Design and Specification 3

Question 1

Allendale Road: What were the clients/project requirements?

Answer 1

The primary requirements were:

1. Provide a new roof covering as water ingress evident.
2. Minimise capital expenditure
3. Provide break out areas to the roofs.

Question 2

Allendale Road: What statutory matters did you consider?

Answer 2

Party Wall - No

Planning - Yes to terrace areas.

CDM - Yes (carried out in house by NCC)

Building Regulations:

A - Was the structure fit for break out areas and balconies?
B - Means of escape, fire detection etc.
C - Moisture (roof)
K - Hand rails for protection from falling (1100mm minimum)
L2B - Upgrade thermal insulation due to replacement of the water proofing layer. (0.18 uValue required)

Question 3

Allendale Road: What drawings did you provide?

Answer 3

1. Roof plan drawing
2. Sections and details which were drafted from Construction Technology book by Greeno and Chudley.

This allowed various suppliers such as Sika, Bauder and Moy to price for the works on a ‘Or Similar’ basis.

They all use similar BBA systems.

Question 4

Allendale Road: Why was the roof thermally insulated?

Answer 4

Its a requirement of Approved Document L2B as more than 50% of the individual roof coverings or 25% of the entire element was being renovated which is classed as a Major Renovation.

(i. e. element being renovated was the water proofing layer)
(0. 18 uValue required)

Question 5

Allendale Road: What other roof covering options did you consider?

Answer 5

1. Liquid applied as its good for detailing and guarantees can be topped up. (too expensive!)
2. Considered single-ply to the roof terraces however, too weak as ‘single membrane’ which wouldn’t be suitable for the works as the building has a lot of plant on the roof.
3. Bitumen system selected as a little more expensive than single ply though a lot more robust which was needed for the plant.

Question 6

Allendale Road: Why did you specify a warm roof system to these areas?

Answer 6

Rejected: cold roof - as wanted to minimise disruption internally.
Rejected: inverted roof - as more costly to repair.

Selected: warm roof - as most economical option.

Question 7

Allendale Road: How had the existing bitumen roof covering fail?

Answer 7

• UV degraded and brittle with age (bitumen subject to extensive thermal changes which can then cause cracks/rips to appear)
• Joints delaminated by water (standard with laps/joints as they’re exposed to wind and rain)

Question 8

Allendale Road: Why did you specify a new modified bitumen roof covering?

Answer 8

A strong surface was required due to the extensive plant on the roof.

The bitumen system is more robust due to having more layers appose to a single layer ‘single ply’ system.

Although cost was a key issue and a single ply was slightly cheaper, the cost difference between that and bitumen system was minimal.

Question 9

Allendale Road: How long does a bitumen roof covering or single ply usually last?

Answer 9

Both are the same being around 30-40 years.

Question 10

Allendale Road: Why did you use tapered insulation?

Answer 10

• Increase the falls of the flat roof as they were poor.
• Phenolic insulation suitable due to this as wool couldn’t attain these falls.
• Polystyrene discounted with the client as flammable.

Question 11

Allendale Road: What thermal requirements were there?

Answer 11

0.18 – Refurbishment requirements for flat roof.

Was classed as a Major Renovation of the thermal element as more than 25% of the surface was being re-covered.

Question 12

Allendale Road: How did you design the build up?

Answer 12

Chudley and Greeno – Construction Tech book, this allowed me to go to different manufactures on a hybrid performance basis.

Question 13

Allendale Road: Explain the difference between cold, warm and inverted?

Answer 13

Cold – Insulated below the roof deck
Warm – Insulated above the roof deck.
Inverted – Insulated above the roof deck but the water proof layer in below the insulation.

Note: VCL’s are all fitted below the insulation on all systems. Though, Bauder states no VCL required for inverted systems.

Question 14

Allendale Road: Tell me about the parapet requirements?

Answer 14

Approved Document K – 1100mm

Question 15

Allendale Road: What other regulations did you consider?

Answer 15

CDM 2015

CAR 2012 – Client had reports on file.

Question 16

Allendale Road: Was your spec prescriptive or performance? And what are these?

Answer 16

Hybrid approach (though, more prescriptive)

Prescriptive provides in depth detail specifying exact items.

Performance on a descriptive and open basis allows contractors to propose solutions -riskier!!

Question 17

Allendale Road: What other roof coverings did you consider and why weren’t these suitable?

Answer 17

Single-ply system was considered; though, the surface is generally weak and wasn’t suitable as there were extensive plant/machinery to the roof.

Liquid - too expensive.

Question 18

Allendale Road: How does blistering occur and what would you advise if its minimal?

Answer 18

Moisture trapped under the covering trying to escape as evaporation occurs.

If small and not causing any issues leave!

If large pop the blister and patch repair, further investigate any breaches which has caused the water to enter.

Investigate where water has entered and resolve.

Question 19

Harley Terrace: What was included in your technical drawings?

Answer 19

Floor plans
SofW

Specified: 
Structural details
Materials
Fittings/fixtures
Electrical installations

Question 20

Harley Terrace: When did Building Control approve the drawings?

Answer 20

Partially approved during pre-construction and fully approved following SE’s input.

Question 21

Harley Terrace: What had caused the bulge to the rear wall?

Answer 21

Failed wall ties which were subject to corrosion from water/moisture entering cavity.

Wrought iron twist/fish tail wall ties were used.

Question 22

Harley Terrace: Why didn’t you carry out the party wall notices etc?

Answer 22

I was conflicted as I was lead consultant and a ‘party’ to the works.

Question 23

Harley Terrace: How did you calculate the provisional sum?

Answer 23

Labour rate on how long I thought it would take.

Question 24

Harley Terrace: What is a provisional sum and what is a PC sum?

Answer 24

PC sum’s are used to cover items/materials.

Provisional sums are used to cover undefined/unknown works.

Question 25

Harley Terrace: What is defined and undefined sums?

Answer 25

Undefined – unknown element of risk

Defined – works described in enough detail which will allow the contractor to allocate enough time for the works.

Question 26

Harley Terrace: What did you discuss with the SE?

Answer 26

Options to repair, see Key Issue 2.

After considering and advising the client the SE provided their report on my recommendation and the clients selected method.

Question 27

Harley Terrace: Why did you obtain a quote from the contractor first?

Answer 27

Best practice and transparent. Reduce disputes if cannot agree price.

Question 28

Harley Terrace: Why didn’t you carry out these investigations before the project?

Answer 28

To reduce costs for initial investigations as there was clear evidence of movement and the client definitively wanted the issue sorting so bundled together.

Question 29

Harley Terrace: How did you release the provisional sums to the contractor in the contract documentation?

Answer 29

By a contract variation/instruction, theres 2 ways:

Option 1
Omit: Provisional sum.
Add: New fixed cost/quote.

Option 2
Omit part of provisional sum or add extra if needed.

Question 30

Harley Terrace: How do wall ties fail?

Answer 30

1. Zinc coatings on old wall ties will fail over time BRE states that these coatings will loose 2.1 microns per year which results in a life expectancy of around 40 years.
2. Mortar carbonates which offered protection to the steel wall tie,
3. As moisture is drawn through the brick as its porous, it will go to the wall tie and thus corrode.
4. Moisture entering the cavity by defective envelopes will speed up the process.

Question 31

Harley Terrace: What are the different types of wall ties?

Answer 31

Twist, double triangle and butterfly.

In this case the wall ties were twists/fish tails with a large mass which had corroded and expanded. If left in these may have caused further corrosion.

Question 32

Harley Terrace: What are the indicators of failed wall ties?

Answer 32

Bulging by lateral movement.

Raised bed joints sometimes.

Question 33

Harley Terrace: Why did you engage with an SE?

Answer 33

Out of my area of expertise and competence.

Provided a high level of service.

Question 34

Harley Terrace: What approved document would you use to specify the new wall ties?

Answer 34

Approved Document A – Modern ties used.

Question 35

Harley Terrace: What did the SE recommend?

Answer 35

Rebuild design/extent, following discussions on site.

Question 36

Harley Terrace: What was the advice in your summary?

Answer 36

Pre-construction:
I advised to use an undefined PC sum due to the works being unknown at that stage. My responsibility to anticipate the works/costs and prepare the client for this.

Construction phase:
I advised the contractor to use part of the sum and I advised the client that the sum would cover it.