How to Plan a Commercial Flat Roof Replacement That Holds Together

Plan a flat roof replacement backwards from the season you want the crew on site. Establish the condition of the whole assembly first, write one scope every bidder prices identically, carry unit prices for what is hidden under the membrane, and agree site logistics before mobilisation. Most overruns are decided before anyone opens the roof.

When should planning start relative to the work?

Earlier than most owners expect. A replacement running in summer or early autumn is normally scoped over the previous winter, tendered in late winter and awarded before the spring rush. By April, GTA roofing crews are largely committed for the good-weather months, and a tender issued in May competes for what is left.

That lead time is about sequence, not queueing. A roof cannot be specified until its condition is known, and its condition cannot be established from the ground. The moisture survey and test cuts should be done when the roof is clear of snow. Material carries procurement lead times. Permits, where they apply, run on a municipal timeline you do not control. Compressing that sequence is where scope quality gets lost.

If the building is on a capital plan, get a written condition assessment a full budget year ahead of the money. A flat roof inspection that records deficiencies by priority, remaining service life and photographs gives you something to put in front of an owner or a board, and it tells you whether the honest answer this year is repair, another maintenance season, or flat roof replacement.

What has to be known before a scope can be written?

A specification written from a walk-around and a satellite image is a guess. Before anyone can commit to an assembly, these facts have to be on paper:

• Deck type and condition. Steel, concrete, or wood plank on older buildings. Steel gauge and flute direction matter for fastener pull-out values and for how leaked water has been travelling.
• How many roof layers are already there. This governs whether a recover is even permitted, and it changes disposal volume dramatically if it is a tear-off.
• Insulation type, thickness and moisture content. Established with test cuts and a moisture survey across the field, not from one wet corner. Wet insulation does not dry inside a sealed assembly and cannot be reused.
• Whether a vapour barrier exists and where. A building with high interior humidity and no air and vapour control layer is a different project from a dry warehouse.
• The drainage layout. Number and position of internal drains and scuppers, whether overflows exist, where water actually stands after rain, and whether tapered insulation is present or the deck itself is pitched.
• The perimeter and every penetration. Parapet heights, base flashing height, cap flashing and reglet condition, curbs, pitch pockets, expansion joints, hatches and skylights.
• Access and structure. Where a crane or hoist can set up, where material can be staged, and whether the structure accepts a thicker assembly.

Test cuts are the part owners resist and the part that pays for itself. A cut the size of a sheet of paper shows every layer, its thickness and its condition, and it is patched the same day.

What belongs in a defensible replacement specification?

The specification is the project. It should be written so that two contractors reading it build the same roof. At minimum it names the assembly layer by layer and the details that surround it.

• Tear-off or recover, and the disposal method.
• Air and vapour control layer, where required, and its termination at walls and penetrations.
• Insulation type, board thickness, number of layers, staggered joints, and tapered insulation layout with crickets between drains and behind curbs.
• Cover board type and thickness.
• Membrane system: two-ply SBS modified bitumen with a granulated cap sheet, heat-welded TPO or PVC, EPDM or a built-up assembly.
• Attachment method and fastener pattern, with the increased density required in the perimeter and corner zones where wind uplift is highest.
• Flashing: base flashing height up the parapet, cant strips at wall junctions, new cap flashing or counterflashing set into a reglet, and how the membrane is terminated.
• Drainage: new clamping rings and target flashings at every drain, sump reconstruction where the drain sits proud, and overflow provision.
• Penetrations: curbs built or raised rather than sealed with pitch pockets wherever the detail allows, plus expansion joint covers.
• Walkway pads on the traffic routes to rooftop equipment.

Ontario Building Code requirements apply to the new assembly, including thermal requirements and wind uplift design for the roof zone. Design snow and rain loads come from the climatic data for your municipality in OBC Supplementary Standard SB-1, and structural questions go to a professional engineer. The commercial roof installation and replacement page sets out how we build that scope.

How should the tender be structured so bids are comparable?

The most common way an owner loses control of a budget is tendering a vague scope and awarding the lowest number. The low bid is often low because it excluded something the others included, and that exclusion becomes a change order in week two.

Structure the tender so that comparison is possible:

1. One written scope, issued to everyone. Not a conversation with each bidder separately.
2. A mandatory site meeting. Every bidder walks the same roof and sees the same conditions. Questions asked there get answered to all bidders in writing.
3. Unit prices for what is hidden. Deck replacement and wet insulation removal per unit area, additional drains, additional length of cap flashing. Agreed before award, so what is found later is priced at a rate you already accepted.
4. Named allowances for anything genuinely unknowable until the roof is open.
5. Submittals for the actual products, so the assembly delivered is the assembly specified.
6. Schedule, working hours and site rules stated in the tender, because a contractor pricing restricted hours and a contractor pricing free run of the site are not pricing the same job.

Ask each bidder for WSIB coverage confirmation and a certificate of insurance. We provide both on request, and a property management office will normally want them on file before mobilisation. Our property manager services page covers the documentation side in more detail.

Which variables move the price and the schedule most?

Roof area is the number everyone starts with, and it is rarely the number that decides the project. The variables that move a commercial replacement most are:

• Tear-off versus recover. Removal and disposal of a saturated multi-layer assembly is labour and volume, and it exposes deck repairs that a recover would have hidden.
• Access. A roof reached by crane over a live street with lane occupancy permits behaves differently from a warehouse with a yard beside it.
• Detail length relative to area. A small roof cut up by curbs, walls, shafts and setbacks holds more site-built detail per unit area than a large open field, and detail is where the labour is.
• Rooftop equipment. Units that have to be lifted, disconnected or curbed higher bring other trades into your schedule.
• Drainage rebuild. Adding tapered insulation, relocating or adding drains, or cutting new scuppers is a design change, not a like-for-like replacement.
• Occupancy constraints. Night work, phasing around production and torch-free assemblies in sensitive buildings all carry into the price.

If you are budgeting before you are buying, the flat roof cost calculator shows how those variables interact. It is a planning tool for a range, not a quote, and it does not replace a site visit.

How do you keep the building running while the roof comes off?

Nearly every roof we replace has people or product underneath it. The controls that make that workable are agreed before the first load of material arrives, not improvised on day three.

Phasing is the core of it. The crew opens only the area that can be made watertight the same day, with a temporary water cut-off at every stop point, so nothing is left exposed overnight. On a large industrial roof that means daily sections that follow the drainage pattern rather than the shape of the building, so rain during the work does not run into an open edge.

The rest is logistics: crane or hoist placement and hours, material staging away from fire routes, dumpster position, drains kept clear through the work, tenant notice before noisy or odorous phases, and a fire watch maintained after torch-applied work is shut down. Where a rooftop air intake sits near the work, it is shut down or filtered during hot work so odours do not reach the occupied space. Our crews follow the Occupational Health and Safety Act and the working-at-heights requirements enforced by the Ministry of Labour, Immigration, Training and Skills Development, with guardrails or travel restraint at every open edge and anchor points chosen for the deck type.

Where do replacement timelines actually slip?

Weather gets the blame and rarely deserves all of it. In practice the schedule slips in a short list of places:

• Permits. Requirements vary by municipality and are confirmed before work begins, not after mobilisation.
• Long-lead material. Insulation, custom metal and specialty membrane carry lead times that can dwarf the installation window.
• Crane and hoist availability on a specific date, especially when a street occupancy permit ties you to it.
• A second membrane layer nobody knew about, doubling tear-off and disposal.
• Deck deterioration found under wet insulation, which is why the unit price matters.
• Other trades. Mechanical disconnects and structural work on a curb sit outside the roofer's control.

Season choice helps too, and our post on choosing the season for commercial roofing works through the trade-offs. To get the condition and the scope established before you commit money, contact our team at 416-456-0777.