Be Your Own Surveyor – Flat Roofs.

Be Your Own Surveyor – Flat Roofs.

Last week I introduced a new blog mini-series about what to look for when viewing property to buy. For simplicity I’ll start at the top and work down. I did pitched roofs last week, I’ll look at flat roofs this week. In subsequent weeks I’ll deal with chimneys and parapets, gutters and drains, electrics, plumbing and central heating, windows and doors, kitchens and bathrooms, walls, boundaries inc fencing and hedging, floors, damp, subsidence and foundations, and anything else you want if you just ask. There is a facility to reply to blogs although it isn’t obvious at first glance – as yet you have to click on the blog titles to see replies. I’m happy to answer questions or deal with specific issues. When I start to run low on material I might collate it all and publish as an inexpensive ebook.

Flat roofs come in two main types. Plywood/ felt flat roofs are most common in modern (from about the 1960s) houses, concrete before that. Some modern buildings can also have concrete roofs.

Plywood/ felt flat roofs are nasty, cheap and temporary. Fundamentally the materials are the same as used for garden sheds, which says it all really. They are best replaced ASAP with something more reliable and permanent if you intend to keep the building, so reckon on this in your costing. If it isn’t blistered or obviously cracked it may go for a bit longer, but they only have a lifespan of seven – ten years, particularly in a sunny location. I wrote extensively on replacing and maintaining these in my Avoiding Problems with Flat Roofs and Repairing Flat Roofs blogs. Some plywood flat roofs are covered in fibreglass (GRP) or even Roofkrete. These are much better and Roofkrete in particular should give you no problems.


Beam and block floor/roof

Concrete flat roofs come in two main sub-types, which are cast concrete and beam-and-block. Flat roofs in older houses (pre 1960s) are often cast. Many were built this way in cheaper areas during or just after the war when timber was in short supply. They can be troublesome, are unsightly, and typically devalue a house by at least 10% against nearby pitched-roof comparables. Unless the area has drastically improved since it was built, the expense of replacement with a pitched roof isn’t usually worthwhile, and is impractical anyway if it’s a semi or terrace. They are much better than plywood and felt, but still need regular maintenance of the bitumen painted layer and the silver paint layer above that, which reflects back harmful sunlight. Any cracks need investigating for suspicious damp stains on the ceilings below. Also note that these roofs usually have little or no insulation, and the only way of insulating them is to attach insulation sheets to the ceilings below. A lot more trouble than rolling fibreglass into a loft!

In more modern houses concrete roofs are usually of ‘beam and block’ construction (picture) where reinforced concrete beams are laid between the walls. The beams are rebated (notched) to accommodate concrete blocks lain in between each beam. Then the surface is screeded. These present similar insulation problems to cast roofs. Maintaining waterproofing should not be an issue in modern concrete flat roofs, as the waterproof layer should be built into the roof below the screed layer and need no maintenance, provided the concrete itself does not become cracked.

All flat roofs with rainwater downpipes that run through the building, as opposed to outside, present big potential problems in case of blockages.


These blogs are not intended to replace the services of a surveyor, engineer or other professional. They may however help save you the expense of a surveyor for a duff building that you won’t buy.


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