How to Calculate How Much Material You Need for Your Lean-To

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How to Calculate How Much Material You Need for Your Lean-To

Choosing the right material for your lean-to roof can be a task in itself, but calculating how much material you’ll need is another level of complexity. In this guide we’re going to talk you through the process of calculating the materials you need for your lean-to roof, to make your next roofing project that little bit easier.

For this example, we're going to use a standard lean-to with a width of 6.9m, a slope length of 3.5m, and a pitch of 15°. We're going to use the Britmet Slate 2000 as our chosen material.

Table of Contents

Bird's Eye View of Lean-To Roof

Measurements You’ll Need

First things first, you will need a take a certain number of measurements for the following calculations to work. These include:

  • Slope Length (Line A)
  • Roof’s Width (Line B)
  • Roof Pitch
  • Rafter Length (or Slope Factor, more info in our rafter length calculator guide)
  • Batten Gauge
  • Cover Width of your Roofing Material
  • Cover Width of your Apron Flashing
  • Cover Width of your Barges
  • Cover Width of Your Ridge Vent

The information around materials above is easy to find on our website, but your roof will need to be measured by yourself or a qualified professional. Learn how to find a good roofer. If you are taking these yourself, it is important to follow all the recommended safety guidelines when working at height and ideally have a spare set of hands on site to help out.

Please bear in mind that the following calculations are only suitable for "standard" lean-to roofs, which are attached onto an existing wall at the highest point of the roof.

Calculating How Much Material You Need



Roof’s Width


Slope Length




Rafter Length (Slope Factor)


Batten Gauge

369mm (0.369m)

Cover Width of Tile

1250mm (1.25m)

Cover Width of Apron Flashing


Cover Width of Barges


Cover Width of Ridge Vent


Step 1: Calculating How Many Tiles are Needed for One Vertical Run

The first step is to multiply your roof’s slope length by the rafter factor:

Next, take this result and divide by your batten gauge:

This tell us how many tiles will be needed in the run, so round it just needs to be rounded up to the nearest whole, meaning 10 tiles.

Step 2: Calculating How Many Tiles are Needed for One Horizontal Run

A slightly simply process this time around, you just need to divide your roofs width by the cover width of your chosen material:

Again, round this up to the nearest whole which in this case makes, 6 tiles.

Step 3: Calculate How Many Tiles are Needed in Total

With are width and length determine simply multiply the results for an estimate of the total number needed to cover the whole area:

This makes 60 tiles in total.

Step 4: Determine How Much Flashing Is Needed

With the number of tiles worked out we will need to move on the other materials you will need to fit onto your lean to. For flashing simply divined the width of the roof by the cover width of the flashing:

Rounded up this makes for 7 Apron Flashings.

Step 5: Determine How Many Barges You Need

Similar to the above, only with the slope length instead of the roof width, divide your length by the cover width of the barges:

Not only do you just rounded up this time, but you also double that figure to account for both the left- and right-hand sides of your roof. This means you would need 4 left hand barges and 4 right hand.

Step 6: Determine How Many Vents You Need

If you’re lean to needs to be ventilated you can get a rough idea of how many you may need by again dividing the roofs width, this time by the cover width of the chosen vent:

This would make for 7 ridge vents, 7 eaves vents and 7 eave felt carriers.

Step 7: Roof Membrane & Fixings

Unfortunately there is no set formula to determine how many fixings you may need, however to give you an rough idea based on this example, 60 Britmet Slate 2000’s would need: 1x 50x1m membrane, 2kg nails and 1 touch up kit.

Step 8: Final Results

Based upon the above calculations, you will need the following materials:



Roof Tiles


Apron Flashings


LH Barges


RH Barges


1m Ridge Vent


1m Eave Vent


1m Felt Carrier


50m Membrane


2kg Nails


Touch-Up Kit


Calculating Material Amounts for Non-Standard Lean-Tos

The above calculations have only been designed to provide an estimate for materials needed for standard lean-to designs. This means a Lean-To attached to one existing wall along the top width of the roof. If your lean-to looks like the below, you can follow the steps we’ve detailed above.

lean to material plan


If, however, your lean-to is differently designed, then the calculations may differ slightly. But don’t worry – we’ve listed some of the most common variations of lean-to design below, along with what adjustments you’ll need to make to your calculations.

Attached to Two Existing Walls (Back and Left)

Lean-to attached back and left

If your lean to is attached to two walls like the above, perform the calculations as normal but replace whatever number of left-hand barges you need with the same number of left-hand slide walls.

Attached to Two Existing Walls (Back and Right)

Lean-to roof attached back and right

As you would expect, what you have to do in this case is the same as above but the inverse. Replace your right-hand barges with right hand side walls.

Attached to Three Existing Walls

Lean-to attached to three existing walls

With lean-tos connected to three existing walls simply replace all calculated bargeboards with side walls or replace the cover width of bargeboards with the cover width of the side walls in the initial formula.

Buying Lean-To Roofing Materials

Once you’re ready, take a look at our fantastic selection of roofing materials perfect for a lean-to. These include polycarbonate roofing sheets, lightweight synthetic slate roof tiles, sturdy metal roofing sheets and more!

If you need any further advice, don’t hesitate to contact us. Our customer service team is here to help with whichever query you may have. Call them on 01295 565 565, email [email protected], or use the handy live chat below.

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