5. Structural Design of Purlin Roof Eaves Flashings

When an eaves flashing of a purlin roof is to be designed, there is a difference in designing it for a roof structure to be built or for a roof structure already existing and to be provided with a flashing in the course of maintenance of the building.

Eaves flashing of a roof structure to be built

In this case it is relatively easy to provide an eaves flashing because rafters can be used which are extended by the size of the eaves flashing. The size of the eaves flashing, however, is to be measured from the outside face of the containing wall!

Figure 15

Laid-out eaves flashing
Spl roof rafter length, y roof rafter extension

The difference (y) can be taken from a detail drawing in the scale 1: 1 or be laid out in the scale 1:1. It is also possible to calculate the eaves flashing.

Figure 16

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When the surface of the attic beam and the front face of the outside wall are square to each other and have a joint point of intersection with the surface of the roof rafter, the calculation is based on the following:

y - extension of roof rafter length
z - horizontal distance of eaves flashing
b - half the width of building
z₁ - secondary size (auxiliary size)
x₀ - joining size (length from inferior purlin line to roof rafter foot)
Spl_R - roof rafter length from calculation
a - surface of attic beam
c - front face of outside wall

Spl = Spl_R + y

Spl_R = b² + h²

Spl - real rafter length
h - roof height

y₁ - perpendicular attachment timber
n - height of inferior purlin above surface of attic beam

o - square attachment timber

Eaves flashing for an existing roof structure

This structural design is mainly used in connexion with maintenance of existing buildings. For example, if a dilapidated cornice is broken involving the danger of falling down, proofing of the building can be achieved again by an extending rafter foot.

Figure 17

(1) dilapidated cornice, (2) new eaves flashing
1 containing wall, 2 roof rafter, 3 cornice, 4 roof base facing, 5 nailed cover strap

Proofing of the building can also be achieved by displacement of the rafter foot.

Figure 18

Re-proofing of buildings
(1) former eaves flashing, (2) new eaves flashing
1 rafter foot, 2 displaced rafter foot, 3 nailed cover strap, 4 containing wall

In both cases displacement of the rafter foot can be implemented by nailing cover straps laterally onto the roof rafter. It is recommended to cover the eaves flashing by a roof base facing.

Why is a roof base facing to be recommended?
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The cover straps to be nailed on must be sufficiently long and be birdsmouthed to the inferior purlin.

Figure 19

1 wooden beam, 2 inferior purlin, 3 roof rafter, 4 cover strap, 5 birdsmouth, 6 length of cover strap, 7 thickness of cover strap,
h height of roof

The width of the cover strap must be equal to the roof rafter height and the thickness should be 40 mm to 50 mm. The length of the nails to be used for nailing the cover strap to the roof rafter should be equal to or greater than the thickness of the cover strap. Keeping an adequate distance of the nails, groups of at least four nails each should be driven-in above and below the inferior purlin.

Figure 20

Arrangement of nail groups
1 former rafter foot, 2 extended rafter foot, 3 front face of outside wall
d diameter of nail shank

In order to be able to fix the boards for the roof base facing, cleats complying with the profile of the roof base facing are to be nailed to the cover straps.

Figure 21

1 front face of outside wall, 2 cover strap, 3 cleat, 4 wooden beam, 5 inferior purlin, 6 roof rafter, 7 nails for cover straps, 8 nails for cleats

For this purpose, nails are to be used which are long enough to penetrate the two timbers and can be clinched with the grain of the wood.

Why must the nails be clinched?
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The two gable sides of the roof base facing are to be closed. For this purpose, pieces of boards complying with the profile of the roof base facing are to be inserted and nailed.

Figure 22

Closing of the roof base facing
1 front face of outside wall. 2 face of roof base facing, 3 bottom of roof base facing, 4 eaves board, 5 inserted board pieces, 6 verge flashing

Why must the two gable sides be closed?
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