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The underside of a concrete floor slab over an open undercroft car park: bare board-marked soffit, downstand beams, a row of columns and pale daylight from the open side.

Application

The concrete soffit: the build-up, in sequence.

A concrete slab with an occupied floor above and an undercroft, car park or open space below. Closed-cell foam is applied to the soffit from below, wrapping the beams and edges, to the depth the U-value sets, and coated where the fire strategy requires a classification.

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Is this your soffit?

01The build-up

The slab stays. The layer goes on beneath it and wraps what hangs from it.

Concrete is dense, cold and always in contact with the outside where the undercroft is open. The layer is applied to the soffit in passes, to the depth the calculation sets, and carried round downstand beams, column heads and edges so the insulated line is continuous. Services are stopped around at a closed edge, or re-hung outside the layer where the project allows. Five things in the section, keyed beneath it.

Schematic / not to scaleKeyed 1 to 5 below

1 / Existing slab and floor above

Stays as it is. Nothing is lifted, cut or fixed through from the floor above.

2 / Prepared soffit

Sound, clean and dry: laitance, dust and loose material off, the surface within the temperature window. Adhesion is decided here, not later.

3 / Closed-cell layer, in passes

To the specified depth, verified as the work proceeds. λ 0.027 W/mK is the NexSeal LE product figure, not a HOLVEM performance figure.

4 / Beams, edges and services

Downstand beams and edges are wrapped so the line is continuous. Services are stopped around at a closed edge, or re-hung outside the layer.

5 / Coating, where specified

B-s2,d0 belongs to the coated NexSeal LE and DC315 system, as tested. Where the coating is not applied, that classification does not carry. Section 03 below.

The building this is written for most: offices over an undercroftClosed-cell system data

02Project records

The sequence, on real soffits.

HOLVEM's own footage and photographs on concrete soffits: the layer applied from below in passes, carried round the frame, the services beneath left clear. Press play to watch.

Project recordSoffit sprayed
A concrete soffit sprayed from belowHOLVEM's own installation footage: closed cell applied to the soffit in passes.
A concrete deck soffit in an undercroft fully sprayed in a continuous cream closed-cell layer, galvanised steel frame and red cable ducts below, the spray rig drums and gun on the gravel floor.
Project recordUndercroft soffit
Concrete deck soffit, fully sprayedThe layer continuous across the soffit and round the frame; the services below left clear.
Project recordSoffit insulation
Soffit insulation, in passesHOLVEM's own footage: the layer built up on a soffit, pass by pass.

All project records

03The sequence

Four states. The slab never moves; the face the air touches does.

Step through the four states on one drawing. Beneath it, the run says where the face the air touches sits: at the cold end while the slab is bare and prepared, at the dry end once the layer is on. Each state is checked before the next begins.

STATE 1 OF 4 · BARESchematic / not to scale

Cold slab · below the dew pointThe dew point · water formsAbove the dew point · dry

Each state is checked before the next begins. Where one cannot be met on the day, the work stops and the reason is recorded; it is not sprayed anyway.

04Fire considerations

Coated, or as applied. Only one of them carries a Euroclass.

A soffit over a car park is a ceiling people walk under, so the fire strategy has a say in what lines it. The layer can be left as sprayed or finished with DC315, and the two perform differently in a fire. We settle which one you need before anything is specified.

Coated · DC315 intumescent

B-s2,d0 to BS EN 13501-1, on the coated system.

The DC315 intumescent coating is applied to the cured layer; the foam depth under it depends on the scope and what the building has to achieve. The classification belongs to the coated NexSeal LE and DC315 system as tested: 35 to 40 mm of NexSeal LE on 9 mm plasterboard with DC315 at 0.7 mm wet film thickness (BTTG report 27/06638/12/24), confirmed for each project against the fire strategy. It is applied as one build-up and recorded as one.

  1. Layer cured and clean, depth verified.
  2. Coating applied to the specified thickness, coverage recorded.
  3. The record in the QA evidence pack at handover.

Uncoated · the layer as applied

Class 1 surface spread of flame to BS 476-7. No Euroclass claim.

Left as applied, the layer carries the product's Class 1 surface spread of flame and nothing more. Where the specification does not require a reaction-to-fire classification, this is a real option; where it does, it is not an option at all.

  1. Layer applied and verified, face left as cured.
  2. No coating, no Euroclass: B-s2,d0 does not carry.
  3. The decision recorded in the specification, so nobody claims it later.

Whether your soffit needs a classification, and which one, is set by the building's fire strategy and the Building Regulations guidance that applies to it. HOLVEM answers what each system achieves and puts it in writing. Fire performance and coatings.

05Which depth fits

Four objectives. On a soffit, the third is usually the one.

Decide what the floor above has to achieve and the depth follows. On concrete it is usually a U-value target, calculated with the beams and edges in it. The table shows what each depth is for; the survey confirms it.

ObjectiveDepthWhat that depth is for
Control condensation25 mmCondensation control on the soffit. The slab stops being the coldest surface the air beneath it meets, so nothing forms on it and nothing drips. On a soffit this is rarely the whole objective; it usually comes with the U-value.
Retain heat50 mmCondensation control plus useful heat retention in the floor above. Cuts the conductive loss through the slab so the floor holds its heat. Not a percentage saving and not compliance on its own.
Reach a calculated U-value75 mm and aboveSet by the U-value calculation: the usual objective on a soffit. A figure for the floor to meet, calculated with the beams and edges in it. A contract-specified 0.18 W/m²K has needed 135 mm on a concrete deck and 140 mm on live retail soffits.
Achieve Euroclass BDepth to suit + DC315The coated system, where the building needs it. DC315 intumescent coating on the cured layer: B-s2,d0 to BS EN 13501-1 belongs to the coated NexSeal LE and DC315 system as tested: 35 to 40 mm of NexSeal LE on 9 mm plasterboard with DC315 at 0.7 mm wet film thickness (BTTG report 27/06638/12/24), confirmed for each project against the fire strategy. Uncoated, the layer carries Class 1 surface spread of flame to BS 476-7, which is not a Euroclass.

Thickness is set by the objective and the calculation, never by the area. Whether your soffit needs a classification is the fire strategy's question; section 03 states what the coated and the uncoated system achieve.

Thickness and objectiveFire performance and coatingsWhat it costs: the cost guide

06The survey

Five things we check before anyone talks about depth.

A survey is one visit, from the undercroft, fitted around the building’s hours and its parking. Nothing is opened up and the car park keeps working. It gives us the five things a depth is confirmed against.

  1. The objective

    A calculated U-value for the floor above, condensation control on the soffit, or both. And whether a reaction-to-fire classification is required.

  2. The slab and its soffit

    Condition, previous coatings, moisture and what the surface will hold. The layer bonds to what is there.

  3. Beams, edges and services

    The beams, edges, columns and services the layer has to wrap or stop at, and what can be re-hung outside it.

  4. Above and below

    Heating and occupancy above, parking below, and the hours the undercroft can be released.

  5. Access from below

    Bays released in turn, containment, and vehicle movements while the work is on.

What you get after the survey

Five things in writing, whether or not you go ahead.

  • A specificationWritten for this soffit, from what we found, with its assumptions named.
  • The depth, and whySet by the objective and confirmed by calculation for the element as built, bridges included.
  • The condensation-risk positionWhether anything will condense inside the build-up, to BS EN ISO 13788 with the BS 5250 climate.
  • The fire positionWhether your fire strategy calls for a classified surface, and the coated or uncoated build-up that answers it. Confirmed against the strategy, never assumed.
  • A firm priceFor your soffit, in writing. What moves it is explained in the cost guide.

Survey, or photographs?Send photographs of the soffit

Tell us about the soffit. The survey follows.A short brief: the building, what you are seeing, and how to reach you. Sent before 3pm on a working day, we are in touch the same day.

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Project records

Where we have done this before.

All project records
Project recordThe Box, Plymouth787 m² of concrete soffit at 120 mm, to a specified U-value of 0.18 W/m²K.Read the record Project recordPondswick substation, Luton127 m² of concrete soffit insulated and coated with DC315 under live switchgear.Read the record

07Straight answers

Ask us the hard ones.

The questions engineers, landlords and facilities managers actually ask on the call, answered the way the specialist answers them. A question we have not answered? Ask it on the call. No obligation.

Can the floor above stay in use?
Yes. The work is done from below, and nothing is lifted, cut or fixed from the floor above. The undercroft is released to us in bays, in turn, and handed back; the shopping-centre record above was done with the centre trading throughout.
How thick does it need to be?
On a soffit the depth is usually set by a U-value target for the floor, calculated for the build-up with the beams and edges in it; a contract-specified 0.18 W/m²K has needed 135 to 140 mm. Where the objective is condensation control or heat retention, the 25 and 50 mm bands apply. The calculation sets it; the survey confirms it. Thickness and objective.
Does the soffit have to be coated?
Only where the specification requires a reaction-to-fire classification. Coated, the NexSeal LE and DC315 system carries B-s2,d0 to BS EN 13501-1 in the build-up tested; uncoated, the layer carries Class 1 surface spread of flame to BS 476-7 and no Euroclass claim. Which one your building needs is the fire strategy's question, and we put the answer in writing before the work starts.
How much does it cost?
On a soffit the depth is usually set by a U-value target, so the price follows the calculation and the building. Where condensation control or heat retention is the objective, the 25 and 50 mm bands apply. Access, the condition of the soffit, the beams and services to wrap, and the hours the undercroft can be released move it as well. What moves the price is set out in the cost guide; send the building in a project brief and you get a real figure for your soffit.
How do you prove the system was installed correctly?
Applied thickness measured and recorded, coverage logged, all compiled into a QA evidence pack handed over at completion. You hold the evidence, not our word.
Isn't spray foam a problem for surveyors and lenders?
Those concerns overwhelmingly involve residential lofts and undocumented installs. Our commercial work is the opposite case: BBA-certified products, a written specification from condensation risk analysis and U-value calculation, and a QA evidence pack at handover. The building file answers the surveyor's question before it's asked.

Your next step

Your building.
Your brief.
A specialist.

You may already know the thickness, the system or the U-value; if so, we price to it and confirm it is the right specification. If not, tell us what you do know about the building and the soffit. "Not sure" is a useful starting point.

Start a project brief About five minutes. Photographs help; nothing else is needed.

Or pick up the phone

Telephone . Before 3pm on a working day, you are contacted the same day. After 3pm, the next working day. A commitment to contact, not a quotation.

Photographs of the undercroft first? Upload photographs of the building