How to Measure Siding Square Footage: Net Wall, Gables, Openings, Waste and Squares, Worked on a 2,000 sq ft House, and Why Floor Area Is the Wrong Number

The method in four steps

1. Gross wall, each storey: walk the outside with a tape and add up the perimeter, then multiply by the wall height. A storey is about 9 ft from the bottom of the siding to the top of the wall once you include the floor framing: 8 ft of stud, a foot of joist and plate. Two storeys is 18 to 19 ft. Measure from the bottom of the existing siding to the soffit if you can get a tape there; otherwise count the courses.
2. Gables: each one is a triangle. Width of the wall at the bottom of the gable, times the height from there to the peak, divided by two. If you cannot get at the peak, use the roof pitch: a 6/12 roof rises 6 in. for every 12 in. of run, so a 28 ft wide gable has a 14 ft run and a 7 ft rise.
3. Openings out: measure every window and door, width times height, and subtract them. Some installers do not deduct openings under 10 sq ft because the cutting and J-channel around a small window costs as much as the siding it replaces; ask which they do, because it changes the number by 5% on a house with a lot of small windows.
4. Waste in, then squares: add 10% for lap siding, 12% for board and batten or a house with a lot of gables and dormers. Divide by 100 to get squares, which is how the supplier sells it, and round up to the next square.

Net wall is the number the price is on. Net plus waste is the number the material is ordered on. The installed rates on the siding cost page carry the waste already, so you price the net and the installer orders the gross; a quote that charges you the installed rate on the net-plus-waste number is charging the waste twice.

A 2,000 sq ft two-storey, worked through

A plain two-storey, 28 ft by 36 ft, 1,008 sq ft a floor, gable roof at 6/12 with the gables on the 28 ft ends. Twenty windows, two doors, one 16 ft garage door in a wall.

So the house with 2,000 sq ft of floor has 2,228 sq ft of net wall and the order is 25 squares. At mid vinyl, $9.50, the siding is $21,166. Price it on the floor area instead and you get $19,000, which is $2,166 short, and that is a plain box with two gables. At fibre cement, $18, the same mistake is $4,104.

Now give the same house a front dormer, a bay window and a two-storey bump-out at the back, which is what most 2,000 sq ft houses built since 1990 have. The net area goes up by a couple of hundred feet, but the cost goes up more than that, because every one of those features is corners, trim, flashing and short pieces: a house with dormers, bays and three materials costs 15 to 25% more per square foot than a plain rectangle at the same area. That is a labour effect, not an area effect, and it is why two installers measuring the same house get similar areas and different totals.

The same floor area in other shapes

A 1,200 sq ft bungalow, 30 by 40, 9 ft walls, 6/12 gables on the 30 ft ends. Perimeter 140 ft x 9 ft is 1,260. Gables: 30 x 7.5 / 2 is 112.5, times two, 225. Gross 1,485. Twelve windows and two doors, about 185 sq ft. Net wall 1,300. The floor area and the wall area are nearly the same on a bungalow, which is why people think floor area works; it only does on this shape.

A 2,000 sq ft storey-and-a-half, 28 by 40 with a full second floor under a steep 12/12 roof and two shed dormers. The first storey is 136 ft x 10 ft, 1,360, but the second storey is mostly roof: two gable ends at 28 x 14 / 2, 196 each, 392, plus the dormer faces and cheeks, say 160. Gross 1,912, less 300 of openings, net about 1,610. Less wall than the two-storey, but all of the second-storey wall is in gables and dormers, which is the slowest siding there is, and the rate for it is at the top of the 15 to 25% range.

A 2,000 sq ft side-split or back-split is the one that fools everyone, because the floor area is on four levels and the walls are on three different heights. Measure each wall separately from grade to soffit; a split has 2,400 to 2,700 sq ft of net wall for 2,000 of floor, and a lot of it is the tall wall at the back.

Eaves, trim and the other measurements

Soffit and fascia are priced by the linear foot of eave with the soffit depth stated, not by the square foot. Walk the perimeter at the roof line, including the gable rakes, and note the overhang depth: 12 in., 16 in., 24 in. The 128 ft perimeter house above has about 150 lf of eave and rake once the gable slopes are counted, and that is the 150 lf in every worked example on this site. Soffit and fascia costs run from that number.

Trim is linear feet too: corner posts are the height of the wall at each outside corner, so a two-storey with four corners is about 76 lf of corner post; window and door trim is the perimeter of each opening, about 14 lf for a typical window. Count the openings and write down their sizes while you are measuring them for the deduction; it is the same list, and the flashing kits are priced per opening off it.

Height matters to the price as well as the area. Note which walls have siding above 20 ft from the ground, because that part of the wall carries the scaffolding line, $1.50 a square foot, and on a two-and-a-half storey it is a third of the wall.

What to have on paper before anyone measures

1. A sketch of each wall with its length, its height to the soffit, and the gable rise, labelled front, back, left, right. A photo of each wall from across the street helps the estimator and settles arguments later.
2. The opening list: every window and door with width and height. It is the deduction, the flashing-kit count and the trim count in one list.
3. Your net wall number, worked the way the table above works it. When the quote says 2,600 sq ft and your sheet says 2,228, ask the installer to walk you through his. One of you forgot a gable or counted the garage door twice.
4. What is coming off, by area and material, because tearoff is a unit rate and you want it priced that way, not as a lump.
5. The eave length and soffit depth, separately, so the eaves are not buried in the siding number.

Put the walls into the exterior cladding cost calculator one at a time and it does the gables, the deductions and the scaffolding line for you. If you would rather have it done from the drawings, the $249 takeoff measures every elevation from your plans and hands you the sheet the installers should be quoting from. For three quotes that disagree about the area, the quote comparison grid is where to start.

Questions people ask

Sources and further reading

• Vinyl Siding Institute: the installation manual every vinyl warranty points to: loose nailing, expansion gaps, laps and the J-channel details.
• Ontario’s Building Code, Section 9.27 Cladding: the first and second planes of protection, sheathing membrane, flashing at openings and transitions, and the clearances from grade and roofs that every siding and stucco job has to meet.
• Your rights when starting home renovations or repairs: Ontario government: the 10-day cooling-off period on a contract signed at your door, the written contract rule, estimates, and the door-to-door rules.