Indicative detailing - The Hempcrete Topic: Designing and Building with Hemp-lime - page 550

difficult to achieve. As discussed in Chapter 21 , it is also necessary to consider the con-
struction process when detailing the eaves, with regard to the advisability of placing the
hempcrete from above.
Airtightness at the eaves is more difficult to achieve than at other junctions. One of the
reasons for this is that at the eaves of most buildings the rafters project through the airtight
line, because the vapour-permeable roof membrane which in most pitched roofs is used on
top of the insulation has to come down and join with the airtight element in the wall. This
usually requires the time-consuming job of taping a hole in the membrane, rather clumsily
(and not very effectively), around each of the rafters. This problem can be solved by us-
ing prefabricated bolt-on rafter ends, as they are fixed after the airtight line has been com-
pleted, meaning that the roof membrane can continue seamlessly down to the top of the
wall (see Figure 28 overleaf). Bolt-on rafter ends also offer the following advantages to a
hempcrete build:
They do not get covered with hempcrete, as they are fitted after the hempcrete is in-
stalled. This is useful if the rafter ends are to be left exposed at the eaves (visible from
the ground outside).
Because they are absent when the hempcrete is placed, the rafter ends don't obstruct
this process as they would do otherwise.
Because they are not a continuation of the main rafters, as would be usual, this allows
the main rafters to be an engineered I-section timber while the rafter ends can be a sol-
id timber. On large roofs this results in a cost saving, while still allowing aesthetically
attractive exposed solid-wood rafter ends. These are often desirable, for example on at-
tractive oak-framed buildings.
They make it a simple process to reduce the pitch of the roof at the eaves (a standard
detail on roofs with a medium-to-steep pitch, which helps to slow rainwater down as it
approaches the guttering).
The rafter ends are very easy to replace, which can be useful since they are most vul-
nerable to the weather and are prone to rot.
The main disadvantages to using bolt-on rafter ends are that:
They are more complicated to build and they are a relatively recent innovation, so not
all construction workers will be familiar with them.
There is a limit to the weight of the roof covering that can be used, since the bolt-on
rafter ends are inherently weaker than a continuous rafter; for example, they would not
be able to support the weight of a green roof.