The Kitchen Extension Flat Roof That Leaks and Sweats

Problem 1: rain penetration at the junction with the house wall. The covering terminates in an upstand of only 75 mm, sealed with mastic and not chased into the wall. Any water running down the house wall in driving rain passes behind the mastic, and in any case mastic on a south- or west-facing upstand will fail within a few years through UV and thermal movement. The stain is a narrow band along the wall line - a linear pattern, which is characteristic of a junction failure, not of a general roof failure.

Problem 2: interstitial condensation in a sealed cold deck. The insulation is between the joists and pushed up against the deck, so in winter the joists and the OSB deck stay cold while the void between the insulation and the plasterboard ceiling is filled with warm, moist kitchen air. The ceiling is not airtight (there will be gaps at the wall junction and around downlights), there is no vapour control layer, and there are no vents in the parapet, so the moisture cannot escape. When the deck falls below the dew point, condensation forms on its underside - the droplets the client describes. The blisters near the outlet are also consistent with trapped moisture beneath the GRP being driven off by sun on a warm day.

Why the outlet is also wrong. Water standing near the outlet suggests the outlet is not in fact at the low point, or that the fall is interrupted; a spigot with no sump also means the water has to be deeper than the outlet lip before it discharges. The pattern of a linear stain at the wall, widespread droplets under the deck and local ponding at the outlet cannot have a single cause.

Diagnostic check. If the liquid under the deck is clear and appears in cold spells, it is condensation; if it is brown and appears after storms, it is rainwater. Both are present here.

Choose a warm-deck, unventilated build-up. With a 600 mm parapet on three sides, cross ventilation of a cold deck is difficult to achieve: vents would have to be cut through the parapets at high level, they can be blocked or ignored on site, and the geometry makes an effective through-path unreliable. A warm deck avoids the need for ventilation altogether and keeps the structural deck warm, so condensation on the timber is prevented by design rather than by airflow.

Proposed build-up, from the joist upwards:

1. 200 × 50 mm joists, plasterboard ceiling and a polyethylene vapour control layer laid over the deck and turned up at all edges; the VCL is the airtightness line and must be continuous and sealed at the wall and parapet junctions.
2. 18 mm OSB3 structural decking.
3. 120–150 mm PIR in two layers with staggered joints, laid over the deck and over the joists, so that the joists are inside the warm envelope.
4. 18 mm ply or OSB carrier deck.
5. Single-ply membrane or GRP, with a minimum fall of 1:40 (GRP generally wants 1:40–1:60; single-ply can be flatter but 1:40 is good practice), formed with firrings if the joists are level.
6. Outlet relocated to the true low point, in the field of the roof and not against the parapet, with a sump and a leaf/ballast guard, plus a scupper or overflow through the parapet set about 50 mm above the finished surface so a blocked outlet cannot allow water to build up to the upstand.
7. Upstands a minimum of 150 mm above the finished surface, with a chased flashing 150 mm into the house wall and into the parapets - not mastic.

Why not simply re-cover the existing deck. New GRP over the same condensation-prone build-up will fail again within two winters, and the client will have paid twice.

Drawing: 1:5 section, A3, cut perpendicular to the house wall through the roof, extending 300 mm up the house wall and 300 mm into the roof. Materials hatched with a key, dimensions to every significant interface, and leader notes. Include the wall to a height that shows the ring beam or floor band so the flashing is clearly independent of it.

Elements the section must show:

• The existing house wall construction, including the outer leaf, cavity and inner leaf, so the flashing is seen to be chased into a mortar joint 150 mm deep and 25 mm deep, with the joint raked and pointed, and a lead or single-ply flashing dressed over a 25 mm upstand.
• The new roof deck, VCL turned up the wall at least 75 mm and sealed, and the insulation continuing to the wall with no gap - this is the cold bridge and the condensation path, so it must be explicit.
• The membrane turning up 150 mm minimum and terminating under the flashing, not face-sealed.
• The cavity of the new wall (if any) closed with a cavity tray above the flashing, with stop ends.
• The plasterboard ceiling line, showing the VCL as a continuous line over the deck and the airtight seal at the wall.
• A dimensioned vertical dimension chain: finished roof surface, top of upstand, flashing height, deck and joist depths.

At the parapet: a second 1:5 detail showing the upstand height, the coping with a drip and a fall, the membrane dressed over the upstand, and the scupper with its invert level relative to the finished roof surface.

What a reviewer must be able to check from these two drawings: that water is shed outward, that the upstand and flashing heights are adequate, that the airtightness and vapour lines are continuous, and that no insulation gap creates a cold bridge.