Roof Timbers: Names, Sizes, Grades and Common Problems

Roof timbers are the hidden skeleton of a pitched roof. They carry the weight of the tiles, the wind and the snow, and they hold the whole roof together. Most people never see them, but the names, sizes and condition of the timber matter when you read a survey, get a quote or plan a loft conversion.

This guide explains the main roof timbers, what they do, what timber is used, how it is graded and treated, and the problems to look out for. For a visual breakdown of where each part sits, see our guide to the parts of a roof.

Roof Timber Names and What They Do

On a cut roof, all of these are formed on site. On a trussed roof, the rafters, ties and braces are built into the factory-made frames. For how they fit together, read our guide to roof construction.

What Timber Is Used for Roofs?

Most modern roofs use softwood, usually spruce, pine or fir, grown in managed forests. It is strong for its weight, easy to cut and affordable. Older and historic roofs often use oak or other hardwoods, particularly on large buildings and listed houses.

Strength Grades

Structural timber in the UK is graded for strength. The most common grades for roof timbers are C16 and C24, and C24 is stronger. The grade is usually stamped on the timber, and the right one depends on the span, spacing and load. A structural engineer or truss designer chooses it, so do not guess.

Timber Treatment

Roof timbers can be treated to protect against rot and insects. Treated softwood is common for battens and for timbers exposed to damp, such as wall plates and fascia boards. Roof battens in particular must be graded and treated to meet the standards for roofing. Our guide to roof battens explains the types and sizes.

Roof Timber Sizes

There is no single standard size for roof timbers, because rafters, joists and purlins are sized for the specific roof. The size depends on the span, the spacing, the weight of the roof covering and the loads from snow and wind. A short-span rafter may be quite slim, while a long-span purlin is much deeper.

Battens are more standard. They are thin strips, commonly in the region of 25 mm by 38 mm or 25 mm by 50 mm, with the size and spacing set by the tile or slate being used. Always follow the tile manufacturer’s guidance and the designer’s calculations.

Moisture, Rot and Woodworm

Timber is strong, but it does not like damp. Common problems include:

• Wet rot: caused by a leak or persistent damp. It softens timber and is common at the eaves, valleys and chimneys.
• Dry rot: a serious fungal decay that can spread through masonry and timber, and needs specialist treatment.
• Woodworm: insect damage shown by small round holes and fine dust. Active infestations need treating.
• Condensation: damp air in a poorly ventilated loft can wet the timbers and cause mould. Our guide to roof ventilation explains how to prevent it.

Signs of Damaged Roof Timbers

• Soft or crumbly timber that can be pushed in with a screwdriver.
• Dark staining, white mould or fungal growth.
• Small holes and fine wood dust below the timbers.
• A sagging ridge or rafters, or a bowed roof line.
• Daylight showing at the eaves or verge.
• A musty smell in the loft.

If you see any of these, have the roof looked at by a roofer or surveyor. Rotten timbers must be replaced or repaired before the roof is re-covered, otherwise the new covering will fail early.

Replacing and Repairing Roof Timbers

• Battens: normally replaced whenever the roof is re-tiled.
• Rafter ends: rotten ends can be cut out and a new piece joined to the old rafter, known as sistering or scarfing, using structural fixings.
• Wall plates: usually replaced in sections.
• Whole rafters or trusses: replaced or reinforced under the advice of an engineer.

Never cut or remove truss members, and do not move purlins or struts without structural advice. A roof acts as a complete system, and removing one part can overload another.

Roof Timber and Eaves Details

The ends of the rafters, the fascia and the soffit are the parts most exposed to water. A blocked gutter or a sagging underlay can soak them for years without being noticed. See our guide to the eaves tray to see how the underlay is supported to protect the rafter ends.

What to Ask a Roofer About Timbers

• Have you checked the rafters, battens and wall plates for rot or woodworm?
• Will damaged timber be replaced or repaired, and is it in the quote?
• What grade and treatment of timber will be used?
• Will the new battens be graded and treated to the proper standard?
• If a structural change is involved, who is responsible for the design?

Frequently Asked Questions

What are the timbers in a roof called?

The main roof timbers are wall plates, rafters or trusses, ceiling joists, the ridge board, purlins, struts, collar ties and battens, plus the fascia, soffit and bargeboards at the edges.

What timber is used for roof rafters?

Mostly graded softwood, usually spruce, pine or fir, commonly in C16 or C24 strength grades. Older roofs often used oak.

What size timber is used for roof battens?

Commonly around 25 mm by 38 mm or 25 mm by 50 mm, though the size and spacing depend on the tile or slate and the rafter spacing.

How do I know if my roof timbers are rotten?

Look for soft, crumbly timber, dark staining, mould, small holes or a musty smell. A roofer or surveyor can confirm the extent of the damage.

Can damaged roof timbers be repaired?

Often yes, by cutting out the rotten section and joining a new timber to the sound part. Serious damage may need replacement and structural advice.

Does roof timber need to be treated?

Many roof timbers are pre-treated, especially battens and those at risk of damp. The right treatment depends on the position and the designer’s specification.

Final Thoughts

Roof timbers do their work out of sight, which is why problems can go unnoticed for years. Learn the main names, check the loft from time to time and look out for damp, rot and woodworm. If a roofer finds damaged timber, deal with it before the roof is re-covered, and always get structural advice before altering the frame.