Replacing the felt and battens on a gable roof is one of the most satisfying jobs a confident DIYer can take on. The roof is simple: two sloping planes meeting at a ridge, with vertical gable ends closing each side. There are no valleys, no hips, no awkward junctions to negotiate. What catches most people out is not the fixing — it is the ordering. Guess the quantities and you either run short halfway up a slope or end up with half a roll of felt festering in the garage for a decade.

This guide walks through measuring a straightforward gable roof accurately, converting those measurements into felt and batten quantities, and knowing what to add for waste. It assumes a single-storey or two-storey house with a conventional pitched roof, accessible safely from a scaffold tower or proper edge protection. If your roof has dormers, valleys or a complex shape, treat this as a starting point and speak to a roofing merchant before ordering.

Before You Climb Up: What You Need

Get the kit together first so you are not shuttling up and down a ladder for a pencil. You will need:

• A 30-metre tape measure — a short 5m tape is useless on a roof slope
• A notebook and pencil, or a phone note you can read in daylight
• A chalk line or marker pen for reference marks
• A spirit level or plumb bob for checking verticals
• Sturdy footwear, gloves, and a safe means of access

Work from a scaffold tower or a properly secured ladder with a stand-off, and never walk on a roof you have not assessed. Old battens and felt can be brittle, and a slipped foot on a wet slate is not worth the risk. If heights are not your thing, this is a job to hand over.

Measure the Roof Slope: Length and Rake

Start with the slope itself. The rake is the sloping edge running from eaves to ridge along the gable end — effectively the length of each rafter plus any overhang at the eaves and gable. Measure it on both sides, because old buildings are rarely symmetrical.

Run your tape from the very bottom edge of the roof covering at the eaves up to the outer face of the bargeboard or gable trim at the ridge. Write that figure down in millimetres. If the roof has a fascia and the felt needs to drape into the gutter, measure to the point where the felt will actually terminate, not where the existing felt happens to end.

Finding the Overall Width

The width of each slope is the distance from the eaves to the ridge measured along the slope — but what you really need for felt is the gable-to-gable dimension. Measure the full width of the building at eaves level, including any gable overhang, then add roughly 50mm per side for the felt to wrap over the gable edge if you are finishing with a bargeboard that will cover it.

A quick sanity check: on a simple gable, both slopes should have the same rake length. If they differ by more than a few centimetres, something is out — re-measure before you commit to an order.

Calculating Felt Quantities

Roofing felt and breathable membranes are sold in rolls with a stated coverage, typically around 1m wide and either 10m, 15m or 50m long. Coverage on the roll is stated for the flat area, but you lose material to laps.

Felt is laid in horizontal courses running across the slope, working up from the eaves. Each course overlaps the one below by at least 100mm, and sometimes more depending on the product and pitch. On a low-pitch roof you may need a 150mm lap. End laps where two rolls meet should also be 100mm minimum, and staggered so they do not line up with the batten fixings.

To work out the number of courses:

1. Take your rake length in millimetres and divide by the effective course width — the roll width minus the lap. For a 1m roll with a 100mm lap, that is 900mm.
2. Round up to the next whole number. That is how many courses you need up the slope.
3. Multiply the number of courses by the gable-to-gable width (plus edge allowance) to get total linear metres.
4. Divide by the roll length, then round up and add one extra roll for waste and cuts.

As a rough example: a slope 4m wide and a rake of 4.5m, using 1m x 15m rolls. With a 900mm effective course, you need five courses. Five courses x 4m = 20 linear metres, plus laps at roll joints and edge allowance, call it 24m. That is two rolls of 15m, with a bit spare. Always round up rather than down — you cannot stretch felt.

A Note on Breathable Membranes

If you are upgrading from traditional bitumen felt to a breathable membrane, the same measuring method applies, but laps are often tighter — many products specify 100mm horizontal and 150mm vertical laps, and some allow less. Read the manufacturer's data sheet rather than assuming. On a cold roof, a breathable membrane needs a clear ventilation path at the eaves and ridge, so check that your existing ventilation details suit it before ordering.

Working Out Batten Quantities

Battens run horizontally across the slope, fixed to the rafters, and the felt sits over or under them depending on your build-up. Their spacing is set by the covering — for concrete tiles it is typically around 345mm or 385mm gauge, and for slates it depends on slate size and headlap. Do not guess this: check the tile or slate manufacturer's fixing guide for the exact gauge, or match the existing spacing if you are re-covering with the same product.

To calculate batten length:

• Divide the rake length by the batten gauge to get the number of batten rows per slope
• Add one for the eaves batten
• Multiply by the gable-to-gable width for each slope
• Add 5–10% for cutting and defects, then round up to full lengths

Battens come in standard lengths, commonly 3m or 4.8m. Work out how your rows divide into those lengths to keep waste down — sometimes buying 4.8m lengths and cutting them sensibly saves a surprising amount. Use graded, treated roofing battens that meet the relevant British Standard; cheap ungraded timber is a false economy on a roof.

Add fixings to your order too: galvanised batten nails or screws long enough to penetrate the rafter properly, and clout nails for the felt. As a general rule, allow a couple of nails per batten fixing point plus a generous margin, and buy by weight rather than counting individually.

Ordering Tips That Save Time and Money

Order everything in one go where you can. Delivery charges and return trips are the hidden cost in a roofing job, and merchants rarely stock the exact product you want in the exact quantity on the day you need it.

Check the pitch of your roof before choosing felt. Very low pitches need a product rated for them, and standard felt will not perform. If you are unsure, take a photo of the roof and your measurements to the merchant — most will happily help you match a system to your pitch and covering.

Finally, store the materials flat and covered before the job starts. Felt rolls left standing on end can deform, and battens left in the rain will be heavier and harder to handle. Stack them on bearers off the ground, and keep the felt wrapped until you are ready to lay it.

A Sensible Last Check

Before you pick up the phone to order, do one pass back over your figures. Check that both slopes give the same rake length, that your lap allowance matches the product data sheet, and that your batten gauge matches the covering you are using. Write the numbers out clearly with units in millimetres so nothing gets confused between metres and millimetres at the counter.

Then add a small buffer — one extra roll of felt and a couple of spare battens will sit harmlessly in the garage, whereas a missing 3m length halfway through a weekend will not. Measure twice, order once, and the fitting itself becomes the easy part.

Photo: Christina & Peter / Pexels