# Loft glossary: the words explained > What joists, rafters, eaves, loft legs, U-values, RdSAP and the other loft words mean, in plain English, each with its source. Source: https://completeloftservices.co.uk/glossary/ Generated: 2026-11-02 --- - [Home](/) - [Advice](/advice/) - Glossary # Loft glossary: the words explained The words you will hear about lofts, insulation and boarding, each in a sentence or two, with where it comes from. Assumed (on an EPC) "Assumed" means the assessor could not inspect that part of the home, so the rating comes from its age and type. "Pitched, insulated (assumed)" is not a measured depth. [More on this](/advice/how-to-read-your-epc-loft-rating/). Source: [GOV.UK, find an energy certificate](https://find-energy-certificate.service.gov.uk/). Read 21 October 2026. Building regulations approval Sign-off from building control that work meets the building regulations. The Planning Portal says a loft conversion needs it, and laying boards over the joists may need a building control application. [More on this](/advice/loft-insulation-regulations-legal-requirement/). Sources: [Planning Portal, building regulations for a loft conversion](https://www.planningportal.co.uk/permission/common-projects/loft-conversion/is-building-regulations-approval-needed-for-a-loft-conversion); [Planning Portal, unimproved loft space](https://interactive.planningportal.co.uk/smart-search-house-elements/unimproved-loft-space). Read 21 October 2026. Closed cell spray foam A spray foam that sets rigid and insulates very well. Its main drawback is that water vapour cannot pass through it. [More on this](/advice/spray-foam-in-your-loft-remove-or-keep/). Source: [Energy Saving Trust, spray foam insulation explained](https://energysavingtrust.org.uk/spray-foam-insulation/). Read 21 October 2026. Cold roof A pitched roof insulated at ceiling level, on the loft floor, so the loft above stays unheated. [More on this](/loft-insulation/). Source: [Historic England, insulating pitched roofs at ceiling level](https://historicengland.org.uk/images-books/publications/eehb-insulating-pitched-roofs-ceiling-level-cold-roofs/heag077-cold-roofs/). Read 21 October 2026. Condensation Water that forms when warm, damp air from the house meets a cold surface, such as the underside of the roof. In a loft, airflow is what carries it away. [More on this](/advice/will-loft-boarding-cause-condensation/). Source: [NHBC, condensation in your roof space](https://www.nhbc.co.uk/kontentdocuments/865dc06d-9f7d-4a86-83a6-ed34e82b9a3b/condensation-in-your-roof-space-2024.pdf). Read 21 October 2026. Crawl boards Boards laid as a walkway across a loft, so you can reach tanks, wiring or stored things without stepping between the joists. Historic England suggests a gap of ideally 50mm or so between the boards and the insulation. [More on this](/advice/is-my-loft-strong-enough-to-board/). Source: [Historic England, insulating pitched roofs at ceiling level](https://historicengland.org.uk/images-books/publications/eehb-insulating-pitched-roofs-ceiling-level-cold-roofs/heag077-cold-roofs/). Read 21 October 2026. Eaves The line where the rafters meet the outside wall. In a loft, it is the low, tight space where the roof slope comes down to the floor. [More on this](/loft-boarding/). Source: [Trussed Rafter Association, buyers' guide](https://www.tra.org.uk/wp-content/uploads/tra_buyers_guide_online_29.05.19.pdf). Read 21 October 2026. Eaves ventilation Gaps or vents along the eaves that let outside air flow through the loft and carry damp away. Insulation and boards should never block them. [More on this](/advice/will-loft-boarding-cause-condensation/). Sources: [NHBC, condensation in your roof space](https://www.nhbc.co.uk/kontentdocuments/865dc06d-9f7d-4a86-83a6-ed34e82b9a3b/condensation-in-your-roof-space-2024.pdf); [Historic England, insulating pitched roofs at ceiling level](https://historicengland.org.uk/images-books/publications/eehb-insulating-pitched-roofs-ceiling-level-cold-roofs/heag077-cold-roofs/). Read 21 October 2026. Energy certificate (EPC) An Energy Performance Certificate rates how energy efficient a home is, from A (best) to G (worst). It lasts ten years, and you can look one up by postcode on GOV.UK. [More on this](/advice/how-to-read-your-epc-loft-rating/). Sources: [GOV.UK, energy performance certificates](https://www.gov.uk/buy-sell-your-home/energy-performance-certificates); [GOV.UK, find an energy certificate](https://find-energy-certificate.service.gov.uk/). Read 21 October 2026. Joist The horizontal timbers that form the floor of the loft and hold up the ceiling of the rooms below. The first layer of insulation sits between them. [More on this](/advice/is-my-loft-strong-enough-to-board/). Source: [Energy Saving Trust, roof and loft insulation guide](https://energysavingtrust.org.uk/advice/roof-and-loft-insulation/). Read 21 October 2026. Loft boarding Boards laid across the loft floor so you can walk on it and store things up there. Done well, it sits above the insulation rather than squashing it. [More on this](/loft-boarding/). Source: [Energy Saving Trust, roof and loft insulation guide](https://energysavingtrust.org.uk/advice/roof-and-loft-insulation/). Read 21 October 2026. Loft conversion Turning a loft into a liveable room, one you use as a normal part of the house, even a spare bedroom used now and then. It is a bigger job than boarding a loft for storage. [More on this](/loft-conversion/). Source: [Planning Portal, building regulations for a loft conversion](https://www.planningportal.co.uk/permission/common-projects/loft-conversion/is-building-regulations-approval-needed-for-a-loft-conversion). Read 21 October 2026. Loft hatch The opening in the ceiling that gets you into the loft. An insulated, draught sealed hatch stops warm air leaking up into the loft. [More on this](/loft-hatches/). Source: [Historic England, insulating pitched roofs at ceiling level](https://historicengland.org.uk/images-books/publications/eehb-insulating-pitched-roofs-ceiling-level-cold-roofs/heag077-cold-roofs/). Read 21 October 2026. Loft ladder A ladder fixed at the hatch that folds or slides away when not in use. The common types are aluminium, timber folding, concertina, which folds up like an accordion, and telescopic, which closes into itself. [More on this](/loft-ladders/). Loft legs Supports that stand on the joists and hold a loft floor up above the insulation, so the insulation keeps its full depth under the boards. [More on this](/loft-boarding/). Loft lining A breathable sheet, a foil sheet or boards fixed across the underside of the rafters, to keep dust and grit off what you store. It is not insulation. [More on this](/loft-lining/). Mineral wool The soft insulation sold in rolls. It is laid between and then across the joists, and it works best when it is not squashed. [More on this](/loft-insulation/). Source: [Energy Saving Trust, roof and loft insulation guide](https://energysavingtrust.org.uk/advice/roof-and-loft-insulation/). Read 21 October 2026. 270mm 270mm is the minimum loft insulation depth the Energy Saving Trust recommends. It is a recommendation, not a depth set in the building regulations. [More on this](/advice/how-much-loft-insulation-do-i-need/). Source: [Energy Saving Trust, how to insulate a loft](https://energysavingtrust.org.uk/how-to-insulate-a-loft). Read 21 October 2026. Open cell spray foam A soft spray foam used as insulation. It is lighter and more flexible than closed cell foam, but it does not insulate as well. [More on this](/advice/spray-foam-in-your-loft-remove-or-keep/). Source: [Energy Saving Trust, spray foam insulation explained](https://energysavingtrust.org.uk/spray-foam-insulation/). Read 21 October 2026. Rafter The sloping timbers that make up the roof itself. The roof covering sits on them, and loft lining is fixed across their underside. [More on this](/loft-lining/). Sources: [Energy Saving Trust, roof and loft insulation guide](https://energysavingtrust.org.uk/advice/roof-and-loft-insulation/); [Trussed Rafter Association, buyers' guide](https://www.tra.org.uk/wp-content/uploads/tra_buyers_guide_online_29.05.19.pdf). Read 21 October 2026. Raised loft boarding A loft floor fixed on legs above the joists, so there is room for the insulation underneath at full depth. [More on this](/loft-boarding/). Source: [Energy Saving Trust, roof and loft insulation guide](https://energysavingtrust.org.uk/advice/roof-and-loft-insulation/). Read 21 October 2026. RdSAP The simpler method assessors use to produce energy certificates for existing homes. It fills gaps with standard assumptions, so the assessor collects less. [More on this](/advice/will-loft-insulation-improve-my-epc-rating/). Source: [GOV.UK, Standard Assessment Procedure](https://www.gov.uk/guidance/standard-assessment-procedure). Read 21 October 2026. Trussed rafter (truss) A light timber frame, usually triangular, made to be set at regular spacing to hold up the roof. It must never be cut or trimmed unless a truss designer says so. [More on this](/advice/is-my-loft-strong-enough-to-board/). Source: [Trussed Rafter Association, buyers' guide](https://www.tra.org.uk/wp-content/uploads/tra_buyers_guide_online_29.05.19.pdf). Read 21 October 2026. U-value A measure of how much heat passes through part of a building, such as a roof or ceiling. The lower the number, the less heat escapes. [More on this](/advice/270mm-vs-300mm-loft-insulation/). Source: [Approved Document L, Volume 1](https://assets.publishing.service.gov.uk/media/662a2e3e55e1582b6ca7e592/Approved_Document_L__Conservation_of_fuel_and_power__Volume_1_Dwellings__2021_edition_incorporating_2023_amendments.pdf). Read 21 October 2026. ## Want to know what your loft needs? The survey is free and takes about twenty minutes. You get a written price before any work starts. [Book a free survey](/contact/).