Re Roofing an Existing Building: Where the Money Actually Goes

The membrane is a minor part of the bill. Stripping, disposal, substrate repair, insulation, details and whatever is standing on the roof dominate the cost.

Owners ask for a price per square metre of membrane and receive quotations that differ by a factor of two. The variation sits in everything other than the membrane: how much has to be stripped, whether the substrate is sound, how much wet insulation must be removed and disposed of, how many penetrations and upstands need rebuilding, and what equipment is sitting on the roof that has to be lifted, supported or relocated.

What this number does and does not include

This covers replacing the waterproofing system on an existing flat or low pitched roof, including preparation, insulation, details and the handling of existing plant and services. It excludes structural strengthening, which is a separate assessment, and it excludes internal repairs to damage caused by previous leaks, which belong to the building rather than to the roof contract.

Stripping and disposal

Removing the existing build up is labour intensive, generates substantial waste and has to happen in weather windows that allow the roof to be made watertight again each day. Disposal cost depends on volume and on the material, and some older materials require specific handling that must be identified before work starts rather than discovered mid strip.

• Pushes it up: Multiple previous layers, saturated insulation holding water weight and restricted access for waste removal.
• Brings it down: A single sound layer that can be overlaid where the system and warranty permit it.
• How to control it: Open up a sample area before pricing so the number of layers is known rather than assumed.

Substrate repair and falls

The deck beneath the waterproofing may be sound, locally degraded or holding water. Screed that has failed must be removed and replaced. Where ponding has been a problem, the falls need correcting, which means either a new screed or a tapered insulation layout. Laying a new membrane over poor falls guarantees ponding and shortens its life.

• Pushes it up: Degraded screed, flat or reverse falls and localised deck deterioration requiring repair.
• Brings it down: A sound deck with adequate falls that only requires cleaning and priming.
• How to control it: Survey falls with a level and record ponding areas before the specification is written.

Insulation

Replacing insulation is the moment to improve thermal performance, because the access cost is already being paid. Wet insulation must be removed, since it will not dry under a new membrane. Thickness affects upstand heights, threshold levels and sometimes door clearances, so an increase has consequences beyond the insulation line itself.

• Pushes it up: Thicker specification, tapered layouts for falls and the knock on effects on upstands and thresholds.
• Brings it down: Keeping the existing thickness where thermal performance is already adequate.
• How to control it: Decide the thermal target before design, since changing it later changes every detail.

Details, upstands and penetrations

Outlets, upstands, movement joints, parapets, plinths, pipe penetrations, lightning conductor fixings and door thresholds are where roofs fail and where the labour concentrates. Each needs a detail drawing and often a proprietary component. A roof with many penetrations costs considerably more per square metre than a clear one of the same area.

• Pushes it up: High penetration counts, low existing upstands requiring rebuilding and complex parapet geometry.
• Brings it down: Rationalising redundant penetrations by removing them rather than detailing around them.
• How to control it: Count and photograph every penetration before tender and remove those no longer in use.

Plant, services and solar equipment

Air conditioning units, ductwork, cable trays, water tanks, satellite equipment and solar arrays all sit on the roof and all have to be lifted, temporarily supported or removed and reinstated. Where plant is in service, this has to be sequenced so the building keeps operating. This line is routinely omitted from first quotations and is routinely substantial.

• Pushes it up: Operating plant that cannot be shut down, solar arrays requiring removal and reinstatement.
• Brings it down: Plant mounted on plinths that allow the membrane to be dressed beneath without disconnection.
• How to control it: Survey everything standing on the roof and price its handling as a named item.

Access, protection and safe working

Materials and waste have to reach and leave the roof, often by crane or hoist, with road closures or parking suspensions in dense locations. Edge protection is required, and where the building is occupied there must be protection against falling objects. Weather windows constrain the programme, since an opened roof cannot be left exposed.

• Pushes it up: Tall buildings, restricted street access, occupied premises below and limited crane positions.
• Brings it down: Open site access, a single crane position serving the whole roof and vacant premises.
• How to control it: Resolve the access method before tender, because it drives both programme and cost.

Testing, guarantee and documentation

Flood testing, inspection by the system manufacturer where the guarantee requires it, and registration of that guarantee. Manufacturer guarantees generally require approved installers and documented inspections, so these steps are conditions rather than optional quality measures. The documentation is what makes the guarantee usable later.

• Pushes it up: Longer guarantee periods requiring more inspection and more stringent installer approval.
• Brings it down: Standard guarantee terms with a single inspection regime.
• How to control it: Make guarantee registration a condition of the final payment.

Costs that never appear on the first page

• Structural repair discovered when the deck is exposed, which cannot be priced before the strip.
• Raising door thresholds and adjusting access where insulation thickness increases upstand heights.
• Temporary waterproofing when a weather event arrives mid programme and the roof must be secured.
• Removing and reinstating a solar array, including re testing and reconnection after the works.
• Internal redecoration where stripping the roof releases water that has been held in the build up.
• Replacement of lightning protection fixings and re testing of the system after the new roof is laid.

Keeping the budget honest

Price a roof replacement from a survey that includes an opening up, a falls check and an inventory of everything standing on the roof, and treat any quotation produced without those as an estimate rather than a price. Hold a contingency for substrate repair, because it is the one item that genuinely cannot be known until the existing covering is removed. Where the building is occupied, add the cost of protection and sequencing rather than hoping it will be absorbed. Approved systems and the requirements attached to them change over time, so confirm current specifications before procuring.

• Before tender: an opening up record showing the existing layers and the deck condition.
• Before tender: a falls survey and a photographic inventory of every penetration and item of plant.
• Before ordering: detail drawings approved for each upstand, outlet and penetration.
• During works: daily confirmation that the exposed area is watertight at the end of each shift.
• At completion: flood test results, manufacturer inspection sign off and the registered guarantee.

Domira works on buildings that already exist: surveying what is installed, specifying the systems that have to be replaced, and running maintenance and refurbishment work around the people using the property. Tell Domira which asset you are responsible for and we will say what we would inspect first.