Timber Roof Truss Design Residential: 5 Critical Checks Before the Roof Goes On
Timber roof truss design residential is typically a specialist contractor design item, but the structural engineer responsible for the primary structure must understand how trusses work in order to review the contractor's design, check lateral stability and ensure the roof is properly tied to the walls below. Getting timber roof truss design residential wrong can lead to roof spread, wall cracking, or in extreme wind conditions, the entire roof lifting off the building.
Timber Roof Truss Design Residential: The 3 Types of Truss
Timber Roof Truss Design Residential: Typical Loading
The loads applied to timber roof truss design residential projects come from several sources. The structural engineer must define these loads and pass them to the truss manufacturer as part of the design brief.
| Action | Typical value | Position | Duration |
|---|---|---|---|
| Tiles and battens | 0.66 kN/m² | Full length of top chord | Permanent |
| Ceiling and insulation | 0.25 kN/m² | Full length of bottom chord | Permanent |
| Water tank (up to 300 litres) | 2 x 0.45 kN point loads | Bottom chord | Long-term variable |
| Loft storage | 0.25 kN/m² | Full length of bottom chord | Long-term variable |
| Snow | Per BS EN 1991 | Full length of top chord | Short-term variable |
| Maintenance access (pitch ≤ 30°) | 0.9 kN concentrated | Mid-span of each top chord | Short-term variable |
| Wind (uplift and pressure) | Per BS EN 1991 | Full length of top chord | Instantaneous variable |
Timber Roof Truss Design Residential: Tension Member Design
The bottom chord of a simple truss is primarily a tension member. Under gravity loads, the rafters push outward at the eaves and the bottom chord ties the two sides together, preventing roof spread. The design tension strength of timber parallel to the grain is calculated to BS EN 1995-1-1 (Eurocode 5).
| Grade | Bending fm,k | Tension ft,0,k |
|---|---|---|
| C16 | 16 N/mm² | 7.2 N/mm² |
| C24 | 24 N/mm² | 8.5 N/mm² |
| GL28c (glulam) | 28 N/mm² | 19.5 N/mm² |
| TR26 (truss grade) | 28.3 N/mm² | 18 N/mm² |
TR26 is a unique strength class specifically for timber within simple trusses used in timber roof truss design residential. It has significantly higher tension properties than standard C16 or C24, which is why truss manufacturers specify it. The applied tension stress in the bottom chord must not exceed ft,0,d.
Timber Roof Truss Design Residential: Lateral Stability and Bracing
Timber trusses have no inherent lateral stability. They rely entirely on bracing systems to prevent them from toppling sideways. In timber roof truss design residential, both the top chord (rafter space) and the bottom chord (ceiling space) must be braced. This is because wind uplift can reverse the stress in the bottom chord from tension to compression, making it susceptible to buckling.
As a general rule for timber roof truss design residential, the maximum distance between trusses should be 1200mm. If trusses are placed at greater centres, plan bracing must be installed in the planes of both the top and bottom chords. This typically takes the form of diagonal struts made from isolated timber elements, fixed onto trusses to provide the lateral stability system. The diaphragm action within the roof structure (provided by tiling battens above and plasterboard below) also contributes to restraint, but only if the battens span continuously over at least three trusses.
Timber Roof Truss Design Residential: Tying Trusses to Masonry Walls
Trusses are lightweight and can be lifted by wind uplift or suction on gable ends. In timber roof truss design residential, every truss must be positively tied to the masonry walls below using galvanised steel straps. The straps are fixed to the inner leaf of the wall and bent over the wall plate and truss bottom chord. This creates a direct tension path from the truss down into the masonry.
Gable end walls require particular attention in timber roof truss design residential. The gable wall reaches the apex of the roof and is exposed to the full lateral wind pressure. It must be restrained by straps connecting the inner face of the gable wall to at least three trusses. The ceiling bracing in the bottom chord plane then distributes this restraint force across the roof structure.
Roof spread and sliding of the roof structure on the wall plate also need to be reviewed. These are resisted by the positive strap connection between the truss heel and the wall plate, combined with the wall plate being bolted to the masonry below.
Timber Roof Truss Design Residential: Frequently Asked Questions
Who is responsible for timber roof truss design residential?
The truss manufacturer designs the individual truss components and connections. The structural engineer is responsible for specifying the loads, checking the manufacturer's design, ensuring lateral stability of the complete roof structure, and detailing the connections between the roof and the primary building structure.
What is the maximum span for a simple timber truss?
Simple trusses with punched metal plate fasteners can span up to about 15m for domestic-scale buildings. Complex trusses with bolted glulam elements can span up to 20m. Hybrid trusses with steel tension members can span up to 80m, although these are rarely used in residential projects.
Can I convert the loft if it has trussed rafters?
Trussed rafter roofs can be converted, but the trusses cannot simply be cut away. The structural engineer must design an alternative load path, typically using steel beams and purlins, to replace the structural function of the trusses that are removed or modified. This requires full building regulations structural calculations.
How much does timber roof truss design residential cost?
At SECalcs, roof structure design packages that include truss specification, bracing layout and wall plate connection details start from £295 for simple domestic roofs. More complex roof forms or loft conversion designs start from £495. Call 07359 267907 for a quote.
→ Planning a loft conversion? See our Loft Conversion Structural Calculations guide. Timber Floor Joist Design → Building Regulations Structural Calculations → Steel Beam Design for Residential →
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