The tie provides support for the ceiling and prevents the feet of the rafterfrom spreading. In order to function properly the connection between the tieand the rafter must be sound.

Tie typically75 x 50mm

Span - up to 5.0 metres or so.

Closed couple roof

Rafters typically100 x 50mm

If the tie is raised slightly there are minor savings in walling as part of theroom is formed within the roof space. In this situation the tie is known as acollar.

Where there is a large overhangit is common practice to providea soffit board.

Flush eaves are relatively cheap butthere is little protection for the wall.

Cavity construction -typical of 1950s and 1960s.

Solid wall construction -up to 1920 or so.

The soffit board can be fixed to the wall or to the rafters. In some houses the soffitwill include a ventilation strip.

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A Very Brief History of Roofs

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Wall plate

Ceilingjoist

Binder

Ridge

Purlin

Purlin strut

Common rafters

Internalload-bearingwall

Tie

Tie

Purlin

Strut

Tie

Strut

The purlin is built into the party walland given additional support by thesmall corbel in the brickwork.Intermediate support is provided byinclined struts.

Corbel

Purlin

Ventilation can be provided throughthe soffit board or above the fasciaboar. In this example the roofincludes vents in the soffit board anda plastic tray to ensure that ventilationis not blocked by the roof insulation.

Tray to prevent quiltblocking airpath.

In a cold ‘breathing’ roof a vapour checkhelps prevent moist air entering the roofvoid. A ‘breathing’ underlay allows anyvapour to escape and the counter-battensprovide a ventilation space.

Vapour check

‘Breathing’ underlay

Counter battens

Rafter

A ‘breathing’roof in the process ofbattening and counter-battening.

Ventilation is normally providedeaves-to-eaves. Eaves-to-ridgeventilation is more effective.

Hipped roofs are an alternative to gable roofs. Larger examples include purlins on allslopes, sometimes supported by struts and trusses.

Jack rafters

Hip rafter

Mansard roofs have two pitches on eachslope. A purlin normally supports the rafterswhere the two pitches meet. These roofsoften include living accommodation in theroof void.

Single and double mansard roofs.

Some roofs slope inwards rather than outwards. These inevitably create potentialproblems. Centre valleys, covered in lead or zinc, need careful detailing if they areto be trouble free.Strut is notched

to provide supportfor purlin.

Timber cover plateseither side of tensionmembers.

150 x 50 purlin

150 x 25 ridge

Trusses at 1.8 or2.4 centres.

A TRADA truss - 1950s

Queen Post truss (left) andKing Post truss (below).

Trusses provideintermediate supportfor purlins without theneed for internal loadbearing walls. They wereonly required in verylarge houses.

Ridge board

Purlins

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Truss clips

A ‘Fink’ truss - themost common pattern.

Gable walls need restraint and arestrapped to the roof structure. Thestraps are usually positioned so thatthey can be nailed to the horizontalbraces

A holding-down strap.

Horizontal brace (binder)

Diagonal brace

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