the half-span by the pitch factor.
   • 30-degree pitch: rise = half-span x 0.58 (2 m x 0.58 = 1.16 m)
   • 45-degree pitch: rise = half-span x 1.0 (2 m x 1.0 = 2 m)
   • 60-degree pitch: rise = half-span x 1.73 (2 m x 1.73 = 3.46 m)
3. Rafter length: Use the relationship: rafter length = half-span / cos(pitch angle). In practice:
   • 30 degrees: rafter = half-span x 1.15
   • 45 degrees: rafter = half-span x 1.41
   • 60 degrees: rafter = half-span x 2.0
4. Add overhang: Add 30–60 cm for the eave overhang beyond the wall.

For our 4 m wide building with 45-degree pitch: rafter = 2 m x 1.41 + 0.5 m overhang = 3.32 m. Cut rafters at 3.5 m to allow trimming margin.

Basic Timber Joints

You need only four joints to build a complete roof frame. Each can be cut with an axe and a chisel (or a sharpened flat stone).

Mortise and Tenon

The strongest traditional joint. A rectangular projection (tenon) on one timber fits into a matching rectangular hole (mortise) in another.

1. Mark the tenon dimensions on the end of the rafter: typically one-third the timber’s width, centered.
2. Cut away the wood on each side of the tenon with an axe, then trim flat with a chisel.
3. Mark and chop the matching mortise in the receiving beam.
4. The tenon should fit snugly — tight enough to require a mallet to drive it home, but not so tight it splits the mortised timber.
5. Drill a hole through both pieces and drive a hardwood peg to lock the joint.

Lap Joint

Two timbers overlap with half the thickness removed from each at the overlap point. Simple, fast, and adequate for most roof connections.

1. Mark the overlap area on both timbers (length of overlap should equal at least 1.5 times the timber diameter).
2. Cut halfway through each timber across the marked area.
3. Split or chisel out the waste wood.
4. Stack the two halves together — they should sit flush with the surrounding timber surfaces.
5. Pin with one or two wooden pegs.

Birdsmouth Joint

A V-shaped notch cut into the bottom of a rafter where it sits on the wall plate. This locks the rafter in place and prevents it from sliding outward.

1. Set the rafter in its final position against the ridge beam and resting on the wall plate.
2. Mark where the wall plate contacts the rafter — draw a horizontal line (matching the plate top) and a vertical line (matching the plate inner face) to form the notch shape.
3. Cut the notch no deeper than one-third of the rafter depth. Deeper cuts weaken the rafter.
4. The rafter should sit firmly on the plate with the notch hooking over the beam’s inner edge.

Pegged Joint

The simplest connection: two timbers are held together by a hardwood peg driven through drilled holes in both pieces. Used to reinforce all other joints and as a standalone connection for purlins to rafters.

• Use dense hardwood pegs (oak, ash, hickory): 2–3 cm diameter.
• Drill or burn the hole slightly smaller than the peg so it must be driven in.
• The peg should extend through both timbers and protrude at least 2 cm on the far side.

Make Pegs in Advance

Split and carve 50–100 hardwood pegs before you start framing. Having them ready speeds assembly enormously. Taper one end slightly for easier driving.

Raising a Roof Frame

This is the most physically demanding phase. Proper sequence and teamwork prevent injury and structural damage.

Step-by-Step: Gable Roof

1. Install the wall plate: Secure a straight timber along the top of each long wall, lashed or pegged to the wall posts. Check that both plates are level with each other.

2. Set support posts: If your ridge beam needs intermediate support, install vertical posts inside the building, rising from a floor beam or stone pier to the planned ridge height. Brace temporarily.

3. Raise the ridge beam: With helpers, lift the ridge beam onto the support posts (and/or gable-end walls). This is the heaviest single lift. Use ramps, levers, or rope-and-pulley if available. Secure the beam to its supports with pegged joints.

4. Install rafter pairs: Start at the gable ends. Lean one rafter from each wall plate to the ridge beam. Cut the birdsmouth at the bottom and a lap joint or mortise at the top. Lift, set, and peg each pair. Move inward, adding pairs at your planned spacing.

5. Add purlins: Once all rafters are in place, lash or peg horizontal purlins across them. Start at the eaves and work upward toward the ridge. Spacing depends on your roof covering material.

6. Install collar ties: Connect opposing rafters with horizontal ties about one-third up from the wall plate. Minimum every third rafter pair. Use lap joints at each connection.

7. Check everything: Before applying roof covering, verify that the ridge beam is straight and level, rafters are evenly spaced, and all joints are tight. Walk the wall plate and push each rafter — nothing should move or creak.

Never Work Alone at Height

Raising a roof frame requires at least 2 people, ideally 3–4. One person cannot safely hold a rafter in position while also cutting and pegging the joint. Falls from even 2.5 meters can be fatal without modern medical care.

Bracing Against Wind

Wind is a roof’s constant enemy. Without bracing, wind loads can rack the frame sideways, loosen joints, and eventually tear the roof off.

Diagonal Bracing

Install angled timbers (braces) from the top of each corner post diagonally down to the wall plate or sill beam. These triangulate the frame — a triangle cannot be deformed without breaking a member, while a rectangle can be pushed into a parallelogram.

• Minimum: four braces, one at each corner.
• Better: additional braces at each intermediate post.
• Brace angle: 45–60 degrees from horizontal for maximum effectiveness.

Wind Ties

In high-wind areas, lash or peg rafters directly to the wall plate at every rafter position (not just relying on the birdsmouth notch). Run a continuous rope or pole along the inside of the rafters near the eaves, tying each rafter to it and anchoring the ends to heavy stakes driven deep into the ground or to the foundation.

Collar Ties

Collar ties are short horizontal beams connecting opposing rafters. Their purpose is to prevent the rafters from pushing the walls outward under roof load (a force called “rafter thrust”).

• Position: One-third to one-half of the distance from wall plate to ridge.
• Frequency: Every second or third rafter pair minimum. Every pair in heavy snow areas.
• Connection: Lap joint or through-bolt with peg at each rafter.
• Sizing: Same diameter as purlins (5–8 cm) is adequate.

Without collar ties, the outward thrust of the rafters slowly pushes the tops of the walls outward. This is one of the most common failure modes in amateur timber construction — the walls gradually lean, joints loosen, and the roof eventually drops.

Key Takeaways

• Select straight, sound timbers and debark immediately. Oak and pine are the most reliable; avoid rotten or curved trunks.
• Size the ridge beam to the building width: 15 cm for up to 3 m, 20 cm for 5–7 m. Support it every 2–3 meters for spans over 4 m.
• Space rafters 45–90 cm depending on roof covering weight. Calculate rafter length from half-span, pitch angle, and overhang.
• Four joints build an entire roof: mortise-and-tenon (strongest), lap (fast and simple), birdsmouth (rafter-to-plate), and pegged (universal reinforcement).
• Raise the frame in order: wall plates, support posts, ridge beam, rafter pairs (ends first), purlins, collar ties.
• Brace against wind with diagonal corner braces and wind ties at the eaves. Triangulated frames resist wind; rectangular frames collapse.
• Install collar ties on every second or third rafter pair to prevent wall spread from rafter thrust.