Timber Frame Structures
Timber frame structures assemble large solid or engineered timber members into a load-bearing frame using cut joints and steel connectors.
- Part
- Joining
- Revised
- 2026-08-11
At a glance
- Family
- Mechanical
- Typical tolerances
- CNC-cut joinery to about ±1/16 in (±1.5 mm) over member lengths of 40 ft (12 m) and more; erection tolerances are set by the project specification
- Lead time
- Months from design freeze to erection, dominated by engineering, shop drawing approval and engineered-timber mill scheduling; CNC cutting of a frame itself takes days
- Materials
- Wood
What it is
Timber frame construction assembles large solid-sawn or engineered timber members into a load-bearing frame, joined either with cut joinery — pegged mortise and tenon, the traditional route — or with steel connectors such as knife plates, concealed hangers, bolts and self-tapping structural screws.
The modern version runs on engineered products. Glulam is built up from laminations of 1-3/8 in (35 mm) nominal thickness for straight members and 3/4 in (19 mm) for tight curves; cross-laminated timber (CLT) uses 3, 5 or 7 orthogonal layers to give panels 4–12 in (100–300 mm) thick that act as both floor and diaphragm.
Fire design is what makes heavy timber viable structurally. Wood chars at a nominal 1.5 in/hour (38 mm/hour) and the char layer insulates the sound wood beneath, so a member is fire-rated by calculating its residual section after the required exposure — an oversized beam is a fire-rated beam, with no applied protection.
How it works
- Engineer the frame and the connections. Member sizes and connection capacities come from the NDS, with CLT to ANSI/APA PRG 320 and glulam to ANSI A190.1. Traditional cut joinery is designed to TFEC 1. Connections, not members, usually govern.
- Model and produce shop drawings. The frame is modelled in 3D and every joint is dimensioned, because cutting is done off the model rather than off site measurements.
- Machine the joinery. CNC timber processing centers cut mortises, tenons, housings, bird's mouths and connector pockets in members up to 40 ft (12 m) or longer, holding roughly ±1/16 in (±1.5 mm). What was once weeks of layout and chisel work is now a machine program.
- Dry fit. Frames are traditionally assembled flat in the shop or on site to confirm every joint closes before the raising.
- Raise and pin. Bents are assembled on the deck and lifted into place. Traditional joints are secured with 1 in (25 mm) hardwood pegs, draw-bored with a 1/16–1/8 in (1.5–3 mm) offset so driving the peg pulls the shoulder tight. Modern frames use bolts, screws and steel plates, tightened snug rather than to a preload.
- Enclose. Structural insulated panels or CLT panels wrap the frame, which stays exposed inside.
Solid sawn versus engineered timber
Solid heavy timber is usually delivered green or partly seasoned at 12–19% moisture content and will check and shrink across the grain as it dries in service — that is expected and generally not a structural problem, but the connections must tolerate it. Glulam, LVL, PSL and CLT are manufactured at 8–12% moisture content, are dimensionally stable, and can span further for a given depth, which is why they dominate long-span and mass timber work.
Design guidelines
Design connections for shrinkage across the grain
Timber shrinks across the grain and barely at all along it. The classic detailing error is a bolted connection with several inches of cross-grain thickness between fasteners: as the timber dries and shrinks, the steel does not, and the wood splits between the bolts. Keep cross-grain dimensions between fasteners short, use slotted holes where movement is expected, and place fasteners so shrinkage relieves rather than builds stress.
Do not torque timber bolts like steel bolts
Bolts in timber connections are snug-tightened, not preloaded. Wood creeps under sustained compression perpendicular to the grain, so any preload applied at installation is lost within months and excessive torque simply crushes the wood under the washer. The values on the bolt torque chart apply to steel-to-steel joints and must not be carried over to timber connections; follow the connection design and the connector manufacturer's installation instructions instead.
Size for the char layer, not for a coating
At a nominal char rate of 1.5 in/hour (38 mm/hour), a one-hour rating consumes roughly 1.5 in (38 mm) from each exposed face, and the member is designed on the residual section with reduced strength factors. That is why exposed heavy timber achieves ratings that exposed steel cannot without protection — but it only works if the member is genuinely oversized from the outset. Retrofitting fire performance into an undersized timber member is not possible.
Detail out end-grain exposure and standing water
Timber decays where it stays wet. Keep end grain off concrete and out of the weather, provide capillary breaks at bearing points, flash exposed beam ends, and design roof geometry so water never sits on a horizontal timber surface.
| Element | Typical figure | Notes |
|---|---|---|
| Glulam lamination, straight members | 1-3/8 in (35 mm) nominal | Thinner laminations for tight radii |
| Glulam lamination, curved members | 3/4 in (19 mm) | Radius-dependent |
| CLT panel thickness | 4–12 in (100–300 mm) | 3, 5 or 7 orthogonal layers per ANSI/APA PRG 320 |
| Traditional peg | 1 in (25 mm) hardwood, draw-bored 1/16–1/8 in (1.5–3 mm) | The offset is what pulls the shoulder tight |
| CNC-cut joinery tolerance | About ±1/16 in (±1.5 mm) | Over member lengths of 40 ft (12 m) and more |
| Solid timber moisture content | 12–19% at erection | Expect checking and cross-grain shrinkage |
| Engineered timber moisture content | 8–12% | Dimensionally stable |
| Nominal char rate | 1.5 in/hr (38 mm/hr) | Per the NDS; design on the residual section |
Inspection
Grade stamps on solid timber and mill certificates on glulam and CLT are the starting point. Moisture content is verified with a meter at delivery and before enclosure. Joint fit-up is checked at dry fit, connector installation is verified against the shop drawings and the manufacturer's instructions, and glulam bond lines are qualified by delamination testing to ASTM D2559 at the mill. Post-erection inspection is largely visual — plumb, level, bearing, and connector installation — with any splitting at connections recorded and evaluated.
Cost drivers
Engineering and shop drawings come first and are a substantial fraction of the cost of a timber frame, because every joint is designed and dimensioned before anything is cut. Connection design in particular drives both cost and schedule; a frame with dozens of unique connector types is far more expensive than one with a repeating detail.
Material follows. Engineered timber is a manufactured product with mill lead times and minimum order quantities, and long-span glulam or wide CLT panels are made to order. Solid heavy timber in large sections is a specialty purchase.
Fabrication itself is comparatively fast once the model exists — CNC timber processing has collapsed what used to be the dominant labor cost. Erection is craned work with a small crew, usually measured in days for a house-sized frame.
- Repeat the connection detail. One well-engineered joint used sixty times costs a fraction of sixty variations.
- Design bays on a regular module so members repeat and the CNC program runs efficiently.
- Choose engineered timber where spans are long — the added material cost is usually less than the cost of the deeper solid section it replaces.
- Fix the design before the mill order. Changes after glulam or CLT is scheduled reset the lead time entirely.
- Detail for weather during construction. Mass timber left open to rain is expensive to dry and to remediate.
Questions
5 questionsWhy are timber bolts not torqued to steel bolt values?
Because wood creeps under sustained compression perpendicular to the grain. Any preload applied at installation relaxes within months, and high installation torque simply crushes the timber under the washer. Timber connection bolts are snug-tightened per the connection design and the connector manufacturer's instructions; steel-to-steel torque tables do not apply.
How does heavy timber achieve a fire rating without protection?
Wood chars at a nominal 1.5 in/hour (38 mm/hour) and the char layer insulates the sound wood beneath it, so the member keeps a load-bearing core. Fire design under the NDS calculates the residual section after the required exposure and checks it with reduced strength factors. The consequence is that the member must be oversized from the start — the rating cannot be retrofitted.
What is the difference between glulam and CLT?
Glulam is a beam or column built from laminations all running the same direction — 1-3/8 in (35 mm) nominal for straight members, 3/4 in (19 mm) for tight curves — and carries load along its length. CLT is a panel of 3, 5 or 7 layers laid at right angles to each other, 4–12 in (100–300 mm) thick, which spans in two directions and acts as a floor, wall or diaphragm.
Why do timber connections split, and how is it avoided?
Because timber shrinks across the grain as it dries while steel connectors do not. A connection with several inches of cross-grain thickness between fasteners is restrained as the wood tries to shrink, and it splits. The fix is detailing: keep cross-grain distances between fasteners short, use slotted holes where movement is expected, and place fasteners so shrinkage relieves rather than builds stress.
How accurate is CNC-cut timber joinery?
Roughly ±1/16 in (±1.5 mm), held over members 40 ft (12 m) and longer. That accuracy is what makes modern timber framing practical: joinery is cut directly from the 3D model, so a frame can be shipped and raised with confidence that the joints will close, rather than being fitted piece by piece on site.