A) level 2 Certificate: Floor and flat roof components

Level 2 Technical Certificate in Site Carpentry

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Floor and flat roof components

Unit 202: Structural carpentry

Level 2 Technical Certificate in Site Carpentry

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Floor and flat roof components

•‘First fix’ flooring and roofing is the name given to the structural
carpentry work which is to be done before a building is made
watertight.

•This type of work usually involves working at height.

•The site carpenter must draw upon a wide range of skills and
knowledge to safely complete these carpentry tasks.

•You must understand how these tasks link with the work of other
trades involved in this stage of construction.

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Timber floor construction

Suspended timber floors used on the ground floor are not as common
as they used to be.

Solid concrete floors are now they norm in new build construction,
although you will need to be familiar with timber floor components and
construction techniques, particularly with refurbishment type work.

The component names used to make up timber floors will be the same
whether they are to be used at ground floor, first floor or above.

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Types of joist

The names given to timber
joists will depend on their
location and use. The
following is a description of
each type and their location
within the floor structure.

1.

Bridging or common joist: a
joist that spans from load-
bearing wall to load-bearing
wall.

2.

Trimming joist: a joist that
spans in the same direction
as a bridging joist, and
supports the trimmer joist.

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Types of joists

3.

Trimmer joist: a
joist that is placed
at right angles to
the trimming joist,
and supports the
cut end of the
trimmed joists.

4.

Trimmed joist: a
bridging joist that
has been cut or
trimmed to form an
opening in the
floor, and is
supported by the
trimmer.

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Joists for floors and flat roofs

Joists are typically supported by the following methods:

•Built in

•Joist hangers

•Wall plates

•Sleeper walls

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•Built-in joists

‒ Traditionally, joists were built into solid walls, which could
lead to dampness from the wall penetrating the ends of the
joists and causing rot and subsequent collapse, particularly
with solid wall construction which was widely used.

‒ Modern built-in methods incorporate the joist ends being built
into the inner leaf of the cavity wall, ensuring the joist ends do
not project into the cavity. The joist ends must be treated with
a timber preservative to protect the vulnerable timber from
dampness and rot.

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•Built-in joists

‒ As the building dries out, joists can shrink, leading to problems
such as heat, sound and air leakage around the ends of the joists.
To prevent this, as the joist is positioned, the end is placed in a
joist cap or shoe which is then built in.

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•Joist hangers

‒A modern method of supporting

the ends of joists is to use

galvanised steel timber-to-wall

(TW) joist hangers.

‒Advantages of this method are

that the joist end does not

penetrate into the wall cavity,

thereby not coming into contact

with any moisture present.

‒It also means that joists can be

installed independently of the

walls being built.

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•Wall plates

‒For timber ground floors and flat roofs, the joists are supported by a
wall plate bedded onto the top of the wall.

‒This provides a level surface to which to fix the joist.

‒Joists are fixed using framing anchors.

‒Where joists from either side meet over a wall plate, they are nailed
together side by side, with both sides overlapping the wall plate by no
more than 150mm – any more overhang than this can lead to deflection
(bending) of the ends of the joists, creating an uneven surface for the
decking and ceiling.

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•Wall plates

‒For timber ground floors and flat
roofs, the joists are supported by a
wall plate bedded onto the top of a
sleeper wall.

‒This provides a level surface to which
to fix the joist. Joists are fixed using
framing anchors.

‒Where joists from either side meet
over a wall plate, they are nailed
together side by side, with both sides
overlapping the wall plate by no more
than 150mm – any more overhang
than this can lead to deflection of the
ends of the joists, creating an uneven
surface for the decking and ceiling.

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•Sleeper walls

‒Timber ground floors comprise of
dwarf sleeper walls that are built onto
the oversite concrete at about 1.8–2m
centres.

‒The bricks in the wall are laid with a
50mm gap between them, allowing air
to circulate in the void under the floor.

‒Airbricks are built into the outside
walls at ground floor level to enable
through ventilation.

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Alternative types of joist

I-beams or eco joists

‒These types of joist are now very common in new builds of both
traditional and timber frame construction.

‒Their construction makes them very light and strong, and they are also
available in long lengths and can be spaced at greater centres than
solid timber joists.

‒I-joists do not suffer problems such as shrinking, twisting, cupping and
bowing, all of which can occur with timber joists, and contribute to
squeaking floors.

‒I-joists also have pre-stamped holes in the OSB (oriented strand
board) web for the installation of services (ie plumbing and wiring).

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Alternative types of joist

I-beam or eco joist

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Alternative types of joist
Metal web joists

‒These types of joist are also popular for new build construction. They are

constructed using stress graded timber flanges that are joined with v-shaped

galvanised steel webs.

‒The webs are fixed through nail plates to the flanges.

‒The construction of metal web joists makes them very strong and a lot lighter than

solid timber joists, with wide timber flanges allowing for easy fixing of decking.

They are available in long lengths, and again can be spaced at greater centres

than solid timber joists.

‒Their reduced timber content means less shrinkage, which provides for a quieter

floor system and their open web design allows for easy installation of services.

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Alternative types of joists

Metal web joists

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Types of strutting

•Deep floor joists are prone to twisting or buckling along their
length. A row, or rows, of struts must be fixed between the joists to
stiffen the floor and prevent this movement. The TRADA span
tables stipulate that joists that span 2.5–4.5m require one row
positioned at mid-span. Joists spanning over 4.5m require two
rows, at 1/3 span positions.

•The main types of strutting used are:

‒ Solid

‒ Herringbone

‒ Galvanised steel

•It is usual to fix strutting just prior to laying the flooring.

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•Solid strutting

‒Timber struts should be at least 3/4 the depth of the joist, and at least
38mm thick, and are cut to fit tightly between each joist.

‒They are secured by skew-nailing.

‒The disadvantage of solid strutting is that as the joists shrink the struts
can become loose.

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•Herringbone strutting

‒Timber battens of a minimum section size of 38 × 38mm are cut to fit
diagonally between the joists, and secured with nails.

‒This method is considered the most effective, as the struts tighten as the
joists shrink.

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•Herringbone strutting can be fitted using the following method.

‒Position and nail a temporary batten next to where the row of strutting is
to be fixed, to make sure the joists are maintained at the right centres.
This is important, as if struts are measured to the wrong centres then the
joists will be out of line, causing problems later on.

‒Mark the centre-line of the row of strutting across the joists with a pencil
or chalk line. Measure the depth of the joists, and deduct 10mm. Take this
measurement and mark a line parallel to the centre-line (eg, if the joists
were 200mm, then 190mm would be the figure used). This measurement
ensures that when the struts are fixed, they will finish below the top of the
joist and above the bottom of the joist.

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‒Place a length of strutting diagonally across two joists between the
parallel lines, and mark off the underside with a pencil.

‒Cut the strut and test fit. If all joist centres are the same, the strutting will
be the same length, and this can be used as a template.

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‒Fix the strutting to either side of the centre-line with nails, and fix the
struts together with a nail through the centre.

‒After the strutting is fixed, solid blocking or folding wedges will be required
between the end joists and the wall to complete the stiffening of the joists.

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•Galvanised steel strutting

‒This type of strutting is quick and easy to install.

‒The disadvantage with steel strutting is that the struts are only available to
fit joists with centres of 400, 450 or 600mm.

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Service provision in joists

•Joists will usually require drilling or notching to accommodate pipes and
wires for services. When I-beams are being used only the pre-stamped
hole positions should be removed.

•For solid timber, the following requirements should be met:

‒Notches should not be deeper than 0.125 of the depth of a joist and
should be positioned between 0.07 and 0.25 times the span from either
support.

‒For example: span = 3.0m, joist depth = 150mm.

‒Safe area for notching is between 0.07 x 3.0m = 210mm and 0.25 x 3.0m
= 750mm.

‒The maximum depth of notch is 0.125 x 150mm = 19mm.

‒So safe areas to notch are from 210 to 750mm from either wall, and up to
19mm deep.

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Service provision in joists

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Web link to floors

http://www.haringey.gov.uk/sites/haringeygovuk/files/note07_-
_domestic_timber_floors.pdf

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Any questions?