What this planner works out
This planner divides your loft area by the area of a single board to give the number of boards needed, before waste.
The arithmetic is simple. The important part of loft boarding is not the board count — it is that boarding straight onto the joists compresses the insulation underneath and creates a condensation risk, which is covered below.
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Before you board a loft
- Measure the area you will actually board, not the whole loft. Boarding the full footprint is rarely necessary and rarely wise.
- Standard loft panels are commonly 1.22 by 0.32 m — three fit the width of a standard sheet. Full 2.4 by 0.6 m boards are cheaper per square metre but far harder to get through a hatch.
- Add 5 to 10% for cuts around trusses, tanks and the hatch.
- Check what the joists are. Ceiling joists in a trussed roof are sized to hold up a ceiling, not to store possessions.
Current guidance is 270 mm of loft insulation. Ceiling joists are typically 100 mm deep. Laying boards directly on them compresses the insulation to a third of its thickness, destroying most of its value, and creates a cold bridge where condensation forms. Raised boarding systems — legs or battens lifting the boards above the insulation while leaving an air gap — are the correct approach.
How this calculator works
An area divided by a board area:
Loft area = length × width
Board area = board length × board width
Boards = loft area ÷ board area, rounded up
Add 5 to 10% for cutsThe calculation assumes a clear rectangle. Trusses, water tanks, chimney breasts and the hatch itself all reduce the usable area and increase cutting waste, so a real loft usually needs the higher end of the waste allowance.
Worked example: a 4 m by 3 m loft
Using the default figures — a 4 m by 3 m area with 1.22 by 0.32 m boards:
- Loft area: 12 m²
- Board area: 0.39 m²
- Boards: 31, before waste
Add 10% and it is 34 boards. But before ordering, work out what sits underneath: 12 m² of 270 mm insulation compressed to 100 mm is a substantial loss of thermal performance across the whole boarded area, and on a house with a well-insulated loft that is a measurable increase in heating cost. A raised system costs more per square metre and does not create that problem.
Common mistakes
- Boarding directly onto the joists. It compresses the insulation and risks condensation.
- Assuming ceiling joists will take storage loads. They are sized to support a ceiling.
- Boarding the whole loft. Board the area you will actually use and leave the rest insulated.
- Blocking eaves ventilation. Boarding to the edge can close the airflow that keeps the roof dry.
- Buying full-size boards. They frequently will not fit through a standard loft hatch.
Frequently asked questions
How much weight can a loft take?
Far less than most people assume. Ceiling joists in a modern trussed roof are designed to carry the ceiling and the insulation, not storage — and cutting or altering trusses is never acceptable without engineering advice. Older cut roofs with deeper joists may take more. If you plan to store anything substantial, or to use the space regularly, have the structure checked.
What is a raised loft boarding system?
Plastic or timber legs that raise the boards above the insulation, leaving the full 270 mm uncompressed and an air gap above it. They typically add 100 to 200 mm of height and spread the load across more of the joist. They cost noticeably more than boards alone and are the approach that current insulation guidance effectively requires.
Will boarding cause damp?
It can. Compressed insulation creates a colder surface where warm moist air from the house condenses, and boarding that blocks eaves ventilation stops the roof space drying. Signs are damp patches on the underside of the boards, mould on the roof timbers and wet insulation. Maintaining ventilation and keeping insulation uncompressed prevents it.
Do I need building regulations approval?
Not for boarding out a loft for storage. Converting a loft into a habitable room certainly does — it involves floor structure, stairs, fire escape, insulation and often structural alteration, and is a completely different project from laying boards over the joists.
Is what I enter stored?
No. Loft measurements are processed in your browser and never transmitted or retained.
Related tools
References
- GOV.UK — Approved Document L on insulation and Approved Document A on structure
- Health and Safety Executive — working at height and safe access to loft spaces
Sources are checked at publication and can change — how I choose and check references.