How Many Roofing Sheets Do I Need? Measuring and Ordering Metal Roof Sheets

Ordering metal roofing sheets is simple once you know three numbers: how wide the roof is, how long each slope is, and how the sheets overlap. Get those right and you will order the correct number of sheets, cut to the right length, with the flashings and fixings to finish the job properly. This guide walks you through the measuring, shows two worked examples, and lists everything else you need on the order.

Short on time? Our roof sheet calculator works out your sheets, flashings and fillers from two measurements and adds them all to your basket in one click.

At Mighty Structural Products we have manufactured metal roofing sheets at our factory in Walsall since 1999, and most ordering problems we see come down to one of the steps below being skipped.

The key fact: every sheet covers 1 metre

Every sheet profile we make gives a 1 metre cover width once it is overlapped with the next sheet. That applies to box profile (32/1000), corrugated, tile effect and pan tile effect sheets.

The physical sheet is slightly wider than 1 metre, because part of it sits under or over its neighbour. Just work in metres of cover: a roof that is 4 metres wide needs 4 sheets across.

Step 1: Measure the width of the roof

Measure along the eaves (the lower edge, where rainwater runs off) from one side of the roof to the other. Then add any overhang you want at each gable end or verge.

• Decide how far you want the sheets to overhang each end, and add that to the width.
• Divide the total width by 1 metre and always round up to the next whole sheet.

For example, a roof 3.3 metres wide needs 4 sheets, not 3. You can trim the last sheet down if needed, but you cannot stretch a sheet that is too short.

Step 2: Measure the slope length

The slope length is the distance from the ridge (or the top edge of a lean-to) down to the eaves, measured along the roof surface, not horizontally. This is the length each sheet needs to be cut to.

• Measure along the top of a rafter from the highest point to the lowest point.
• Add the overhang you want at the eaves so rainwater drips clear of the wall or into the gutter.
• If there is an apex, measure each side separately. They are often the same, but not always.

If you cannot get onto the roof safely, you can work the slope length out from the horizontal distance and the rise using Pythagoras (slope = the square root of horizontal squared plus rise squared). If in doubt, send us your measurements through the quote form and we will check them.

We cut sheets to any length from 0.2 metres, in millimetre increments, when you order online. Order the exact length you need.

Step 3: Check whether you need end laps

If your slope is longer than the maximum single sheet length, you will need two or more sheets per run, overlapped at the ends. The maximum lengths you can order online are:

Where sheets are end lapped, the upper sheet sits over the lower sheet so water runs over the joint, not into it. The end lap needs to land on a purlin so both sheets can be fixed through it. The right lap length depends on your roof pitch, so check it in our fitting guide or call the team on 01922 649795 before you cut your order.

To work out lengths for a lapped run, add the end lap to the total slope length, then split it into sheets that fall within the maximum length and finish on a purlin. If your slope is long and you want to avoid laps altogether, box profile is the profile that goes longest in one piece.

Worked example 1: single garage lean-to

You are re-roofing a single garage with a lean-to (mono-pitch) roof. It slopes from the back wall down to the front.

• Width along the eaves: 2.8 metres
• Overhang wanted at each side: 100mm, so add 200mm
• Total width: 3.0 metres
• Slope length from top edge to eaves along the rafter: 5.3 metres
• Overhang wanted at the eaves: 100mm
• Total sheet length: 5.4 metres

Sheets across: 3.0 metres divided by 1 metre cover = 3 sheets. If the width had come to 3.05 metres you would round up to 4.

Sheet length: 5.4 metres. That is within the 12 metre maximum for box profile and the 6 metre maximum for pan tile effect, so either can be a single sheet. Corrugated and tile effect go to 5 metres online, so in those profiles you would need an end lapped run, or you could call us to discuss.

Order: 3 box profile sheets at 5,400mm.

Flashings for a lean-to: a wall abutment flashing (or similar top flashing) where the roof meets the back wall, barge flashings down both sides, and an eaves detail at the front. The top edge of a lean-to that butts a wall does not need a ridge.

Worked example 2: apex shed or small barn

You are roofing a timber-framed apex building with two equal slopes.

• Length of the building along the eaves: 7.6 metres
• Overhang at each gable: 150mm, so add 300mm
• Total width to cover (per slope): 7.9 metres
• Slope length from ridge to eaves, per side: 3.6 metres
• Eaves overhang: 150mm
• Sheet length per side: 3.75 metres

Sheets per slope: 7.9 metres divided by 1 metre = 7.9, rounded up to 8 sheets.

Total sheets: 8 sheets per side multiplied by 2 slopes = 16 sheets.

Sheet length: 3,750mm. This is under the maximum length for every profile, so no end laps are needed whichever you choose.

Order: 16 sheets at 3,750mm, plus ridge flashing to run the 7.9 metre length of the apex, and barge flashings for all four gable edges (two per gable end, each running the slope length).

If one side of the building faces a different direction or one slope is longer, measure and order each side separately.

Step 4: Add the flashings

Flashings seal the edges and joints of the roof and make the job look finished. Browse our flashings to match the colour and finish of your sheets. The main ones are:

• Ridge flashing: caps the top where two slopes meet on an apex roof. Measure the full length of the ridge including gable overhangs.
• Barge flashing (verge): covers the exposed sheet edge at each gable end. Measure each slope length. An apex roof has four barge edges; a lean-to has two.
• Eaves flashing or filler detail: finishes the lower edge. Measure the full eaves length on each slope.
• Wall abutment flashing: for lean-to roofs that meet a wall at the top or side.

Flashings come in set lengths, so divide each run by the flashing length and round up, allowing for the overlap where one piece joins the next. Check the product page for the length supplied.

Step 5: Work out your fixings

Sheets are fixed through to the purlins or battens with roofing screws fitted with sealing washers, and side laps are stitched together along their length. You will find suitable screws in our fixings range.

The number of screws depends on your profile, your purlin spacing and how exposed the site is. Follow the fixing pattern in our fitting guide, and buy a few spares.

If you are unsure about purlin spacing for your building, do not guess. Call the team on 01922 649795 and we will talk it through.

Step 6: Foam fillers and accessories

Box profile, corrugated and tile effect sheets have a shaped profile, which leaves gaps where the sheet meets a flat surface such as the eaves or the underside of a ridge. Profiled foam fillers close those gaps to keep out birds, insects, wind-driven rain and draughts.

• Fit eaves fillers under the sheets along the lower edge.
• Fit top fillers between the sheets and the ridge or abutment flashing.
• Make sure the filler matches your sheet profile. A box profile filler will not fit corrugated sheets.
• Count fillers by length along each eaves and ridge run, rounding up.

You will find fillers, sealant tape and other extras in our accessories. If you want natural light inside a garage, shed or barn, consider replacing one or more sheets with a matching skylight panel in the same profile.

Do not forget condensation

Metal sheets can collect condensation on the underside when warm, moist air inside the building meets the cold sheet. On an unheated garage, workshop, stable or storage building, that can mean drips onto whatever is below. Every sheet we make is available with an anti-condensation fleece backing, in every finish.

Ordering checklist

1. Width of each slope including verge overhangs, divided by 1 metre, rounded up.
2. Slope length of each side including eaves overhang, in millimetres.
3. End laps worked out if any slope is longer than the maximum sheet length for your profile.
4. Profile, finish (plastisol, polyester or galvanised) and thickness (0.5mm or 0.7mm).
5. Anti-condensation backing, yes or no.
6. Ridge, barge, eaves and abutment flashings measured by run.
7. Screws and stitching screws, checked against the fitting guide, plus spares.
8. Foam fillers matched to your profile.

Delivery, collection and payment

Delivery usually takes 7 to 10 working days (10 to 20 working days to Scotland). The delivery charge depends on your postcode and is shown at checkout. You can also collect free of charge from our factory at Unit C, Walstead Road, Walsall WS5 4PG. See our delivery page for details. You can pay by card, Apple Pay, Google Pay, Klarna, PayPal or bank transfer.

Ready to order?

Once you have your measurements, choose your profile: box profile, corrugated, tile effect or pan tile effect, then add your flashings, fixings and accessories. If your roof is larger or more complicated, send your measurements through our get a quote form or call us on 01922 649795 and we will help you put the order together.