Timber Structural Notes and Specifications

PorthouseDean’s structural drawings include a standard timber specification that applies to all timber structural elements unless explicitly noted otherwise. This article explains each requirement and why it matters.

Structural Timber Grade

All structural timber: C24 to BS EN 338.

C24 is the standard structural grade for domestic projects in the UK. Lower grades (C16) are weaker; higher grades (C35+) are available but not required for standard spans and loadings. C24 provides the balances between availability, cost, and structural performance needed for domestic construction.

Visual grading marks should be visible on delivered timber. If unmarked timber is delivered, request confirmation of grade from the supplier before installation.

Treatment

All structural timber must be factory treated to the appropriate Use Class before delivery to site. For domestic structural applications above damp-proof course level, this is typically Use Class 2 (UC2) — inside a building with risk of occasional wetting, but not permanently wet.

Do not treat timber on site with brush-applied preservative and expect this to substitute for factory treatment. Factory treatment achieves full penetration of the preservative under pressure or vacuum-pressure processes.

Multiple Pieces (Built-Up Members)

Where two or more pieces of timber are used together to form a single structural member (e.g. a double or triple joist):

– All pieces must be M12 bolted at 500 mm centres through the full depth
– Use flat washers under bolt heads and nuts to prevent pull-through
– Bolt holes should be drilled to 1 mm clearance

This ensures the combination acts as a single composite member rather than individual pieces that can separate under load.

Joist Hangers

Where joists terminate into a beam or wall rather than bearing on top:

– Use joist hangers with side flanges (the flanges that grip the sides of the joist, not just the underside)
– Side-flanged hangers provide rotational stability and prevent the joist rolling under load
– Size hangers to suit the joist width and depth — do not use hangers that are too narrow or too deep

Bearing Into Masonry

Timber bearing into masonry walls must have a minimum 100 mm bearing length. This ensures adequate distribution of reaction at the joist end into the wall below.

Do not cut joists so that they bear on less than 100 mm — the reduced bearing area increases local stress and may crush the timber or masonry at the support.

Post and Column Restraint

Timber posts and columns must be strapped at both top and bottom to the adjacent structure. Restraint straps must be:

– At the top of the post, connected to the beam or plate above
– At the base, connected to the floor or foundation below
– Sized to transfer the design lateral restraint force

Without top and bottom strapping, freestanding posts can become unstable under lateral loads.

Fixing to Masonry

Where structural timber is fixed to masonry walls using mechanical anchors:

– Use M12 anchors at 450 mm vertical centres
– Anchors should be chemically bonded or mechanical expansion type, as specified
– Do not use screws or plastic wall plugs for structural connections

Loft Conversion: Clearance to Ceiling Joists

Where new loft floor joists run at a level above existing ceiling joists, a 25 mm minimum clearance must be maintained between the underside of the new loft floor structure and any existing ceiling joists. This prevents load being transferred accidentally to elements that were not designed to receive it.

Insulation and Ventilation Note

Joist sizes specified by PorthouseDean are structural sizes — they are not increased to accommodate insulation or ventilation requirements. Do not use the structural joist depth as a guide to the total floor or roof construction depth. Consult your architect to confirm the total build-up including insulation.

Contact us if your project has timber elements that need clarification.