Estimating and Project Support
Material Takeoffs for roofing contractors
The maths in a takeoff is the part nobody gets wrong. What decides the number is everything that was assumed before the maths started.
Quantities, and what each one rests on
A list of materials with quantities against them, in the units you order in rather than the units the drawing happens to use. That is the part everybody expects.
Beside it, the assumption behind each quantity, stated. Waste factor applied. Pitch used. Layers counted, or the number assumed and marked as assumed. What the existing surface was taken to be. Which areas are replacement and which are overlay.
And a note of where the dimensions came from — a field measurement, a report, a set of drawings, or somebody’s recollection. Those are not equally reliable and the sheet should not pretend they are.
A correct calculation on a wrong premise
Give two experienced estimators the same roof and the same tape, and they will produce different bills of material. Not because either can add up badly, but because a takeoff is mostly inference. The measurable part — length, width, the geometry of the planes — is the smallest contributor to what ends up being ordered.
Everything else is a judgement standing in for a fact nobody has yet established. How much comes off in waste on this crew, cutting this material, on this pitch. Whether there is one layer up there or three. Whether the deck under it will take a fastener. Whether that flat section at the back is the same assembly as the rest. Each of those changes the order, and none of them is visible in the finished quantity.
Which produces the peculiar failure of this desk. A takeoff is almost never discovered to be wrong on paper — it is discovered on a Tuesday morning, on a roof, when the material runs out or the wrong material turns up. By then the reasoning that produced it has evaporated, and there is no way to tell whether the shortfall came from the waste factor, the layer count or a section nobody had accounted for.
So the deliverable is not really the quantity. It is the quantity with its reasoning still attached, so that the next argument about it is about an assumption somebody can accept or replace, rather than about whether the estimator is any good.
One assumption the law will not let you make
Most assumptions in a takeoff are commercial and you are free to get them wrong at your own cost. There is a class where that stops being true, and it is worth knowing because it turns a quantity question into a legal one. On covered buildings, what the existing material IS has to be established rather than presumed. Under 40 CFR 61.145, the owner or operator must “thoroughly inspect the affected facility or part of the facility where the demolition or renovation operation will occur for the presence of asbestos, including Category I and Category II nonfriable ACM.”
Older roofing assemblies are exactly the place this arises. A tear-off quantity assumes the material coming off is ordinary waste; if it is not, the quantity is the least of what changes, because the handling, the disposal and the people permitted to do the work all change with it.
The rule does not reach everything, and the page would be misleading if it implied otherwise. The definition at 40 CFR 61.141 provides that “Facility means any institutional, commercial, public, industrial, or residential structure, installation, or building (including any structure, installation, or building containing condominiums or individual dwelling units operated as a residential cooperative, but excluding residential buildings having four or fewer dwelling units)”. Ordinary house work sits outside that definition; larger multi-unit and commercial work often does not.
Both provisions bind the owner or operator of the renovation, not this brand. They are quoted here for a single narrow reason: they are the clearest illustration that a takeoff rests on a claim about what is already on the building, and that on some buildings the claim has to be checked by somebody qualified before anyone counts anything.
This describes a federal standard in general terms to illustrate a class. It is not legal advice, it makes no determination about any building, material or project of yours, and other rules may reach the same work. Those questions belong with your own counsel and a qualified inspector.
Between the measurement and the order
This desk sits inside Estimating and Project Support, and the boundary governing the department as a whole is set out there rather than repeated here. What is particular to this page is its position: downstream of however the roof was measured, and upstream of anything anybody does with a price.
What estimators ask about quantities
Our takeoffs come back and the crew still runs short. Why?
Usually because the quantity was right for the roof somebody assumed and wrong for the roof that was there. A second layer nobody counted, a pitch steeper than the one used, a section of decking that had to be replaced rather than reused. The arithmetic was fine. The input was a guess that nothing on the sheet identified as a guess.
What do you need to produce one?
Dimensions from a measurement or a drawing, and answers to the handful of questions that decide the rest: what is up there now, how many layers, what the pitch is, and what is being replaced versus covered. Where an answer is not known we write down the assumption we used instead of quietly picking one.
Why write the assumptions down if they are only estimates?
Because that is what makes the number arguable. A quantity on its own can only be accepted or rejected. A quantity with its basis attached can be corrected in the one place that is actually wrong, by somebody who knows the roof better than the sheet does. It also means a takeoff that turns out badly teaches you something.
Do you set the waste factor?
We apply the one you use and we record which one was applied. Waste is a commercial judgement about your crews, your material and how much you are willing to send back, and it varies legitimately between companies. What is not legitimate is a waste figure nobody can find afterwards, applied differently on two jobs for no stated reason.
Can you do takeoffs from photographs?
For sanity checks and for questions about condition, often. For quantities, not on their own — a photograph establishes what something looks like, not how big it is. If a measurement exists we work from it; if one does not, the honest first step is getting one rather than inferring dimensions from an image and presenting the result as measured.
Do you tell us what the materials cost?
No. Pricing is yours, and it moves for reasons that have nothing to do with the roof — supplier terms, timing, what you have in the yard. This desk produces the quantities and the reasoning behind them; the money that gets attached to them is a separate decision made with information we do not hold.
The renovation standard cited here
- 40 CFR 61.145 — Standard for demolition and renovation — U.S. Environmental Protection Agency, via eCFR
- 40 CFR 61.141 — Definitions — U.S. Environmental Protection Agency, via eCFR
Take your last short order and work backwards
If nobody can now say which assumption produced the shortfall, that is the thing to fix, and it is not the arithmetic.