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NVQ Level 2 How to erect structural carcassing

ValleysThe next section deals with valleys, which are formed when two

sloping parts of a pitched roof meet at an internal corner.

Marking out for valleysValleys can be worked out in the same way as hips, using either a

ready reckoner or geometry. Here we will look at geometry.

Working out the angles for valleys is similar to doing so for hips

except that the key drawing is not a triangle but a plan drawing of

the roof.

First you need to fi nd out the valley rafter true length, plumb and

seat cut. Start by fi nding the rise of the roof and drawing a line

this length at a right angle to the valley where it meets the ridge.

Join this line to the point where the valley meets the wall plate.

This will give you the true length of the valley rafter as well as the

plumb and seat cuts.

As with the hip rafter there

are two other angles to fi nd

for a valley rafter: the dihedral

angle and the edge cut.

The dihedral angle for the

valley is used in the same way

as the hip dihedral. Figure

12.35 shows you how to work

out the dihedral angle.

Figure 12.33 Plan of a roof

Figure 12.34 Valley true length

Figure 12.35 Dihedral angle hip

Hip

Valley

Plumb cutvalley rafter(PCVR)

Seat cut valley rafter(SCVR)

True lengthvalley rafter

Rise

Plumb cutvalley rafter(PCVR)

Seat cut valley rafter(SCVR)

Rise

90 90

True lengthvalley rafter

Dihedral anglevalley rafter(DAVR)

Most drawings use abbreviations or symbols to avoid cluttering the drawings and to make them easier to read

Did you know?

The fi rst and last rafters are placed 50 mm away from the wall to prevent moisture that penetrates the outside wall coming into contact with the rafters, thus preventing rot

Did you know?

You will need to understand the contract and specifi cation fully before you begin work on preparing a valley. These are formal documents that often have special terminology (words and phrases). You will need to read these carefully, using detailed reading strategies, to ensure that you have all the information you need

Functional skills

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NVQ Level 2 How to erect structural carcassing

The fi nal angle to fi nd is the edge cut for the valley rafter, as

follows:

1 Mark on the rise and true length of the valley rafter.

2 Draw a line at right angles to the valley where it meets the wall

plate and extend this line to touch the ridge at A.

3 Set your compass to the true length of the valley and swing an arc towards the ridge at B.

4 Join up the line A–B to give you the edge cut.

The fi nal part of valley

geometry is to fi nd the true

length and edge cut for the

cripple rafters, as follows:

1 Draw out the roof plan

as usual, then to the side

of your plan draw out a

section of the roof.

2 Set your compass to the

rafter length and swing an

arc downwards.

3 Draw line A downwards

until it meets the arc, then

draw a line at right angles

to line A until it hits the

wall plate, creating line B.

4 Draw a line from where

line B hits the wall plate

up to where the valley

meets the ridge. This will

give you the appropriate

true length (TLCrR) and

edge cut (ECCrR).

Setting out a valleyThere are four steps to follow

when setting out a valley.

Step 1 Fit the wall plate and

mark it out with the position

of the common rafters.

Step 2 Fit the common

rafters and ridge.

Step 3 Fit the hip and jack

rafters.

Figure 12.36 Edge cut

Figure 12.37 Cripple length and angle

Swing an arc – set a compass to the required radius and lightly draw a circle or arc

Key term

Plan

Edge cut valleyrafter (ECVR)

90

Rise

TLVR

BA

º

Rafte

rs

Line B

Development ofsurface

True length cripplerafter (TLCrR)

Edge cutcripple rafter(ECCrR)

Line A

Figure 12.38 Step 2 Common rafters fi tted

Figure 12.39 Step 3 Hip and jack rafters fi tted

When cutting or fi tting wood make sure that you are using the correct PPE. You will need to be particularly careful when setting valleys to make sure your hands are not harmed. Use safety gloves at all times

Safety tip

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NVQ Level 2 How to erect structural carcassing

Chloe and Tyrone have been tasked with cutting in a garage roof with gable ends. They work out the angles and lengths, cut the pattern rafter, which they use to mark out all the others, then fi t them. They are almost fi nished when the foreman turns up and tells them to stop and check it for level. Chloe and Tyrone put a level on the ridge board and fi nd that it is way out of level.

How could this have happened? What could have been done to prevent it? Chloe and Tyrone should have been looking at checking this work at a more regular intervals – what tools and resources could they have used for this? How could they have made sure that they were doing enough to check? Is it their responsibility for the mistake in the work? The foreman has only just checked their work now near the end.

Is there anything Chloe and Tyrone could do to fi x this?

Working life

Step 4 Fit the valley and cripple

rafters, taking the true lengths and

bevels from the drawings.

An alternative to using valley rafters

is to use a lay board. Lay boards

are most commonly used with

extensions to existing roofs or where

there are dormer windows.

The lay board is fi tted onto the

existing rafters at the correct pitch,

then the cripple rafters are cut and

fi xed to it.

Figure 12.41 Valley lay board set-up

Lay board – a piece of timber fi tted to the common rafters of an existing roof to allow the cripple rafters to be fi xed

Key term

Lay board

Cripplerafters

Figure 12.40 Step 4 Valley and cripple rafters fi tted

FAQ

Check it out

Why are fl at roofs only guaranteed for a certain amount of time?

Most things that you buy or have fi tted have a guarantee for a certain amount of time and

building work is no different. If the fl at roof had a guarantee for 50 years, the builder would

be responsible for any maintenance work on the roof free of charge for 50 years. Since the

average life of a fl at roof is 12–15 years, the builder will only offer a guarantee for 10 years.

Which type of fl at roof decking is the best?

There is no specifi c best or worst but some materials are better than others. All the materials

stated serve a purpose, but only if they are fi nished correctly, for example a chipboard-covered

roof that is poorly felted will leak, as will a metal roof if the screw or bolt holes are not sealed

correctly.

How can I get the lead on a fl at roof to fi t into the brick/block work?

The lead is fi tted into a channel or groove that is cut into the brick/block work by a bricklayer.

The groove is cut using either a disc cutter or a hammer and cold chisel. Once the groove is

cut the lead is fed into the groove, wedged and sealed with a suitable mastic or silicon.

1 Describe the differences between a single and double roof, using diagrams to clearly

show the different components of each.

2 Describe the method used for fi tting a wall plate.

3 Draw a diagram showing where a pitch line should be marked. Describe what the

purpose of the pitch line is.

4 What is the dihedral angle (backing bevel) and what purpose does it serve?

5 Describe the two methods used to form a valley. Use diagrams to illustrate the different

steps. State clearly what tools would be needed to complete these exercises.

6 What four different materials can be used when decking a fl at roof? What benefi ts does

each type of material bring when used for decking?

7 What purpose does Pythagoras’ theorem serve for carpenters? Draw a diagram

demonstrating how a carpenter would use this equation.

8 Explain the difference between a hip and a gable end. Why would there be differences in

use between these two types of end? Use a diagram to demonstrate the different uses of

these ends.

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