How to felt a flat or shed roof

Why felt a roof?

Roofing felt is a popular material due to the fact it’s easy to install and cost-effective, it is mainly used on outbuilding and shed roofs. Roofing felt is commonly in the form of bitumen felt shingles which are designed to cover and protect structures from external dangers such as wind and rain.

The advantage to using roofing felt is that it’s cost-effective, lightweight, simple to install, easy to maintain, and widely available. The typical lifespan of roofing felt is 10-20 years with proper maintenance and care, but can decrease if not looked after.

Equipment and preparation

In order to get started on felting your shed or flat roof, the first step you need to take is measuring the area of your roof, so you know how much felt you’ll need to cut:

For a flat roof, multiply length and width, you’ll need 2 layers of underlay felt and 1 layer of felt for a flat roof in order to keep it watertight.

For an apex roof, multiply the roof's length and multiply it by the slope height. You’ll need three equal sheets per side of the apex.

It’s good practice to measure the area twice so you can be certain how much felt you will need, you’ll also need to leave about 50-75mm on the bottom for an overhang for apex roofs.

Always read the manufacturer’s instructions for the felt, but the equipment you’ll need is as follows:

• A hammer
• Stanley knife
• 50mm brush
• Flat pry bar
• Galvanized clout nails
• Felt adhesive
• Tape measure
• Suitable lengths of roof felt
• Gloves
• Suitable adhesive
• Mineral cap sheet
• Underlay felt

You will also need to check the condition of the apex or flat roof, if it’s not in a good condition then this will need to be rectified before it is felted, otherwise, it will lead to future issues. You will also need to clear the roof of any existing bargeboards, nails, and existing felt, if the bargeboards and finials are in good condition then they can be reused.

Felting a shed roof

1.  Lay your first piece of felt

Lay your first piece of felt across the bottom of the slope, leaving the overhang mentioned above, then nail in the top edge with the galvanized clout nails, leaving 50mm gaps. Fold the edges of the felt and nail them into place using clout nails.

2.  Lay the second piece

Take your second cut out and lay it across the roof, ensuring there is an overlap on the first roll of felt, then nail the top of the felt into place. Once nailed, you can apply the felt adhesive on the joint between the first and second layer using the brush, nail the joint for extra security.

3.  Repeat this process

Repeat this process across the first and second sides of the roof have been completed, leaving room for the capping sheet.

4.  Add your capping sheet

Now it’s time to add your capping sheet, place it across the top of the roof, ensuring there’s an overlap of 75mm between the two sides of the roof, apply adhesive on the overlaps and nail it down at 50mm intervals at either side.

Felting a flat roof

1.  Lay the first underlay felt layer

Lay down your first layer of underlay felt on the deck by starting at the lowest edge of the roof and rolling it out at right angles to the direction of the fall of the roof and make sure it’s aligned. Once aligned, nail using clout nails from the centre and work outwards leaving 50mm gaps.

2.  Apply adhesive

Apply adhesive onto the first layer, this should be an even coat across otherwise this will cause issues with future layers.

3.  Lay your second layer

Lay the second layer of underlay onto the adhesive and ensure any joints are overlapped and it’s flat against the first layer, then nail down using clout nails leaving the 50mm gaps as before.

4.  Apply more adhesive

Spread out more adhesive on the second layer, making sure there is an even coat same as the first layer.

5.  Lay your final layer of felt

Now it’s time to lay the final layer of felt onto the flat roof, lay it as you have the other layers before and nail with 50mm gaps. Once laid you can smooth this out by walking on it, ensuring it’s tight.

6.  Add a final layer of adhesive

Adding a final layer of adhesive will ensure that maximum weatherproofing is in place and no water will pass through.