C&G PowerPoint Form an opening in a roof with a dormer

Level 3 Diploma in Site Carpentry

© 2013 City and Guilds of London Institute. All rights reserved. 1 of 10

Unit 306: Carry out first fix roofing and stairs

Forming an opening in a roof and constructing a dormer



IntroductionAims

•Understand how to form openings in a roof and construct a dormer

•Describe the procedures for forming openings in roofs

•Describe construction details of a dormer



Forming openings in a roof

A roof may have components such as chimneys or roof windows. Forming these creates extra work because the roof must have an opening for them to be fitted. The work involves cutting out timbers in the roof and putting in extra support to carry the weight of the roof over the missing timbers. This is known as trimming out.



Forming a chimney opening

Framing anchoror joist hanger

Trimmer

Bevelled housing joint

TrimmerTrimming Trimming

Trimmingrafter

TrimmerTrimmedrafter

Roof slope

Trimmedrafter

Trimmedrafter

Rafter

Source: BarkingDog




Chimneys are rarely used in new house construction as there are more efficient and environmental ways of providing heating, but most older houses will have chimneys and often these roofs may be altered.

When constructing such a roof, the chimney would already be in place, so timbers would be cut to fit the roof, omitting the rafters around the chimney. Rafters and trimmers are positioned with a 50mm gap between the chimney and the timbers to prevent the spread of fire.




Source: BarkingDog



Forming an opening for a roof light or dormer window

We’ll look at creating openings for:•a new roof•an existing roof.

Source: Tim Taylor



Dormer roofs

Dormer roofs come in a number of styles.

Source: BarkingDog



Dormer construction

Source: Shutterstock



Construction procedureThe procedure for constructing dormers is as follows:

1.Form opening2.Form sides3.Form roof4.Fit cladding5.Fit roof tiles6.Insulate sides and ceiling7.Plasterboard sides and ceiling



Dormer constructionA vertical section through the side wall:

Source: Concerca



U and R values

R-value (thermal resistance)Thermal resistance is measured in m2K/W and is equal to the thickness of the material (in metres) divided by the conductivity of that material. The resistances of each material within an element are added together to determine the overall resistance of the element. The higher the R-value, the more efficient the insulation

U-value (thermal transmittance)The U-value is a measure of heat loss in a building element such as a wall, floor or roof. It can also be referred to as an ‘overall heat transfer co-efficient’ and measures how well parts of a building transfer heat. This means that the higher the U-value the worse the thermal performance of the building envelope. A low U-value usually indicates high levels of insulation. This is useful because it is a way of predicting the composite behaviour of an entire building element rather than relying on the properties of individual materials



The U-value is expressed as watts per square metre, per degree Kelvin, W/m2K. It is calculated from the reciprocal of the combined thermal resistances of the materials in the element, air spaces and surfaces. The effect of thermal bridges, air gaps and fixings is also taken into account .

U-values can be calculated using the following formulas:

Firstly calculate your total Thermal Resistance (R-Value)

Thickness of product in meters ÷ Lambda (Thermal Conductivity)

Calculating U-values



ExampleInsulation board 100mm = 0.1 ÷ 0.022 = 4.54 (R-value)Glass fibre roll 100mm = 0.1 ÷ 0.044 = 2.27 (R-value)

Then, simply add together the R-Values of each product used in the application (including any air gaps) and apply the following formula:1 ÷ Total of all R-Values

ExampleInsulation Board 100mm + Glass Fibre Roll 100mm =Total Combined R-Value = 6.81U-Value = 1 ÷ 6.81 = 0.147 W/M2k

N.B. The above examples do not allow for any thermal bridges, air gaps and fixings etc.



Any questions?

C&G PowerPoint Form an opening in a roof with a dormer

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