Building Pathology - OFS Flashcards
What is clinker? How did you establish it was clinker?
- 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
What are the defects associated with clinker?
Damp conditions produce sulphuric acid from coal in clinker which corrodes steel joists, leads to loss of section
It is a deleterious material
What is a deleterious material?
- 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
What are some examples of deleterious materials associated with the building age / type?
- Asbestos
- Lead paintwork/lead pipework
- Filler joists
What are the remedies to filler joist construction?
- 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
What were the sources of internal water ingress?
- Leaking rainwater goods
- Cracked / crazed asphalt roof coverings
- Cracking to parapet walls
- Poor flashing termination of dormer windows
Why was a structural engineer was required?
To confirm my advice in respect of roof replacement and parapet wall rebuilding
How did you propose to rebuild the parapet walls?
- 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
What defects did you identify? How?
- 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
Can you describe delamination of steel joists? How did you establish this?
- 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
Why was a timber frame used over steel or traditional construction?
- Quick construction time
- Manufactured off site, quality control etc. no on site cutting or storage
- Lower embodied energy
How did you select the joists and centres for Roof 1?
- 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
What is the difference between C16 and C24?
- 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.
How were the joists supported?
- 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
How did the contractor carry out the joist & deck installation?
- 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
What was the roof build-up?
- 48x220mm timber joists
- 70mm firings
- 18mm plywood deck
- VCL
- 120mm PIR insulation
- Underlay sheet
- Cap sheet
How did you select the specific felt system?
- 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
What is PIR Insulation?
- Polyisocyanurate (poly-iso-cyanurate)
- More efficient than PUR insulation
- A type of rigid board insulations, sandwiched between aluminium foil facing
Did you check any specific certification?
- 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
How was the U-value calculated?
- Provided by IKO, the manufacturer of the roof coverings and warranty provider.
What type of plywood did you specify? Why?
- 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
What are the types of plywood grade / class?
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
How did you ensure the design allowed for a safe working load?
- Incorporated appropriate plywood (grace C) and timber (C24) within design
- Proposed my design to structural engineer who confirmed appropriate loading
With regard to OFS, can you reference any supporting documentation that provides guidance on crack identification & classification?
- BRE Digest 251 – classification of 0-5
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.
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.
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.
How would you monitor cracks?
- Installation of tell-tales
- Reading between fixed points with a Vernier gauge
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
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
