Can You Mix Old and New Slates on One Roof?

Does mixing cause any technical problem

Not if the slates match where it counts. A slate roof depends on each course lying flat on the one below and overlapping correctly. If a new slate is thicker than its neighbours, it sits proud and can be lifted by wind, and the slates above it ride up over its edge and leave a gap. If it is thinner, it can sag or sit in a hollow where water collects. A different size changes the gauge, so the courses no longer line up, and a gap shows on the lap. Matching the length and width of the existing slates, and choosing a thickness within a small tolerance, avoids all of this.

The fixings must also be compatible. Old slates are often nailed with iron or galvanised nails that have corroded, and the new ones should go on with copper, stainless or aluminium nails or hooks. Mixing metals in contact can cause corrosion, so the repair uses fixings that will last. Our guide to slate hooks versus nails explains the options.

The colour and weathering problem

Natural slate weathers. Slates that have spent decades outdoors lose their fresh sheen, pick up lichen and soften in tone. A new slate of the same type looks cleaner and, depending on the quarry, sometimes a different shade. A scatter of bright slates stands out on an old roof like a patch on a coat. Over a few years, it fades, but it may never match exactly if the stone is from a different source. Welsh, Cornish, Spanish and Brazilian slates all differ in colour and texture, and a mix of origins looks uneven. See Welsh versus Spanish slate for the typical differences.

The practical fixes are reclaimed slates, which are already weathered, and placing new ones among the old in the least visible spots, such as the back slope or the lower edge. Our page on reclaimed slate explains where they come from and what to check.

Repairing versus re-covering a slope

If only a few slates have gone, mixing is the obvious answer. If a large share of a slope has been replaced with new slate over the years, it can look like a patchwork. A good approach is to put all the new slate on one slope, keeping the front elevation consistent, or to re-cover the worst slope fully, using the best of the stripped slates on the visible side and new ones on the hidden side. A careful roofer will stack the stripped slates, sort them by size and condition, and use the sound ones again. That is the same method used in a full re-slate where old material is reused.

Do not mix slate with a different kind of covering to fill in holes. Concrete tile or fibre-cement among natural slate looks poor and sheds water differently. The cost of the right slate is small compared with the cost of a bad patch.

Rules, listed buildings and conservation areas

On a listed building or in a conservation area, like-for-like repairs are usually accepted, but a visible change in material or appearance may need consent. Mixing a modern man-made slate into an old natural roof is the kind of thing a conservation officer will notice. Ask the council's conservation officer about it before you buy anything that differs from the roof you have. Our guide to slate roofs on listed homes sets out the usual expectations.

Sorting and sizing stripped slates

If a roof is being stripped and part of it reused, sorting is a skill in itself. The roofer sorts slates into piles by size, thickness and condition, rejecting cracked or delaminating ones. The best go back on the front slope, and those in fair condition go on the back or are kept as spares. Sound nail holes are checked, and slates with holes too near the edge are discarded or re-holed. Done carefully, a mixed roof is stable and attractive, with new slates distributed so no single patch draws the eye.

A final check is done from the ground once the repair is finished, standing back far enough to see the whole slope. Any slate that catches the light differently, or any course that no longer runs straight, is easy to adjust while the access is still in place and impossible to fix cheaply once it is taken down.

We match slates by hand, measuring a sample, noting the thickness and checking colour against the roof in daylight. The survey is free, and the pictures show the area to be repaired and the slates we propose to use. Your price is fixed and written down. More on this type of work is in the slate roofing section of the site.