14. Construction Tech and Environmental Sciences

Question 1

How would you determine what foundations are required?

Answer 1

I would discuss with a structural engineer. However, note there are a wide range of foundation types. These vary depending on:
- Nature of load requiring support
- Ground conditions
- Presence of water
- Space availability
- Accessibility

Question 2

What are the main types of foundations?

Answer 2

The main foundation types are categorised as shallow foundations or deep foundations.

Question 3

What are shallow foundations?

Answer 3

Typically, these are used when the loads imposed by the structure are low relative to the bearing capacity of the surface soils. They include:

1) Strip foundations - These provide a continuous strip to a linear structure such as a wall.
2) Pad foundations - These are rectangular or circular pads used to support localised loads such as columns.
3) Raft foundations - These are slabs that cover a wide area, often the entire footprint of the building. These are suitable when ground conditions are poor and settlement likely.

Question 4

What are deep foundations?

Answer 4

These are necessary when the bearing capacity of the surface soils is not adequate to support the loads imposed by a structure and so these need to be transferred to deeper layers with higher bearing capacity.

Piles - These are generally classified as:

1) End bearing piles (where most of the friction is developed at the toe of the pile, bearing on a hard layer).
2) Friction piles (where most of the pile bearing capacity is developed by shear stresses along the sides of the pile, suitable when hard layers are too deep)
3) Sheet piles / Diaphragm walls - These are made by excavating a deep trench that is prevented from collapsing by being filled with engineering slurry such as bentonite. The trench is filled with reinforced concrete panels. The joints between which can be watertight.

Question 5

Can you name and describe different types of floors?

Answer 5

1) Solid ground - These require little maintenance and less prone to movement. Built up using:
- Sub-base - Well compacted rubble or loose stone based material
- Hardcore - Material to make the require level and create a solid base
- Damp proof membrane - Impervious layer such as heavy duty polythene sheeting
- Concrete bed - Providing a solid level surface
- Insulation - To limit heat transfer with the ground
- Screed - Sand and cement mix laid to prepare for installation of floor covering
- Floor finish - Such as vinyl or carpet

2) Suspended timber floor - Usually constructed using timber joists suspended from bearing walls.
- Joists are typically laid across the shortest span.
- Ventilation provided to the void between the floor by placing air bricks (or bents) to external walls
- Can cause draughts. However, can be avoided by installing an airtight breather membrane to maintain an airtight seal.

3) Suspended concrete floors - These are similar to timber suspended floors. It can span greater lengths and offers better sound insulation.
- Voids can be created by beams or ribs, to house services as well as providing support for suspended or attached ceilings.

4) Ribbed floors - The use of narrow-space shallow ribs, rather than wide spaced deep beams. Troughed floors are ribbed in one direction, whereas coffered floors are ribbed in two directions. Ribbed floors have greater span and load potential per unit weight than flat slab construction.

5) Hollow Pot floors - This is a ribbed cast in-situ floor with permanent formwork in the form of hollow clay or concrete pots. This creates a flat soffit, allowing the direction application of a plaster finish or drylining. The pot voids can be used to contain small diameter services within the overall slab depth. The most common form is a one way spanning floor, although two way spanning is also possible.

Question 6

Can you name and describe different types of walls?

Answer 6

1) Cavity wall - A wall constructed from two skins of masonry. The outer skin can be brickwork or blockwork. The inner skin is generally blockwork. Separated by a cavity to prevent the penetration of moisture and to allow for the installation of thermal insulation.

2) Internal load-bearing wall - A wall that is also required to transfer loads from other parts of the structure to the foundations.

3) Partition wall - Non-load bearing wall that separates the internal spaces of a building.

4) Curtain wall - Non-structural cladding system for the external walls of a building.

5) Parapet wall - Uppermost reaches of a wall that extends above the roof level and provides a degree of protection to roof, gutters, balconies and walkways.

6) Party wall - A wall that stands on the land of 2 or more owners or a wall that is on one owners land but is used by 2 or more owners to separate their buildings.

Question 7

Can you name and describe the different types of roofs?

Answer 7

1) Pitched roofs
- These are generally split into warm and cold style construction.
- Timber joists are sized using Approved Doc A and TRADA span tables. These are either C16 or C24.
- Joists are strapped down to the wall plates.
- Ventilation needs to be considered if insulating at ceiling joist level. Vapour Control Layer if at rafter level.
- Insulation is required to hit the required U-Values of 0.16 as illustrated in Approved Doc L.
2) Flat roofs
- These are generally split into warm and cold system construction. There is also an inverted flat roof system.

Question 8

Explain the construction of a warm flat roof?

Answer 8

A warm deck is modern and preferable.
1) Insulation is placed above the joists and main deck. This means the whole roof structure is insulated.
2) The benefit of this is that the chance of condensation to the roof structure is reduced.
3) Unlike a cold deck, there is no need to vent the roof void.
4) If you are installing a warm roof over an existing roof, it is possible to leave the original roof covering in place to act as the vapour barrier as long as it is in good condition.

Ceiling
Joists
Timber deck
Vapour control layer
Insulation
Selected roof covering

Question 9

Explain the construction of a cold flat roof?

Answer 9

A cold roof is more common and traditional.

1) Insulation is placed directly above the ceiling boards.
2) A cavity is left between this and the roof deck.
3) The air in this area is essentially outside and cold. This must be well ventilated to avoid condensation occurring.

Ceiling
Insulation (between the joists)
Joists
Timber deck
Selected roof covering

Question 10

Explain the construction of an inverted roof?

Answer 10

The key difference between an inverted roof and other roofing systems is the position of the waterproofing membrane.

The ‘upside down’ sequence means the deck is waterproofed first, then insulated and finally finished with paving or ballast.

The waterproofing membrane is between protected from the elements and other factors such as UV radiation and rapid changes in temperature.

Question 11

Explain the construction of a cold pitched roof?

Answer 11

Roof joists - Support the weight of the roof.
Breather membrane - Keeps out the rain and offers a secondary protection to any water that gets through the outside layer of the roof.
Battens - These are fixed to the joists with nails.
Tiles

Question 12

Explain the construction of a warm pitched roof?

Answer 12

Roof joists
Breather membrane
PIR Insulation (or another type of insulation)
Battens
Counter battens (to provide a void if necessary)
Tiles

Question 13

Name 3 different defects that may occur on a pitched roof?

Answer 13

1) Slipped/broken tiles
2) Tiles supported on timber battens can rot (if exposed to excessive moisture and rain)
3) Roof spread - Caused by overloading the roof with a heavier material
4) Missing mortar
5) Moss on roof / blocked valleys
6) Detailing around openings may fail (such as around chimney breasts, vent or overflow pipes)
7) At junctions such as valleys and perimeter walls where flashings can fail

Question 14

Name 3 different defects that may occur on a flat roof?

Answer 14

1) Poor detailing - Perimeter flashings etc. may become vulnerable.
2) Poor design - Weak points where services pass through a covering
3) UV Degradation - Sign of an ageing roof which can lead to crazing or ‘alligatoring’
4) Blistering
5) Damage caused by foot traffic or maintenance activity
6) Moisture retention
7) Crazing - Which is small cracks

Question 15

Rougham - You mentioned the roof suffered from several defects, what were these and what would the remedial action be?

Answer 15

Rougham had a bitumen felt flat roofing system. It suffered from ponding water that would suggest poor falls. These can be solved several ways:
1) Joists cut to falls with flat soffits
2) Joists laid to falls with slopping soffit
3) Firings (tapered strips fixed above the joists) with joist run
4) Firings against joist run

Question 16

What types of flat roof material are there?

Answer 16

1) Asphalt Roofing
2) Liquid Waterproofing
3) Single plywood
4) Bitumen Felt

Question 17

What are the advantages of each roofing system?

Answer 17

Liquid Waterproofing
- Lightweight and flexible
- Can be used to overcome complex details

Question 18

Sir Roger Manwood School - What other considerations did you need to make when assessing whether a ridgelight was required?

Answer 18

-Timescales for removing and replacing the ridgelight system.
-Futureproofing the school and whether this would be needed in the future.
-Potential disruption to education.

Question 19

Sir Roger Manwood School - What is load bearing capacity?

Answer 19

The maximum weight a roof can support without compromising its structural integrity. Typically measured in pounds per square foot. This includes live loads, dead loads, span and pitch of the roof.

Question 20

Sir Roger Manwood School - Explain the existing roof system

Answer 20

The existing roof system was a cold deck with mineral wool insulation placed above the ceiling joists.

Question 21

Sir Roger Manwood School - Explain the proposed roofing system

Answer 21

Cold deck to match existing. I reviewed the relevant guidance and discussed the requirements with a roofing manufacturer. In terms of the construction this would include
1) Roof joists - Agreed size with the structural engineer
2) Breather membrane
3) Battens
4) Tiles

Question 22

Sir Roger Manwood School - What are the key risks associated with a cold deck?

Answer 22

Condensation

Question 23

Sir Roger Manwood School - How do you mitigate condensation in a cold deck?

Answer 23

The key to mitigating condensation in a cold deck is to allow adequate ventilation of the roof void.

This can be done by two methods either installing ventilation openings at the eaves of the fascia (needs to be continuous and 10mm - BS5250), or, installing a breather membrane. Breather membranes allow air to escape from the inside of the roof void whilst preventing air entering.

Two documents to consider with ventilation:
1) Approved Doc C - Site preparation and resistance to containments and moisture.
- States that thermal transmittance of moisture and containments should not exceed 0.35 U-values
2) BS5250: Management of moisture in buildings - code of practice
- States ventilation openings need to be continuous and 10mm

Question 24

Sir Roger Manwood School - Could you have converted the cold roof into a warm roof?

Answer 24

Yes, this would be possible by removing the insulation (at ceiling joist level, blocking ventilation and installing PIR over the outside of rafters). However, this would lead to additional cost implications - which the client was not interested in.

Question 25

Sir Roger Manwood School - What is a ridgelight?

Answer 25

Glazing structure

Question 26

Sir Roger Manwood School - How is it detailed in the roof?

Question 27

Why did you advise your client a structural engineer was required?

Answer 27

The works would involve structural alterations. A structural engineer would be required to assess the proposed loads of the roof coverings.

Question 28

Sir Roger Manwood School - What input did you have on design and specification of the replacement / removal of ridgelight?

Answer 28

• Roof joist sizing’s confirmed.
• Made note it was a cold pitched deck.
• Produced a design based on this.
• Discussed with roofing manufacturer to specify products.
• Provided the structural engineer with data sheets to ensure happy with proposed loads.

Question 29

Sir Roger Manwood School - How did you know the roof was a cold deck?

Answer 29

Insulation was at ceiling level.

Question 30

Sir Roger Manwood School - Explain the design that you carried out?

Answer 30

• Upgrade insulation.
• Breather membrane fixed to rafters.
• Battens.
• Clay tiles.

Question 31

Sir Roger Manwood School - What other considerations did you need to include in your design?

Answer 31

• Ensure the roof design was sufficient to reach the required U-values.
• Ensure adequate drainage and waste disposal.

Question 32

Sir Roger Manwood School - What guidance did you follow?

Answer 32

• Approved Doc C - Ventilation requirements for ensuring moisture levels are below 0.35
• Approved Doc H - Gutter sizes and outlets dependent on maximum effective roof area. 53m2 so gutter size diameter must be 115mm and outlet 63mm
• Approved Doc L
  Ensuring roof is required u-value

Question 33

Sir Roger Manwood School - Why did you recommend the ridgelight should be removed rather than repaired?

Answer 33

• Widespread failure.
• Flash banding to temporary seal broken panes.
• Client confirmed ridgelight not being used for natural light so served little purpose.
• Futureproofing the school.

Question 34

Sir Roger Manwood School - What cost benefit analysis did you present your client with?

Answer 34

• Used BCIS and SPONS to estimate cost for replacing with tiles.
• As the ridgelight was bespoke and one of a kind, I discussed the measurements with a window manufacturer who provided estimated costs.

Question 35

Sir Roger Manwood School - Please draw a section detail of the pitched roof construction you designed

Question 36

Tell me about the impact of different design solutions and construction processes on cost and programme?

Answer 36

-Aware that different design solutions can impact on the construction process.
-Aware of the importance of key meetings with the client to discuss and sign off designs at different stages of the project.
-Changing the design can impact the construction process through cost and time.
-For example, client changes when onsite.

Question 37

Tell me about your advise on the selection and application of particular processes within your area of expertise.

Answer 37

Talk about roofing project and why you would decide on a specific roof system dependent on use of the roof.

Question 38

How have you considered practical alternatives to those specified by the professional team?

Answer 38

Knockhall Primary School - Talk about using a catenary system as opposed to excavating a trench to the school playground to install electrical mains cabling.

Question 39

Rougham Primary School - Describe the construction of the new roofing system you specified?

Answer 39

I specified a 3-layer bitumen felt system. This consisted of a vapour control layer, insulation followed by a a capping sheet.

Question 40

Rougham Primary School - What type of bitumen felt did you specify?

Answer 40

Elastomeric bitumen capping sheet - Installed to allow structural movement without fracture and mineral finish to provide protection against UV degradation.

Question 41

Rougham Primary School - What is the difference between a cold and warm roof system?

Answer 41

Key difference is where the insulation is placed. In a cold roof, the insulation is between the floor joists. In a warm roof, this is placed between the rafters.

Question 42

Rougham Primary School - What is the difference between a warm and inverted roof system?

Answer 42

An inverted roof is essentially an upside down warm roof.

-Waterproofing layer in a warm roof is above the insulation.
-Inverted roof the waterproofing system is directly above the roof deck. The insulation is the last layer.

Question 43

Rougham Primary School - What is the angle threshold of a flat roof. When does it become pitched?

Answer 43

-Anything more than 10 degrees is considered pitched.