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The finished soffit at Derriford District Centre: a continuous pale foam layer across the underside of the ribbed metal deck around the building's perimeter.
Project recordThe finished soffit

Project record · Metal deck soffit · Hybrid build-up

Derriford, Plymouth: a ribbed metal soffit taken to U 0.18 with two foams and no fixings.

An exposed galvanised metal deck soffit around the perimeter of a new retail centre, with a design U-value of 0.18 W/m²K to meet. Boards could not follow the rib profile or the access. We bonded 25 mm of NexSeal LE closed-cell to the deck and built 160 mm of SucraSeal open-cell over it, from scissor lifts, in two stages.

0.18W/m²K
Design U-value, met
185mm
25 mm closed-cell base plus 160 mm open-cell
2 stages
Sprayed from scissor lifts
0
Fixings into the deck

01The building

A new building with a soffit that boards could not follow.

A district retail centre in Plymouth, new, with an exposed galvanised metal deck soffit running round its perimeter. The design called for a U-value of 0.18 W/m²K through that soffit. The deck is deeply ribbed, the access is at height, and the ribs leave a board system with voids it cannot close.

The target. 0.18 W/m²K is a compliance figure, not a comfort one. It has to be calculated for the real build-up, with the ribs and the fixings in it, and then achieved on site to the depth the calculation needs.

The substrate. A ribbed galvanised deck has a profile depth that a flat board bridges rather than follows. Every bridged rib is a void, and every void is a thermal short-cut and a condensation risk behind the insulation. A bonded, sprayed layer follows the rib.

The access. The soffit sits at around 3 m above an external perimeter, worked from scissor lifts, in Plymouth weather. The method had to tolerate that, and stop when the weather said so.

Building
District retail centre, new build
Client
Derriford District Centre; Aldi
Location
Plymouth, Devon
Substrate
Ribbed galvanised metal deck soffitExternal perimeter, worked at about 3 m
Application
25 mm Enverge NexSeal LE closed-cell160 mm Enverge SucraSeal open-cell over it
Design U-value
0.18 W/m²K, met

02What we specified, and why

A thin bonded skin against the steel, then the depth the U-value needed.

The closed-cell does the bonding and the vapour control against cold metal. The open-cell does the depth, economically, to the figure the calculation set. Together they reach 0.18 without a single fixing into the deck.

  • The base: Enverge NexSeal LE closed-cell, at 25 mm. Bonded to the galvanised deck, following every rib, so the metal is sealed and the build-up has its vapour-control layer against the cold side. NexSeal LE declares μ 62 to BS EN 12086, used in the condensation-risk analysis for this soffit.
  • The depth: Enverge SucraSeal open-cell, at 160 mm. Built up over the cured base in passes to the thickness the U-value calculation required, filling the ribs and giving the soffit its thermal depth. Open-cell system data.
  • Why hybrid for a U-value. 0.18 through a metal deck needs real depth. Closed-cell to that depth is costly and unnecessary; open-cell alone would not bond and seal the steel. Each layer does what it is for, and the calculation carries both. U-values and Approved Document L.
  • No fixings. A sprayed, bonded build-up has nothing driven through it. The finished soffit is seamless, and the deck is untouched.
Base layer
Enverge NexSeal LE closed-cell, 25 mm
Build-up
Enverge SucraSeal open-cell, 160 mm
Substrate
Ribbed galvanised metal deck, underside
U-value
0.18 W/m²K design, met by the build-up
The closed-cell base layer on the ribbed galvanised deck: a thin cream skin following each rib of the soffit before the open-cell depth goes on.

Stage one: the base against the steel. 25 mm of closed-cell following every rib of the deck, the surface the 160 mm of open-cell would be built on. Project record.

Photographs · Project record

The deck, the base, the depth, the finish.

The bare ribbed soffit, the closed-cell base, the open-cell build-up and the finished perimeter.

Before: the exposed ribbed galvanised metal deck soffit around the building perimeter.
Project recordBefore: the deck
Before: the rib profile of the deck close up, the voids a board system would bridge.
Project recordBefore: rib profile
During: the closed-cell base layer bonded to the ribbed deck.
Project recordDuring: base layer
During: open-cell foam being built up over the base from a scissor lift, containment sheeting around the work.
Project recordDuring: open-cell
After: the finished soffit, one continuous layer around the perimeter.
Project recordAfter: finished

Photographs from the job, as it ran. Labelled Project record because they are.All project records

03How the work ran

Two stages from the lift, paused when the weather said so.

An external soffit at height is worked in containment, from scissor lifts, in two passes of two different foams, and only when the wind and the rain allow. The record says so, and so do we.

  1. Survey and calculation

    The deck measured, the U-value run.

    Rib profile, perimeter length and the build-up calculated to 0.18 W/m²K with the deck and the layers in it. The 25 plus 160 mm specification came out of that.

  2. Containment

    Sheeting up, lifts in.

    Containment sheeting around the work zone at around 3 m, scissor lifts positioned along the perimeter, adjacent finishes masked.

  3. Stage one

    Closed-cell to the deck.

    NexSeal LE sprayed to the ribbed deck, following the profile and sealing the metal, then left to cure.

  4. Stage two

    Open-cell to depth, weather permitting.

    SucraSeal built up over the base in passes to 160 mm. Work paused in wind and rain and resumed when conditions allowed, so the layer went on as designed.

04The outcome

The design U-value met, with a seamless soffit and nothing fixed through it.

The U-value is the figure the design set and the build-up was calculated and applied to meet. The rest is what a sprayed system gives: no joints, no fixings, no voids behind the layer.

U-value through the soffit

0.18W/m²Kdesign, met

Calculated for the deck and the two layers, and achieved at the depth the calculation set.

Fixings into the deck

0bonded, not fixed

A seamless soffit with nothing driven through the layer or the metal.

The ribs

Followedno voids

Every rib filled by the bonded base and the depth over it; no bridged profile behind the insulation.

Weather

Respectedpaused in wind and rain

The build-up went on only in conditions that let it cure as designed.

Where this record comes from

Calculated and applied, not metered. A U-value is calculated for the build-up to BS EN ISO 6946 by a competent person; the site work achieves the depth the calculation needs. HOLVEM supplies the calculation; the assessor holds the compliance position.

Yours will differ. A different deck, target or exposure gives a different build-up. U-values and Approved Document L.

05Where this record leads

If this looks like your building.

The building type, the compliance route this soffit met, and the application page for a profiled metal deck.

Your next step

Your building.
Your brief.
A specialist.

If you already know the U-value or thickness you need, tell us and we price to it. If you do not, tell us what you know about the building and the problem, and we work it out with you. Someone who does this every day reads it and comes back to you.

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Or pick up the phone

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