Betws-y-Coed · Eryri · Est. 2015
Welsh slate, pole-lathe ash and iron-glazed stoneware, made in Eryri
We are a makers’ cooperative in the old village school at Betws-y-Coed, working Penrhyn and Ffestiniog slate, coppiced ash and local mineral glazes by hand.
BS 5534 slate headlap and batten gauge calculator
Choose a traditional Welsh slate, your rafter pitch and your exposure. You get the minimum headlap, batten gauge, slate count and nail length for a double-lap roof.
Countess 20"×10" at 30°, Moderate exposure: batten at 216 mm gauge with 75 mm headlap.
Below 25° in Severe Eryri Upland exposure sits outside ordinary double-lap practice. Take specialist advice before specifying.
- Minimum headlap
- 75 mm
- Batten gauge
- 216 mm
- Slates per m²
- 18.2
- Copper nail length
- 30 mm
- Nails per m²
- 37
Indicative values for planning. Check the final design against BS 5534, your slate supplier’s data and the wind-uplift loads for your site.
How a Welsh slate roof keeps the rain out
A slate roof sheds water by overlap rather than by sealing. In a double-lap roof every point is covered by at least two thicknesses of slate. The headlap is the distance a slate’s head is covered by the slate two courses above it. A steep pitch drains quickly and needs less headlap. A shallow pitch in driving Atlantic rain needs more.
The batten gauge follows from the headlap: slate length minus headlap, divided by two. That is why a Countess 20"×10" slate at 30° is battened at 216 mm, not at half its length.
Exposure matters as much as pitch. A sheltered street in the Conwy valley and a bare slope facing the Atlantic can sit twenty miles apart and behave like different countries. Wind drives rain up the slope and under the tails of the slates, so we add twenty mm of headlap in the Severe Eryri Upland setting. Try both and compare.
| Rafter pitch | Headlap, Moderate (mm) | Gauge, Moderate (mm) | Headlap, Severe (mm) | Gauge, Severe (mm) |
|---|---|---|---|---|
| 20° | 100 | 204 | 120 | 194 |
| 25° | 85 | 211 | 105 | 201 |
| 30° | 75 | 216 | 95 | 206 |
| 35° | 70 | 219 | 90 | 209 |
| 40° | 65 | 221 | 85 | 211 |
| 45° | 60 | 224 | 80 | 214 |
Four disciplines, one yard
Slate masonry
Hand-riven Penrhyn heather-blue and Ffestiniog blue-grey for roofs, hearths and cills.
Read the monographStick chairs
Steam-bent ash crests, drawknifed legs and wedged through-tenons from coppiced Eryri woodland.
Read the monographMineral glazes
Celadon and tenmoku built from slate dust, wood ash and river-bed iron.
Read the monographVisit and commission
Find the Hen Ysgol workshops, check lead times and start a commission.
Open the guideNotes from Bryn and Lowri
A good slate tells you where it wants to part. Most of my work is learning to listen, and then hitting it once, in the right place.
Everything in my glaze bucket came out of this valley except the clay. I like that a pot can taste of where it was made.
Beyond our own benches we keep a hand-written Eryri Maker Directory of blacksmiths, weavers, stonewallers and basket-makers we would trust with our own jobs. Ask us for an introduction.