Video Transcript

What This Detail Is – And What You’ll Learn Today

Today’s detail is a masonry cavity wall parapet with a warm flat roof build-up — insulation above the roof deck, a single-ply waterproof membrane, and a coping stone at the wall head.

By the end of this episode, you’ll understand:

• The role of the cavity tray and weep holes at the parapet
• How copings and support plates protect the wall below
• What each layer in the flat roof build-up does and how they connect
• Why upstand height and membrane laps are crucial for waterproofing
• How the thermal and moisture lines run continuously across the junction

You can download the free student pack that accompanies this episode – more about that later.

Glossary of Terms

• Warm roof — A roof build-up where the insulation is placed above the structural deck, keeping the deck and structure warm and reducing condensation risk.
• Vapour control layer (VCL) — A layer beneath roof insulation that restricts vapour from moving into the colder parts of the roof build-up.
• Upstand — A vertical or raised portion of the roof edge that allows the waterproof membrane to turn up, preventing water ingress at the junction.
• Coping stone — The capping at the top of a parapet wall that sheds rainwater to both sides, protecting the wall below. You can also get coping stones that shed to one side only too. 
• Cavity tray — A damp-proof barrier that catches any water entering the cavity and directs it to the outside through weep holes.
• Weep holes — Small openings allowing any water or condensation behind the outer leaf or under the coping to drain out safely.
• Support plate / flashing — A metal or rigid support beneath the coping, this closes the cavity and ensures the DPC remains stable. 

Walk Through the Detail

Let’s look at how this junction is built up, from inside to outside, and bottom to top.

Starting at the internal face, we have 12.5 mm + 40mm insulated plasterboard as the internal wall finish, attached to the blockwork inner leaf with dot and dabs. The cavity between inner and outer leaves is fully filled with insulation to maintain the wall’s thermal performance.

At the top of the wall, the cavity is closed with a cavity tray that includes weep holes at regular intervals. The tray catches any water that may find its way behind the coping or through joints and channels it safely outwards. 

Above this, the coping stone spans across the cavity. It’s bedded onto a DPC supported by a metal plate — this was one of the student’s main questions. The metal plate or flashing closes the cavity and stabilises the DPC. The coping itself has drips on both sides, designed to throw rainwater clear of the wall.

Below the coping, we see the roof build-up. This is a warm roof, meaning the insulation sits above the deck. Here we have an 18 mm plywood deck on joists, covered by a vapour control layer, then two layers of rigid insulation, and finally a single-ply waterproof membrane.

Where the flat roof meets the parapet, an insulated upstand above the base of the horizontal insulation is installed to improve thermal performance. The waterproof membrane continues up the face of the upstand and laps over the insulation, connecting neatly to the flashing and coping arrangement. The waterproof membrane upstand must be a minimum of 150mm from the finished roof level.

Student Questions

1. Is that a metal plate beneath the coping stone?
   Yes — beneath many copings you’ll find a support plate or flashing, often stainless steel or aluminium. It closes the cavity and helps support the DPC across the cavity.
2. What do the thick black lines represent?
   The thick black lines in this drawing are showing the flashing that laps over the upstand to prevent water ingress. Each flat roof waterproofing membrane will require specific abutment pieces and flashing requirements. 
3. Is there a chamfer here?
   Yes, a chamfer or fillet is installed, depending on the requirements of the flat roof waterproofing material and manufacturer recommendations, it helps direct water away from the edges or corners of the roof toward the drainage. 
4. What does this represent?
   The small element behind the insulated plasterboard is an indicative dot and dab adhesive that is used to fix the insulated plasterboard to the wall. 

Key Points for This Detail

• Waterproofing continuity: The upstand must extend at least 150 mm above the roof surface. The membrane should lap securely under the coping or termination flashing.
• Cavity drainage: Always include weep holes above the cavity tray to let moisture escape.
• Thermal continuity: The roof insulation should meet the cavity insulation to prevent a cold bridge at the parapet. If the two insulations can’t physically meet, there should be a suitable overlap between the two to minimise cold bridging. In this case both the cavity insulation and the roof insulation extend beyond the finished roof.
• Coping design: Provide drips both sides to shed water clear of the wall face. Support plates or flashings must be non-ferrous and durable.
• Vapour control: Ensure the VCL under the roof insulation is continuous and sealed at joints.
• Coordination: Check heights and upstand dimensions against manufacturer guidance and insulation thickness.
• Maintenance: Parapets are high-risk for water ingress — ensure good access and durable materials.

As always Project specifics matter. Substrate, moisture, exposure, finishes, acoustic and fire requirements, warranties/insurer requirements and manufacturer guidance can all shift the final arrangement. 

There is always more than one way to design a detail.