Building Terms Glossary

Also: 110V, 110 volt site power, site voltage, CTE transformer, centre-tapped earth transformer

UK professional tool hire companies predominantly supply tools at 110V (centre-tapped earth transformer) rather than 240V domestic supply, as a site safety measure that limits shock voltage to 55V. Homeowners hiring a breaker without a 110V step-down transformer cannot use hire-grade 110V tools. A genuine friction point: 'no 110V converter = friction with hire'.

Also: roof valley

Where two roof slopes meet, requiring weatherproofing

Also: VE, value optimisation

The practice of substituting equivalent performance at lower cost when a budget category is trending over. For example, using radiators instead of underfloor heating, or standard tiles instead of large-format porcelain.

Also: VCL, vapour barrier, vapour check

A membrane or sealed layer that restricts the passage of water vapour through a building element. PIR foil facing acts as a VCL only when all joints are taped with foil tape.

Also: variation order

A change to the agreed scope of work, usually with cost implications

Also: VAT Notice 708, Notice 708, HMRC buildings and construction VAT notice

HMRC's guidance notice setting out the VAT liability of construction work, including that work to an existing building is normally standard-rated and when the reduced or zero rate can apply.

Also: VAT Notice 708/6, Notice 708/6, energy-saving materials VAT notice

HMRC's guidance notice on the reduced and zero rates that apply to installed energy-saving materials such as insulation, including the rule that a single supply takes the rate of its principal element.

Also: VAT on building work, VAT on a loft conversion, standard-rated building work, zero-rated work, reduced-rate VAT

Which VAT rate applies to a building job. Work to an existing home is normally standard-rated. The reduced 5% rate and the zero rate apply only in narrower cases, such as qualifying conversions that change the number of dwellings, new dwellings, or specified energy-saving materials installed under the zero-rate relief. An ordinary loft room added to a house that stays one dwelling is normally standard-rated. A firm below the VAT registration threshold charges no VAT, which is why quotes can differ by the VAT amount alone.

Also: VAT threshold, £90,000 VAT threshold, mandatory VAT registration threshold

Builders earning above a certain turnover (currently £90,000) must register for VAT and add 20% to their invoices. A builder not charging VAT isn’t necessarily dodgy: they may simply be below the threshold.

Also: VAT record, VAT column, construction VAT records

The practice of maintaining a dedicated VAT column in a cost tracker, recording VAT paid per line item. Creates a VAT-ready record and helps verify that VAT-registered builders invoice correctly.

A German certification mark indicating tools are tested to IEC 60900 for 1,000V AC use on live circuits. Relevant to wire strippers and voltage tester leads.

Also: VELUX, Velux UK, velux.co.uk, Velux GGL, Velux GGU, Velux Certified Installer

Market-dominant branded roof window manufacturer in the UK. Their size code system (CK02, FK04, MK06, etc.) is the de facto industry standard. Their underfelt collar, flashing kit, and installation system are specifically referenced in DuPont's Tyvek technical manual. Offer a Velux Certified Installer programme.

Also: gravity-fed system, tank-fed system, open-vented system, low-pressure hot water system

A hot water system fed from a cold water storage cistern in the loft, with a vent pipe discharging over that cistern. Pressure at the taps comes from the height of the water above the outlet (about 0.1 bar per metre), which is why a loft ensuite fed this way usually needs a pump. Removing the loft cistern means moving to a mains-fed alternative (combi boiler or unvented cylinder) or relocating the tank.

Also: soffit board, soffit, vented soffit, ventilated soffit, hollow soffit board

The board that closes the underside of the roof overhang between the fascia and the wall face. The ventilated version has slots or a continuous strip giving the eaves airflow that Building Regulations require for a cold roof void (commonly a 10mm or 25mm continuous gap). Supplied as flat 9mm uPVC sheet or as pre-vented hollow soffit.

The planned approach to providing fresh air while maintaining energy efficiency

Also: vermiculite, vermiculite insulation, pebble-like loft insulation

A light, pebble-like loose-fill loft insulation, grey-brown or gold. Some vermiculite was contaminated with asbestos, but not all of it, and it cannot be identified by eye. HSE's guidance names loose-fill asbestos in lofts but does not name vermiculite; US Environmental Protection Agency guidance says to assume vermiculite contains asbestos and leave it undisturbed. Get a sample tested by an accredited laboratory before any work disturbs it.

Also: vertical damp proof course, abutment DPC

A strip of DPC material fixed vertically where the new extension wall abuts the existing house wall, sealed with mastic, to prevent moisture transferring between old and new masonry.

Also: voltage drop limit, 5% voltage drop, Reg 525.1

BS 7671 Reg 525.1 limits voltage drop to 5% of supply voltage for power circuits over the full circuit length, directly relevant to long cable runs to kitchen islands.

Also: non-contact voltage tester, NCV tester, voltage pen, volt stick

A pen-shaped device that detects live electrical cables without touching bare conductors. Hold it near a cable or socket and it beeps or lights up if voltage is present. A critical safety tool — always test before working near electrics.