Hip rafter

What is a hip rafter in a hipped roof?

A hip rafter is the diagonal rafter at the external corner of a hip roof, where two roof slopes meet. In Slovak practice it is called nárožná krokva. It runs from the corner of the wall plate up toward the top of the roof, cutting across the corner of the building in plan.

The hip rafter is the spine of the hip. The shorter námetky (jack rafters) frame into it from each side, so it carries the load they bring from both slopes. Common rafters run straight up the slope. The hip rafter runs diagonally across the corner, so at the same pitch it is longer than any common rafter on the same roof.

How does a hip rafter differ from a valley rafter?

The two members look alike in plan because both run diagonally, but they sit at opposite corners. A hip rafter sits at an external corner, where the roof falls away from the building. A valley rafter (úžľabná krokva) sits at an internal corner, where two slopes meet in a trough. The geometry is similar, but the water behaviour is not.

Water on a hip runs outward and is shed down the hip line. Water in a valley is gathered from both slopes and runs down the trough, so it is concentrated. A valley therefore needs a continuous waterproof lining or valley gutter, and a hip does not. The table below sets the three members side by side.

Member	Position in plan	Where water goes	Load it carries	Main detailing concern
Common rafter	Square to the eaves, up the slope	Down the slope into the gutter	Its own bay of roof	Spacing, bearing at both ends
Hip rafter	External corner, diagonal	Shed outward down the hip	Its own load plus the jack rafters framed into it	Support at foot and top, hip covering, ventilation
Valley rafter	Internal corner, diagonal	Gathered and concentrated into the trough	Its own load plus jack rafters from both slopes	Continuous waterproofing, valley lining, drainage capacity

Why does a hip rafter carry more load than a common rafter?

Two effects combine. The hip rafter is longer, so it spans farther between its supports than a common rafter does. It also gathers the load of every jack rafter framed into it. Each jack rafter delivers part of the roof weight and snow into the hip, and the hip collects these from both slopes. Its tributary area is therefore much larger than that of a single common rafter.

In practice the hip rafter is usually a heavier section than the common rafters, and its bending and deflection are checked more carefully. It is not sized from a rule of thumb. The cross section depends on the span, the timber grade, the snow and wind loads, and the weight of the covering, so it belongs in the structural design. Loads on a hipped roof are rarely even across the slopes, and the engineer checks the hip for the combination that governs.

How is a hip rafter supported at its foot and at its top?

A hip rafter needs a firm support at both ends. It cannot lean on neighbouring rafters for stability the way a run of common rafters does, because each of its jack rafters pushes on it at an angle. At its foot, it bears on the wall plate at the building corner, where the two wall lines meet. The bearing must be solid, and the connection must resist the outward thrust of the roof.

At its top, the hip rafter meets the ridge beam or ridge board. On larger hips it may be carried by a corner post or a diagonal strut instead. The top support stops the hip from sagging under the jack rafters. Without it, the hip deflects and the covering above it cracks or leaks. The exact arrangement depends on the roof span and is set by the structural engineer.

How is the hip line detailed for ventilation and covering?

The hip line is the most exposed part of the roof. It takes rain and wind from two slopes, and it is where the covering changes direction. Two concerns dominate: the roof cavity must still ventilate along the hip, and the covering must shed water cleanly over the joint.

For ventilation, the air path from the eaves must reach the ridge and the hip without being blocked. The ventilated roof cavity carries moisture out of the timber, and along a hip this is normally handled with a hip or ridge ventilation profile that lets the air leave at the top. Without a continuous route, moisture collects in the hip rafter and the jack rafters that bear on it, and the timber decays slowly where nobody can see it.

For covering, the hip is closed with hip tiles (valbové tvárnice) or a metal hip cap, bedded and fixed so that water runs down the hip without entering the joint. The product and fixing depend on the covering, and the installer should follow the manufacturer's instructions.

Hip line detail	Purpose	What to check at design stage
Hip tiles or metal hip cap	Sheds rain over the joint	Matches the covering and pitch, fixing per manufacturer
Hip or ridge ventilation profile	Lets the roof cavity exhale at the top	Continuous path from the eaves, no blocked insulation
Hip rafter bearing at foot	Carries the corner load to the walls	Solid bearing, tied into the wall structure
Hip rafter support at top	Stops the hip from sagging	Ridge beam or corner post, sized by the engineer
Underlay and flashing at the hip	Secondary water protection	Lapped correctly over the lower course

What should an architect check on a hipped roof design?

The architect should treat the hip as a deliberate design element, not a detail left to the carpenter. These checks cover the points that most often cause trouble on site.

• The hip rafter has its own section in the structural drawings, prepared by the engineer, rather than being copied from the common rafters.
• The foot and top supports are drawn, with the wall plate and ridge arrangement shown.
• The hip line has a ventilation route from the eaves to the ridge, shown on the section.
• The covering manufacturer's hip detail is specified, together with the underlay and flashing laps.
• The use of the attic is decided early, because the hip geometry limits headroom in the corners.

What is a common misconception about hip rafters?

A common misconception is that a hip rafter is simply a longer common rafter. It is not. Its geometry is diagonal, it carries jack rafters, and it needs its own supports. A second misconception is that the hip is a minor detail for the covering. Hip lines are a frequent source of leaks, because every cut tile and joint there is a place where water can enter. A hip roof is also not inherently stronger than a gable roof, only more complex.