Building Pathology - OFS Flashcards
1
Q
What is clinker? How did you establish it was clinker?
A
- A fused material produced at high temperatures – a waste material from coal-fired glass furnaces
- Produced by heating limestone and clay to liquefaction around 1,400-1,500 degrees
- Composition of the slab typical for age of building
- Could have sent the clinker to a lab for testing, however I considered the time and cost implications and advised the client this was not necessary
2
Q
What are the defects associated with clinker?
A
Damp conditions produce sulphuric acid from coal in clinker which corrodes steel joists, leads to loss of section
It is a deleterious material
3
Q
What is a deleterious material?
A
- Materials that are:
o Harmful to health
o Harmful to safety
o Harmful to the environment
o Not suitable for their intended purposes
o Pose a risk where they have been used
4
Q
What are some examples of deleterious materials associated with the building age / type?
A
- Asbestos
- Lead paintwork/lead pipework
- Filler joists
5
Q
What are the remedies to filler joist construction?
A
- Localised breaking out and coating with resin repair systems
- Replacement of the joists where serious section loss has occurred.
- Regular inspection and keeping the joists dry
6
Q
What were the sources of internal water ingress?
A
- Leaking rainwater goods
- Cracked / crazed asphalt roof coverings
- Cracking to parapet walls
- Poor flashing termination of dormer windows
7
Q
Why was a structural engineer was required?
A
To confirm my advice in respect of roof replacement and parapet wall rebuilding
8
Q
How did you propose to rebuild the parapet walls?
A
- Protect internal rooms an ensure no water ingress – top hat scaffold
- Remove soldier course down to top layer of brickwork
- Remove creasing tiles and mortar
- Carefully take down brick courses to an acceptable level – i.e. a safe level that is structurally sound
- Install new brickwork to match similar – key in with matching mortar – NHL 3.5
- All soldier course brickwork to be mechanically fixed and installed using NHL 5 mortar
9
Q
What defects did you identify? How?
A
- Undertook opening up works using a jackhammer
o Revealed double-skin brickwork with the steel joist embedded – no joist hangers or waterproofing present - Identified severely corroded steel with delamination
- Observed severe cracking above the steel on the asphalt roof covering, and through the parapet soldier course
- Identified slipped, missing and cracked ridge and roof tiles
- Cracking and blisters across each of the asphalt roof coverings
- Observed all defects via safe access from scaffold platforms
10
Q
Can you describe delamination of steel joists? How did you establish this?
A
- Delamination – separation of layers of the steel joist due to corrosion
- Delaminated section was embedded into inner brick skin – I judged this to be very risky in respect of being structurally sound
- Internally, acro props had been installed behind plasterboard – opening up works revealed this
- Structural engineer confirmed my diagnosis
11
Q
Why was a timber frame used over steel or traditional construction?
A
- Quick construction time
- Manufactured off site, quality control etc. no on site cutting or storage
- Lower embodied energy
12
Q
How did you select the joists and centres for Roof 1?
A
- C24 is strongest and for external uses
- Centres – checked the NHBC guidelines which advises:
o Minimum centres of 450mm for 15mm deck
o Minimum centres 600mm for 18mm deck - Selected 400mm centres due to potential future loading of roof
- Checked all proposals / measurements with structural engineer before proceeding
13
Q
What is the difference between C16 and C24?
A
- Used to grade softwood, under BS EN 338
- C24 can take a higher point load and has fewer, and smaller knots in the timber
- Timbers are evaluated after being kiln dried to 20% moisture content.
14
Q
How were the joists supported?
A
- Installed timber wall plate bedded into mortar on top of the inner skin using galvanised holding-down straps using M10 steel bolts
- Joists installed onto joist hangers, fixed onto the wall plate using M10 steel bolts
15
Q
How did the contractor carry out the joist & deck installation?
A
- Joists as above
- 70mm timber firings
- Deck fixed onto joists using screw fixing of 53mm allowing embedding into joist of 35mm
- Minimum fastener edge distance 8mm
- Minimum bearing distance 18mm
- Panels fastened to joists at 300mm centres
16
Q
What was the roof build-up?
A
- 48x220mm timber joists
- 70mm firings
- 18mm plywood deck
- VCL
- 120mm PIR insulation
- Underlay sheet
- Cap sheet
17
Q
How did you select the specific felt system?
A
- Checked requirements in Approved Document L (conservation of fuel and power) to ensure U-value met
- Selected 3-layer system for full waterproofing – incorporating VCL, underlay and felt cap sheet
- Felt chose for ease of installation, allowing to chase into parapet brickwork
- Approached a manufacturer to ensure these requirements were met and instructed them to design a system that would be incorporated within the tender documents
- 20 years guarantee - BBA (British Board of Agrément) certified
- Ensured roofing manufacturer was BBA (British Board of Agrément) certified
18
Q
What is PIR Insulation?
A
- Polyisocyanurate (poly-iso-cyanurate)
- More efficient than PUR insulation
- A type of rigid board insulations, sandwiched between aluminium foil facing
19
Q
Did you check any specific certification?
A
- Adhered to Approved Doc L – 0.16 U-value
- For installation – checked Safe2Torch guidelines, hot works permit when on site
- Ensured appropriate edge protection present for installation
- Specified a 25-year insurance backed guarantee was in place
- Ensured contractor used approved installer for roofing system
- Building regulations sign off
20
Q
How was the U-value calculated?
A
- Provided by IKO, the manufacturer of the roof coverings and warranty provider.
21
Q
What type of plywood did you specify? Why?
A
- 18mm plywood
- Class II (Humid use) – Structural EN 636-2 S, grade C
- Protective against potential water ingress, for use externally
- Plywood is covered by roof covering, but still at risk of wetting
22
Q
What are the types of plywood grade / class?
A
Class
- Class I – dry interior use
- Class II – humid areas, occasional wetting
- Class III – unprotected exterior use, frequent wetting
Grade
- AB grade – high quality, small pint knots, no variation in colour
- B grade – change in colour, smooth knots 15mm-35mm
- BR ‘veneer’ grade – thinner sheets, knots 7-8mm
- BB grade – larger knots than B, suitable where appearance not important
- C grade – used when strength important, discolouration, open knots
23
Q
How did you ensure the design allowed for a safe working load?
A
- Incorporated appropriate plywood (grace C) and timber (C24) within design
- Proposed my design to structural engineer who confirmed appropriate loading
24
Q
With regard to OFS, can you reference any supporting documentation that provides guidance on crack identification & classification?
A
- BRE Digest 251 – classification of 0-5
25
What grades of cracking are given in BRE digest 251?
- Grade 0 – Hairline cracks less than 0.1mm. no action required
- Grade 1 – Fine cracks up to 1mm, treated using normal redecoration
- Grade 2 – Cracks up to 5mm, doors & windows may require easing, some external repointing may be required to maintain weather tightness.
- Grade 3 – Cracks between 5-15mm, weather tightness impaired, may require opening up to repair, repointing and small sections of masonry replaced.
- Grade 4 – 15-25mm cracks, extensive damage which requires breaking out and replacing sections of walls. Services disrupted, walls leaning or bulging
- Grade 5 – greater than 25mm, structural damage, danger of instability, beams lose bearing, walls require shoring, windows broken with distortion.
26
What are the three classifications of cracks?
- Classification one: Failures affect appearance only
- Classification two: Fractures and cracks cause damage to the function of the wall i.e. weatherproofing and insulation.
- Classification three: unacceptable risk of failure unless work is carried out.
27
How would you investigate cracking in a building?
- Record the damage
- Ascertain the history of the site,
- Record position, species and approximate age of nearby trees and shrubs
- Record position and condition of nearby drains
- Establish that cracking is consistent with foundation movement
- Monitor
- Investigate sub-soil by means of trial pits to reveal depth of foundations, presence of clay and roots below the foundations.
28
How would you monitor cracks?
- Installation of tell-tales
- Reading between fixed points with a Vernier gauge
29
What did you specify for pitched roof renewal?
- Marley vapour permeable membrane with BS 5534 25x38 treated battens
- Acme clay classic plain tiles – hung on 2no. nail fixings (aluminium – do not rust)
- Double course to eaves and ridges
- Tile & half’s to alternate courses at abutments and verges
- Marley 10mm eave vent system to eaves
- Marley dry vented ridge system with segmental ride
- Code 4 lead soakers, code 5 cover flashing to abutments
- Code 5 lead flashings to chimneys and back gutter
30
you undertook various re-roofing and refurbishment works. Were building regulations applicable? At what point are you required to update the roofing insulation?
- Building regulations were applicable – the works had an approved inspector employed
- Insulation needs to be upgraded to current standard if you are replacing 50% or more of the existing roof covering
o Repairing / recovering 25% or less usually don’t have to submit a building regulations application
