Decorating a Loft Conversion in London: What You Need to Know

Loft conversions are one of the most technically demanding first-decoration jobs in London

Every week we are asked to decorate newly completed loft conversions in London — and every week we encounter the same set of problems caused by the construction sequence rather than by any failure of the builder. Lofts are unusual spaces: a mixture of sloping and vertical surfaces, complex geometry around windows, new plasterboard that is often still drying, and a set of material junctions that require careful thinking before any paint is opened.

This guide sets out what distinguishes a loft conversion decoration job from a standard room, and how to get it right first time.

The substrate question: cold roof vs warm roof plasterboard

The first thing a decorator needs to know before specifying primers and finishes is the construction type.

Warm roof (insulated rafter) construction places the insulation above or between the rafters, with plasterboard fixed directly to the underside of the rafters or to counter battens. The plasterboard surface is typically at ambient temperature and behaves like any other new plasterboard in the house.

Cold roof construction — still found in many London loft conversions, particularly where the structural conversion was done with a traditional cut-roof approach — places insulation between and/or below the rafters, with plasterboard on the cold side of the insulation layer. In this configuration, the plasterboard can experience significant temperature cycling and, importantly, condensation risk at the plasterboard face. If there is a vapour barrier, it must be intact — any gaps allow warm moist air to contact the cold back of the plasterboard, which can cause moisture damage, staining, and eventually mould at the plasterboard face.

Before decorating a cold roof loft, check:

• Is there a vapour control layer on the warm side of the plasterboard? If not, insist the builder remedies this before you paint.
• Is there any visible moisture or staining at the plasterboard face? If yes, the roof needs investigation, not painting.

Mist coats on new plasterboard: not optional

New plasterboard, whether direct-fixed or skim-coated, must receive a mist coat before any finish paint is applied. A mist coat is a diluted first coat — typically 10–20% water added to a standard white emulsion — that penetrates the porous surface and creates a base that binds subsequent coats.

Without a mist coat:

• The porous plasterboard face draws the binder out of the first finish coat unevenly, leaving a patchy, variable sheen that no amount of further coats will fully resolve.
• On skim-coated surfaces, dry plaster will pull the moisture from full-strength emulsion too rapidly, leaving a weak, powdery first coat.

On skim-coated walls and ceilings in a new loft, use Dulux Trade Vinyl Matt diluted 10–15% as the mist coat. Allow to dry fully (typically four to six hours in warm conditions) before applying the first full coat. Two full coats of finish emulsion after the mist coat will give a consistent, opaque result.

On bare unskim'd plasterboard — less common but sometimes seen in budget conversions — apply a dedicated plasterboard primer (Dulux Trade Plasterboard Primer or equivalent) before any emulsion.

Sloping ceilings: the practical challenges

Painting a sloping ceiling in a loft is physically demanding and technically different from a flat ceiling. The challenges:

Cutting-in at the ridge and eaves. The junction between a sloping ceiling and the vertical wall at the top (ridge board junction) and the bottom (knee wall junction) must be cut in accurately by brush before rolling. On a sharply raked ceiling, keeping a clean line without drips running down the slope requires a loaded but not over-loaded brush and a steady hand.

Roller direction on raked surfaces. Roll in the direction of the slope (top to bottom) rather than across it. Rolling across a raked surface creates lap marks that are difficult to eliminate.

Sheen choices. Flat matt emulsions hide surface imperfections — the variations in plasterboard flatness that are common in loft conversions — better than eggshell or silk. On sloping ceilings where the natural light from Velux windows will rake across the surface at low angles, any surface imperfection is highlighted. A dead flat emulsion (Farrow & Ball Estate Emulsion, Zinsser Perma-White Matt, or Dulux Trade Flat Matt) minimises this.

Velux soffits and reveals

Velux windows and other roof lights create reveals and soffits that are among the most fiddly areas to paint in any loft. They are typically formed in plasterboard, timber or a combination. Key points:

• Plasterboard reveals need the same mist coat treatment as the walls. They are small in area but highly visible — a poor finish around the window is immediately apparent.
• Timber soffits need to be primed with an oil-based or shellac-based primer before emulsion is applied over them. Timber soffits painted with emulsion without priming will absorb unevenly and may show resin bleed through knots.
• The frame itself. Velux window frames are factory-finished in white. Do not paint the factory-finished frame surface — you will void any warranty and the frame finish is superior to anything you can apply with a brush on site. Mask the frame before painting the surrounding reveals.
• Condensation at the reveal. Velux reveals in cold roof constructions can suffer condensation in winter. If you see staining at the junction of the reveal and the frame, investigate the thermal bridge before decorating — painting over condensation damage will fail within one season.

Knee walls and low-section storage areas

Knee walls — the short vertical walls at the eaves where the sloping roof meets the floor level — are frequently underestimated. They are awkward to reach, may have restricted headroom, and are often fitted with access hatches to the eaves storage beyond.

• Paint the accessible face of the knee wall in the same finish as the rest of the room.
• Access hatch doors should be painted both sides — the inward face (visible when open) and the outward face.
• Storage areas behind knee walls are sometimes skim-coated and painted, but more commonly left as bare plasterboard or OSB. If they are to be painted, a simple white emulsion over a mist coat is sufficient. Use a mould-resistant emulsion if there is any risk of condensation in the eaves void.

The correct decoration sequence for a loft

1. Inspect substrate: check moisture, check for cold spots or condensation, confirm vapour barrier integrity.
2. Apply mist coat to all new plasterboard and skim surfaces. Allow to dry.
3. Fill and sand any visible jointing, screw indentations or imperfections.
4. Spot-prime any knots or resinous timber in soffits and reveals.
5. First full coat: walls, ceiling and reveals. Cut in by brush at all junctions; roll all flat areas.
6. Allow to dry (minimum four hours, ideally overnight).
7. Second full coat. Inspect for missed areas in raking light from the Velux windows.
8. Prime and first coat all timber joinery (window boards, door linings, skirtings).
9. Second coat joinery. Satinwood or eggshell for all timber in living spaces; gloss only if specifically requested.
10. Touch-up any nibs or brush marks identified on final inspection.

Talk to us about your loft conversion

If you have a newly completed loft conversion or are planning to start decorating one, contact us for a free quote. We will inspect the substrate, identify any moisture or preparation issues, and provide a detailed quotation covering the complete decoration package.