Design & Specification - OFS Flashcards
1
Q
What are the British Standards for pitched roof works?
A
- BS 5534: 2014 – Slating and tiling for pitched roofs and vertical cladding
- BS 5250: 2011 – Code of practice for condensation control in buildings
2
Q
What was the 2018 update to BS 5534?
A
- Ensure underlays are installed with a maximum drape of 15mm
- Avoidance of underlay exposure for more than a few days during installation
- Checking of underlay zonal classification label
- Standard states this may not be suitable for roofs on historically or architecturally important building.
3
Q
OFS - What did you specify for pitched roof renewal?
A
- 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
4
Q
Why was this considered over refurbishment/localised tile repairs?
A
- Pitched coverings were approx. 25% patch replaced – safe assumption that the system is failing, and roof tiles need to be replaced in their entirety
- Health and safety incident whereby a roof tile slipped into the courtyard – assessment made of condition of existing battens / tiles
5
Q
What were the existing roof coverings?
A
- Tile coverings
- Flat roofs – concrete, asphalt, rubble / liquid membrane
6
Q
What type of pitched roof was it?
A
- Mansard & pitched – 1898 Victorian building
7
Q
What was the pitch, what head lap did you specify?
A
- High pitch with 65mm head lap
- Pitch was between 48-51 degrees
8
Q
Did this incorporate additional insulation?
A
- Yes – incorporated insulation at loft level (not rafter) to achieve 0.16 U-value due to consequential improvement
- Insulation – Knauf loft roll 44 270mm insulation
9
Q
What are the types of roofing underlay?
A
- Breathable & non-breathable
10
Q
What is the difference between a breathable and non-breathable underlay, and when would these be used?
A
- Non-breathable are non-permeable (e.g. bituminous or polypropylene)
o Most cost effective and functional
o Low level (eaves) and high level (ridge) ventilation systems
o Locations where roofs require counter-battens to ventilate the batten void - Breathable (low resistance) – lightweight, durable, easy to install
o Vapour permeable – used as roof ventilation but must be used with high-level ventilation according to BS & NHBC
o Vapour and air permeable
11
Q
What are the British Standards for roofing underlay?
A
- BS 5534
o Ensure underlays are installed to max 15mm drape – prevents transfer of wind loads (ballooning) roof covering
o Avoid leaving underlays exposed – over exposure to UV light or freeze/thaw could lead to degradation of underlays – underlays should not be exposed for more than a few days - BS 5250
o In case of LR vapour-permeable underlays, designers must ensure manufacturer’s stated water vapour resistance values are suitable for application
o These underlays should comply with BS EN 13859-1
12
Q
What did you specify for the window replacement?
A
- Window schedule, corresponding to numbered windows on elevations and plans.
- This corresponded to a tabulated schedule setting out the window, openings, materials, sizes and profile
- Specified timber sash, double glazed units, with sliding mechanism, window restrictors, fitch sash window lock
13
Q
OFS - Was any statutory permissions required?
A
- Building Control
- Planning permission – was enquired, and a meeting with planning officer was arranged – was advised spec was in line with regulations so no planning permission required
- Windows – self certification
14
Q
What was the cause of the parapet wall cracking?
A
- Rainwater goods original – not wide enough to accommodate more rain
- Caused breakdown of asphalt roof coverings & walls – water ingress internally
- Corroded steel filler joists which expanded their volume and pushed brickwork outwards
- Significant cracking
15
Q
Describe the specification for the parapet wall rebuilding
A
- Remove soldier course across all elevations down to creasing tiles, keep intact bricks for reinstallation – any defective bricks replaced with matching
- Remove creasing tiles and mortar and dispose
- Take down brick courses to 2no. courses beneath cracking
- Rebuild brickwork using NHL 3.5 mortar, with the same or matching bricks
o Provisional rate allowed for, for each parapet wall section - Install new creasing tiles & soldier course with NHL 5 mortar and mechanical fixings
16
Q
How did you specify the overlaying of the flat roof coverings?
A
- Ensure building remains watertight at all times – areas of the roof remained in a single day must be done so ensuring the provision of night joints
- Heat up lumps, rolls, blisters, cracks, etc. in asphalt and iron out to provide a smooth surface
- New 3-layer waterproof system to be installed with insulation
- New system incorporating new internal gutter channels using lead chutes linking to existing cast iron rainwater goods
- In accordance with the manufacturer’s specification
17
Q
How did you specify the replacement of the flat roof covering?
A
- Ensure provision of night joints
- Top hat scaffold required
- Strip asphalt from roof, including to upstands and main roof field down to concrete deck
- Break out clinker concrete slab & safely remove filler joists
- Install new timber joists (75mm x 220mm) at 600mm centres, ensure for hard packing e.g. slates or tile to bed into mortar in brickwork
- Joists placed on fixed wall plate bedded into mortar on top of the inner skin and strapped using galvanised holding-down straps
- Install roof deck of exterior grade 18mm plywood, embedding into brickwork
- 3-layer waterproof system with 130mm insulation to achieve 0.16W/m2K U-value
18
Q
What advice did you give prior to specifying the works to ensure the health and safety of the leaseholders?
A
- Discussed site compound with the tenants – it was agreed to have this on the pavement at the front of the building
- Tenants required access to the rear for fruit and veg packing business
- Safe access for fire escape allowed at rear
19
Q
How did you specify repairs to cracking?
A
- Installation of helical bars – threaded bars embedded into mortar joints
- Rake out or cut slots into horizontal mortar beds minimum 500mm either side of crack
- Clean slots and thorough soak substrate within the slot
- Inject a bead of resin along back of the slot
- Push helical bar into grout to obtain good coverage
- Use further bead of resin over exposed bar, finishing 12mm from face
- Repoint mortar bed and make good vertical crack
