Solar Panels on 1930s Semi-Detached Homes

The standard suburban semi

More of these were built in the 1930s than almost any other house type, and they turn up across every town we work in. Cavity walls, a cut timber roof at a conventional pitch, clay tile originally, a bay to the front and a chimney stack on the party wall. The plan is repetitive enough that we can usually anticipate what a survey will find — which is exactly why the exceptions matter.

Roof geometry is the main constraint

Two variants dominate. The gabled semi presents a straightforward rectangular slope front and back, and is the easier of the two to design for. The hipped semi — the more common of the pair — has the roof returning down over the side elevation, which cuts the corners off both main slopes and shortens the ridge.

That geometry costs panels. An array has to keep clear of the hip lines, and the usable shape narrows as it rises, so the top row is shorter than the bottom. On a hipped semi we frequently end up recommending two smaller arrays on adjacent slopes rather than forcing one onto a single face.

The bay window roof and the porch canopy are not usable. They are too small, too shallow and structurally not intended to carry anything.

The party wall stack

Almost every semi of this age has a stack on or near the party wall, and often a second on the outer gable. The party wall stack sits at the top of the slope and its shadow sweeps across the roof through part of the day, landing on the middle of the array rather than the edge.

We assess this across the year at survey. Where a handful of panels are affected for a couple of hours, optimisers or microinverters contain the loss. Where the stack sterilises a wide band, the sensible design leaves that band empty and accepts a smaller array.

Covering and structure

Original clay tiles that have survived ninety years are usually in reasonable condition, though nibs break and fixings corrode. Many of these roofs were re-covered in concrete tile in the 1960s or 70s, and that generation of concrete tile is now itself old enough to be brittle and crazed. Sarking felt is frequently absent or perished.

Underneath is a cut roof: rafters bearing on a wall plate and supported mid-span by purlins, with struts down to internal walls. Rafter centres vary, sections vary, and any past alteration — a loft conversion, a removed chimney breast, a new dormer — has changed the load path. Rail mounting handles the irregular centres; the alterations need looking at directly.

Electrical implications and design

Consumer units are usually a later replacement and often adequate, but frequently full. Meter positions vary between an external box and an under-stairs cupboard, and that determines the cable route from the loft. Where a battery and an EV charger are wanted alongside the array, the main fuse rating becomes the question rather than the board.

An array over 16A per phase, roughly 3.68kW single phase, is a G99 application requiring approval before installation rather than a G98 notification afterwards. On a hipped semi the usable area often keeps a single-slope system below that; a two-slope design frequently pushes above it, and that needs planning into the programme.

Access and limitations

Side access is the advantage a semi has over a terrace. There is usually room to stand a scaffold down one flank and across the rear, and material access is straightforward.

The limitations are geometry and shade. A hipped semi with a party wall stack and mature trees in a south-facing garden will support a modest system. Worth having, and worth sizing honestly before anyone gets excited about the roof area.