Can you insulate a flat roof from above?

A flat roof can be insulated in three places. Between the joists is the traditional cold roof, with a ventilated void above. Under the joists is an internal lining. On top of the deck is a warm roof, where rigid boards sit above the structure and below the waterproof covering.

The warm roof has a real advantage when you are already stripping the old covering. The joists and deck end up on the warm side of the insulation, so they stay above the temperature where condensation forms. A vapour control layer goes under the insulation, the boards go over it, and the membrane goes on top. Nothing inside the house is disturbed.

Rigid boards of polyisocyanurate (PIR) or similar are common because they give a good thermal performance for their thickness, which matters when height is tight. The product and thickness are chosen to suit the U-value being aimed for.

The 2021 edition of Approved Document L, with its 2023 amendments, treats a flat roof's new waterproof covering as renovation work on a thermal element. Renovate over half the element, or carry out a major renovation, and the guidance is to bring the whole roof up to a U-value of 0.16 W/(m²·K). New or replacement roof elements have to reach 0.15 W/(m²·K) under Table 4.2.

There is some room for judgement. A note to the table accepts a lesser standard when reaching the full figure would eat into head room, and the guidance still wants the insulation plus any air gap to match the rafter depth. In practice, this means your building control body and your installer will agree what is reasonable for your roof.

Our guide to laying a new roof over an old flat roof covers a related decision. An overlay adds layers, but on its own it does nothing for thermal performance.

Boards that give a good U-value are thick, and every millimetre adds to the roof level. The middle of the roof does not mind; the edges do.

• Upstands. The membrane must still turn up a wall to a safe height above the finished surface. Extra insulation takes some of that height away, so the flashing may need to be raised or chased in higher.
• Door thresholds. A door onto a balcony or roof needs a step or threshold clear of the new surface. If the roof rises, the step shrinks and rain can splash over it.
• Window heads and sills. A roof that rises past a window head, or leaves too little height beneath a sill, needs a different detail.
• Edge trims and gutters. The drip edge must still discharge into the gutter at the right line.

These are worked out at the survey with a tape measure, before the quote. Our page on the flat roof on a dormer shows how tight these heights become on a small roof.

A warm roof is not always possible. If the height gain is too great, a cold roof can be rebuilt with insulation between the joists and a ventilated void above, following BS 5250 principles. The void must be properly vented, and blocked vents are a common cause of condensation and rot.

A third route is to insulate under the ceiling instead, which avoids raising the roof but cuts room height and puts the structure on the cold side. It is simpler for some, and more hit and miss for others. Ventilation and a vapour control layer have to be right.

We usually favour a warm roof when the height works, because it leaves the timber warm and the ceilings untouched. Where it does not, we explain the alternatives in the written quote and why we suggest one over another.

Insulation boards need a sound, flat deck. Rotten boards come out first, and the falls are checked. Tapered insulation can be used to create or improve the fall, which helps a roof that was laid dead level. The design target is 1:40, leaving a finished minimum of 1:80.

The covering goes over the boards and must be compatible with them. Felt, EPDM, GRP and single-ply all work above insulation if the system is specified for it, and fixings or adhesives must suit the board. Mechanical fixings pass through the insulation to the deck, and the layout is calculated for the exposure.

Walk-on use needs boards with enough compressive strength, or a protective layer over the top. Tell us when we survey, and we will check the build-up fits the use you have in mind.

The survey is a tape-measure job as much as a visual one. We note the height at every upstand, door and window, then check the deck from above and below. Pictures of your roof go with the written quote. Notifiable jobs have the Building Regulations notification filed by us, with the consent position explained.

The 10-year workmanship guarantee is a written one, and the site is left as we found it. Call 01793 626096 or use WhatsApp or the contact form to arrange a visit to your flat roof. Our page on how many layers of felt a flat roof needs may help if you are choosing the covering as well.