Need to know what we mean by ‘Warm Roof’, or what a ‘Core Sample’ is? Here at we aim to demystify the process of having your school roof repaired, so we’re happy to oblige with this glossary of flat roofing terms.

Here are some useful flat roofing terms and their explanations, to help you navigate the language your roofing specialists may use. Click the links to go to the definition you want – or have a browse.

Is there a word we haven’t included? Is there something you need explaining? Just add a comment below or email us at [email protected] and we’ll add to this post. Thanks in advance for your ideas and suggestions.

Build up

The combination of materials that make up a roof. Commonly consists of the deck, vapour control layer, insulation and waterproofing membrane. Build ups can be ‘cold roof’, ‘warm roof’ and ‘inverted roof’ and will vary due to how they control the transfer of heat and moisture.


The Construction Design and Management (CDM) Regulations 2015 are the regulations which are designed to ensure the health and safety of everyone involved or affected by a construction project. Designers, Clients, Contractors are all required to fulfil their obligations under the CDM Regs. Read more about the CDM regs and Health and Safety during construction projects.

CIF – Condition Improvement Fund

The government’s Condition Improvement Fund, administered by the Education and Skills Funding Agency, is designed to provide capital funding to keep academy and sixth form college buildings safe and in good working order. Find out more here.

Cold bridging

A cold bridge is a point where the structure of a roof or wall protrudes through or into the insulation so that it transfers heat to the outside. Cold bridges can create cold spots on the inside of a wall and roof and produce condensation and heat loss. Modern roofs are designed to avoid cold bridging.

Cold Roof

A roof build up in which the insulation is laid under the roof deck and within the structure (for example between timber joists). Cold roofs are commonly not used today because they create cold bridges, require a ventilation space to allow moisture vapour to escape, and take up more depth. Warm roofs are more common.

Composite Panel

A composite panel is a construction element made up of a number of elements joined together and used in the construction of roofs and/or walls. In the case of roofing composite panels often combine a deck element with insulation and a Vapour Control Layer. They can make the process of replacing or constructing a roof quick and efficient if they are installed correctly. Here is a project which used composite panels in the refurbishment.

Construction Assurance Advisor

A construction assurance advisor works for the client (in the case of academy school roofs, an Academy or Multi Academy Trust) and ensures the client can and does meet all its obligations under the Construction Design and Management Regulations 2015, including carrying out site inspections and monitoring the school construction site to ensure it is being run safely. Make sure you have the advice and assurance (and the legal protection) that you need when a construction project is being carried out on your school. We wrote more about the CAA role here.

Construction Phase Plan

A key CDM document is the Construction Phase Plan, which the contractor produces to show all aspects of how they are going to do the work. This plan, which must be agreed by the team, covers the programme and work stages, welfare facilities and operational arrangements, communications arrangements and how all risks are going to be managed. We wrote about how one of our contractors used a Construction Phase Plan here.


A contractor was traditionally called a ‘builder’. The contractor undertakes to carry out the construction project in accordance with a written contract. On large projects there may be a Main Contractor and several SubContractors. Roofing projects will normally be carried out by a specialist Roofing Contractor.

Core Sample

“Taking a core sample” is the process of cutting out a small section of the roof to expose the different layers back to the deck. This allows us to check what the roof is made of, check for moisture in the construction and inspect the deck and other layers for any faults or alterations. A surveyor will take several core samples across your roofs, in areas which will indicate in his or her experience, the state of the roof as a whole. Find out how core samples were used on a recent project here.


The roof deck is the timber (wood) or metal surface which supports the roof insulation and waterproofing elements. The deck usually spans between walls, transferring the weight of the roof to them. If the space below the roof is large the deck may sit on joists or beams. See some photos of a metal deck here.

Dew Point

The Dew Point is the point within a roof build up where the temperature falls to such a degree that any moisture condenses at that point. This is known as ‘Interstitial Condensation’ as it can occur right within the structure, even between layers of membrane which should be adhered together. An Interstitial Condensation Risk Analysis may plot the Dew Point on a graph, showing precisely where the temperature drops below the point at which water vapour will condense.

EFA – Education Funding Agency

The Education and Skills Funding Agency (EFA or ESFA) is the body assigned to fund education and skills for children young people and adults. It administers the Condition Improvement fund (see CIF)


Flat roofs are not flat, they have falls or slopes which allow the water to drain away and prevent ponding. The way the roof falls determines which way the water drains off into gutters and outlets. The amount of fall of a roof is described as its pitch.


The part of the roof build up which helps keep the building warm in winter and cool in summer. Insulation keeps in heat in the same way as a wooly jumper does, by trapping air. Common insulation materials include wood fibre, mineral fibre, and various foams. Insulation comes in boards, blankets (soft batts or rolls) and as loose-fill and blown-in materials. Insulation can also be used to create falls on a roof.

Interstitial Condensation

Interstitial Condensation is the process by which water vapour within a roof or wall structure condenses. This happens because the temperature of the structure drops between the inside (usually warmer) to outside (usually colder)of a building, and if water vapour is allowed to travel through the structure, the moisture in the air can condense out as the temperature drops. The point at which this happens is called the ‘dew point’.

If interstitial condensation occurs, especially in a very warm interior like a school full of active students, it can produce a lot of water in the structure which will leak into the building. Interstitial condensation is often a problem in schools built in the latter part of the 20th century, and can often be mistaken for a leaking roof. Interstitial condensation is usually avoided by including a Vapour Control Layer (VCL) in the roof build up, which controls the amount of water vapour that can pass into the structure.

Inverted roof

A roof build up in which the waterproof layer is laid under the insulation. This is commonly used with a waterproof layer that needs protection from traffic or UV, and on roof terraces where paving or gravel provides the protective layer.


The process of laying over a new membrane to a flat roof instead of stripping the roof, insulation and even the deck back and replacing the whole thing. Overlays are considerably cheaper than a complete strip, assuming the condition of the roof makes it possible. They can extend the life of your roof by many years.


When water sits on a roof it ‘ponds’ producing areas of standing water that can attract algae and other unpleasant effects. Read more about the hazards of a leaking or inefficient roof here.


The pitch of a roof is the angle of slope of the roof in either degrees (e.g. 30˚) or a ratio such as 1 in 40 (1:40) or 1 in 80 (1:80).  All flat roofs and gutters should have a pitch in order to drain away water.

Risk Assessment

The process under Health and Safety legislation whereby hazards and their risks are evaluated, so as to eliminate as many hazards as possible and reduce the risks of the rest. Read about school risk assessment and leaking roofs here.


A specification is the document which sets out all the instructions on how the roof is to be repaired or replaced, including all materials, operational issues and health and safety. We include a full specification within our roof surveys. Find out more here.

VCL- Vapour Control Layer

A Vapour Control Layer is a thin layer of material designed to control the amount of water vapour (or moist air) that can travel through the roof build up. The VCL is usually placed directly below the insulation in a warm roof. Without a VCL water vapour can travel unchecked into the insulation, where it may condense as the temperature drops, producing interstitial condensation, which can sometimes be mistaken for a leaking roof.

Warm Roof

A roof build up in which the waterproof layer is laid over the top of the insulation and the insulation is laid over the top of the roof structure and deck, keeping the entire structure ‘warm’. By covering the structure with insulation, a warm roof avoids ‘cold bridging’ where an element of the structure transfers heat across the insulation.


The element of a roof which prevents liquid water from entering the roof build up. Waterproofing technology has developed considerably in recent decades. Common waterproofing types include torch-on roofing, liquid waterproofing and single ply membranes. Find out more here.

Help us Expand our Flat Roofing Terms Glossary

Is there a word we haven’t included? Is there something you need explaining? Have you come across some confusing flat roofing terms? Just add a comment below or email us at [email protected] and we’ll add to this post. Thanks in advance for your ideas and suggestions.

Receive Blog Posts by Email:

About the author – SIG D&T Team

We are the team at SIG Design and Technology. We supply school roof refurbishments and operate a successful bid process in consultation with specialist education surveyors for the replacement or refurbishment of roofs and pitched roof areas. With over 120 branches nationwide, SIG Roofing is the largest supplier of roofing materials in the UK, providing our customers with impartial advice on the right roofing solution for their projects.

Registered Office: SIG Trading Limited, Adsetts House, 16 Europa View, Sheffield Business Park, Sheffield, S9 1XH. Registered in England No. 01451007 VAT No. GB 487 01733