Shingle Weight Per Square, and What Your Deck Must Carry

A shingle roof is a load, and most homeowners never see the number. Before the crew nails anything, the shingle weight per square and the deck underneath decide whether the new roof sits on sound framing. This page explains the figures a St. Louis homeowner can check on the wrapper, the product sheet and the permit file, and what to ask before the order goes in.

What a square is

Roofers measure and sell shingles by the square. One square is 100 square feet of roof surface, before waste is added. A roof with 2,000 square feet of surface is about 20 squares. Shingle weight is quoted per square, so the same number of squares of two different products can weigh noticeably different amounts. Keep that unit in mind when you read a quote, because the bundle count and the weight are both tied to it.

Laminated shingles are usually sold in three bundles per square, and three-tab shingles are commonly packaged the same way. The bundles look alike from the driveway, but the weight per square changes with the product line and the thickness of the laminate.

Typical weights, and why the range is wide

Published weights for asphalt shingles vary by manufacturer and product line. As a rough guide, three-tab shingles weigh less per square than laminated, or architectural, shingles, which are built with a thicker layered profile. Laminated products commonly fall somewhere in the range of about 230 to 300 pounds per square, and three-tab products run lighter than that. These are general ranges, not a spec for any one roof. The number that matters is the figure printed on the product sheet for the exact shingle being installed, and that sheet belongs in your file.

Weight also depends on what goes under and around the shingle. Underlayment, ice and water membrane at the eaves and valleys, and any ridge or hip product all add to the load. A roof that uses heavier underlayment or more membrane carries more weight than the shingle figure alone suggests.

What the deck is carrying now

Residential decking in St. Louis is usually plywood or oriented strand board, often 7/16 inch or 1/2 inch in houses built during the last several decades, and rafters or trusses carry it. Older houses in St. Louis County may have plank decking, and that changes the picture. Whatever the material, the framing was designed for a certain dead load, which is the permanent weight of the roof itself, plus the snow and wind loads the local code requires. The shingle layer is a meaningful share of that dead load.

A laminated square spread across a square foot works out to roughly 2 to 3 pounds per square foot. That sounds small, but it applies to every square foot of the roof, on a framing system that was never meant to carry an unlimited stack. Most sound residential framing has margin built in, and a single replacement with the old shingles removed does not usually strain it. The concern grows when the existing roof is already sagging, when layers have built up over decades, or when the rafters are undersized for the span.

Adding a layer changes the number

Weight comes up on many St. Louis roofs because of the overlay. Some owners ask to cover an old roof instead of tearing it off, to avoid the cost of removal. An overlay puts a second full layer of shingles on the deck, so the weight of the old roof is added to the new one. Building codes generally limit how many layers may remain and set conditions under which a tear-off is required, and those rules are applied through the local permit process. In St. Louis County, the municipal permit desk is the place to confirm what the building department will accept before a crew starts.

There is also a practical reason to be careful. Under a second layer, decking damage, soft spots and rot stay hidden, and the crew cannot fasten a sound new layer to a deck they cannot see. A roof that has already been overlaid once is a strong reason to ask for a full tear-off, because the weight, the deck inspection and the fastening all improve.

Signs the framing is already working hard

Before you order any shingle, look at the roof from the street and then from inside the attic. A dip or wave along a ridge or rafter line, cracking where a ceiling meets a wall, and doors that stick in the upper rooms can all point to movement in the framing. None of these means the roof cannot be replaced. They mean the new roof should be specified with the framing in view, and the crew should inspect the deck before the new shingles go on.

The attic check is the cheapest test. Look at the underside of the decking for dark stains, soft spots or fungal growth, and check the rafters or trusses for cracks at notches and bearing points. Photograph everything before the roof comes off, because those photos become part of the written record of what was found.

What to ask the roofer

A roofing proposal should name the product being installed and state its weight per square, or at least point to the manufacturer's sheet that gives it. Ask whether the existing layers will come off, and if the proposal calls for an overlay, ask which permit approval covers that choice. Ask what the crew will do when they find soft decking, and whether that work is in the price or is an allowance line priced on the day.

1. Which product is being installed, and what is its weight per square according to the manufacturer's sheet?
2. How many layers are on the roof now, and will all of them come off?
3. What decking material is under the old shingles, and what happens if it is soft?
4. Does the permit desk require a tear-off in this case, and has the proposal checked that?
5. Who inspects the deck before the new underlayment goes on, and will you get photos of it?

Keep the product sheet with the signed scope. If a later question comes up about the load, the fastening or the deck condition, that sheet and the photographs are the record of what was installed and why.

Where the office fits in

If the framing question is still unclear, call the Keys office at (314) 220-2333 before you sign. The crew can walk the roof and the attic with you and explain what they see. Any written scope should state the product, the number of squares, the layers to be removed and the decking plan, so the figures on your quote match the figures printed on the wrapper and the product sheet.