Liquid-Applied Polyurethane Balcony Flashing Seam Calculator

SettingsSettings for this calculationUS

Market

Imperial · sales tax

Who is doing the work?

Waste is set to 8% by hand. Pick a tier above to replace it, or keep your own figure.

Total Seam/Detail Length

The combined length of all seams, drains, and edge details needing reinforcement.

Add up every location that needs reinforcement fabric or flashing strip in the liquid-applied system — perimeter edges, drain details, expansion joints, wall-to-floor transitions, and any substrate seams.

Waste Allowance (%)

Extra material to allow for overlaps, cutting waste, and detail transitions.

Reinforcement fabric and flashing strips need overlap at splices and extra length at corners and transitions, so a waste allowance beyond the raw measured seam length is typical.

Seam flashing needed

88.56 ft

High confidence

Then change the inputs to see how far the answer moves.

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How this was calculated

Formula source(s)

Total seam flashing = total seam/detail length × (1 + waste allowance), the standard method for estimating reinforcement fabric or flashing strips at seams, drains, and edge details in a liquid-applied waterproofing system

Inputs used

Total Seam/Detail Length: 82 ft
Waste Allowance (%): 8

Final result88.56 ft

What this calculation does not cover

A length is not an order. Reinforcement fabric comes by the roll in a fixed band width, and the width follows the detail — how far the band runs up the upstand and back out onto the deck, commonly around 100 mm (4 in) each way at a wall junction and more at a drain. The fabric is also only half the material: it has to be laid into wet resin and then over-coated, and detail work consumes resin far faster per square metre than the flat deck does. Order the field coating off the deck area and the fabric off this figure, and the detailing is the part left short of resin — which is where the system is already thinnest.
Fabric will not turn a sharp corner. Every internal angle — deck to upstand, upstand to door cill — needs a fillet or cove formed first, or the reinforcement bridges the angle and tents, leaving a void behind it with no bond on either side. That void is invisible under the top coat and it is the first thing to split when the deck moves. The filleting material, and the primer the substrate needs before any of it, are in neither this length nor its waste allowance.
A movement joint counted as plain seam length gets the wrong detail. A joint designed to open cannot simply be reinforced across it: the membrane has to be free to stretch there, which means a bond-breaker tape or a loop of fabric laid with slack, and materially more length at that joint than the straight run someone measured. Reinforce a moving joint rigidly and it tears along exactly the line it was there to protect.

Estimated cost — your price

Price per ft ($)

This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.

Schematic, drawn to the proportions you entered — not to scale on screen.

Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.

Sources checked 2026-09-06 · in the site-wide review of 2026-09-06 · v1.0.1

Regulatory standards & verification citations1

Total seam flashing = total seam/detail length × (1 + waste allowance), the standard method for estimating reinforcement fabric or flashing strips at seams, drains, and edge details in a liquid-applied waterproofing system

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Your workspace

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Now that you have the number

These guides cover the work this quantity is for — the first ones run this calculator inside the section that raises the question.

Balcony Flashing: Where the Deck Meets the Walluses this calculator

The doorway is the one junction on a building where a wet outdoor floor meets a dry indoor one at the same level. Detail it outward from there.

How to calculate liquid-applied polyurethane balcony flashing seam in 3 steps

Total Seam/Detail LengthThe combined length of all seams, drains, and edge details needing reinforcement.

Waste Allowance (%)Extra material to allow for overlaps, cutting waste, and detail transitions.

Seam flashing neededThe tool computes the seam flashing needed from those figures and shows the formula, its sources, and a confidence rating alongside it.

Total Seam/Detail Length	Seam flashing needed (ft)
40 ft	43.2
60 ft	64.8
80 ft	86.4
100 ft	108
120 ft	130
140 ft	151
160 ft	173

Total Seam/Detail Length

Seam flashing needed (ft)

40 ft

43.2

60 ft

64.8

80 ft

86.4

100 ft

108

120 ft

130

140 ft

151

160 ft

173

Frequently asked questions

Where does a balcony actually fail?

At the threshold, almost always. A balcony is the one place a building deliberately drains TOWARDS itself at a single point: the door opening, where the deck finish, the waterproofing, the door cill and the wall construction all arrive at the same line and where the level difference available is measured in tens of millimetres because somebody has to walk over it. Everything else on a balcony is a plain falling surface. That is why this quantity is driven by the perimeter conditions and upstand heights rather than by the deck area, and why the upstand at the door is the dimension worth arguing about — a deck that is a hundred millimetres short of the threshold is one wind-driven rain event away from a wet ceiling below.

What counts as a 'seam or detail' in this calculation?

Any location in the liquid-applied polyurethane balcony system that needs reinforcement fabric or a flashing strip — perimeter edges, drain details, expansion joints, wall-to-floor transitions, and substrate seams should all be included in the total measured length.

Why add a waste allowance to the raw seam length?

Reinforcement fabric and flashing strips need overlap at splices and extra length where they turn corners or transition between details, so the material required is somewhat more than the raw measured seam length.

Does this calculator size the field membrane coating itself?

No — this only estimates the seam reinforcement/flashing material at details. The liquid-applied field coating volume for the balcony surface area should be calculated separately.