Are concrete tiles too heavy for an old Stroud roof?

Two official sources set the threshold and word it a little differently. Under the Planning Portal's wording, approval is triggered once the new covering's weight is up by 15 per cent or more. Approved Document A says more than 15%, a hair looser, so we treat the figure as a prompt to check, not a pass mark. Approved Document A, the structural part of the regulations, puts the line at an increase of more than 15%, and adds that it may matter whether the covering being removed is the original one.

We read the rule as around 15% heavier or more and confirm the position with building control, since either wording leads to the same step: someone qualified looks at the roof structure before the first tile goes on. If the timbers cannot take the added load, whether because members are overstressed or because deflection has set in, Approved Document A expects strengthening to follow. Where the job is notifiable, we explain the steps and file the notice for you.

Weight per square metre differs between makers and ranges, so we compare the data sheets for the old covering and the new rather than guess. A thin natural slate can weigh noticeably less than a concrete tile, and that is the commonest route to crossing the threshold. Stone is a different case: Cotswold District Council's 2000 guidance puts stone slate at about 1,000 kg per 10 m² and calls it very heavy. That means swapping stone for concrete may lighten the load, though the look changes completely and the consent question is a serious one.

Swapping the other way, from clay plain tile to concrete, can add weight even though both are called tiles. The Planning Portal says the pitched-roof checks apply wherever the replacement tile is significantly heavier or lighter than what it replaces. Lighter coverings raise their own questions about fixing and wind uplift, so a change either way should be checked, not assumed.

For a replacement that is much heavier or lighter, the Planning Portal suggests a structural engineer or surveyor looks first. In practice we ask for an engineer where the old roof shows any sign of stress, where the new covering is clearly heavier, or where earlier alterations have removed or cut timbers. Signs include a dipping ridge, bowed rafters and cracked ceilings below. Photographs from the survey, with the loft timbers where we can reach them, go to the engineer with the brief.

The engineer looks at the rafters, purlins and wall plates, and sometimes the walls that carry them. We can arrange loft access and a ladder for the visit. If you already have an engineer or a surveyor you trust, we work with their report and price from it.

The result is a short written opinion: the roof can take the load as it is, needs strengthening, or should take a lighter covering. We then include any strengthening in the quote. Our answer on roof structure defects in older houses explains the faults we look for, and a sagging roof ridge is the symptom that most often prompts the question.

Weight is one question and appearance is another. Changing from slate or stone to concrete tile alters how a house looks. Re-roofing like for like, in a way that does not materially affect external appearance, is not usually development. Putting concrete tile where slate was normally does alter appearance. Ask Stroud District Council's planning team early, particularly if the house is in one of the district's 41 conservation areas or is listed. A listed building needs consent for changes that affect its character.

We begin from what is on the roof already, which means a new stone roof for an old one and slate for slate. If you have a good reason to change, such as a failed slate roof and a tight budget, we explain the consequences in the survey report and put each option in the written quote.

A heavier tile affects more than the rafters. Walls, wall plates and lintels over openings all carry load down, and an old wall with soft mortar may show cracking if the load rises. The engineer looks at those too when there is any sign of movement.

Weight shows up at ground level as well. Scaffold, skips and pallets of tiles going up a narrow Stroud lane all need planning, and we price the access in the quote rather than leaving it for the day.

Battens come next. Batten size and spacing are set by the tile's gauge, and old battens are often too light or too closely spaced for a new product. Fixings change too, and BS 5534 and the tile maker's data sheet set how each tile is held. Our answer on rotten roof battens shows what a tired batten looks like.

Underlay and ventilation come into play as well. A new covering on an old roof is a chance to replace a failed underlay and to check the loft for condensation. You hold a 10-year workmanship guarantee on paper for the finished job, and the garden is left tidy. You can read more about the service on our pitched roofing page.