ᐅ Flat roof leaking: repair or build a pitched roof structure over it?

The flat roof on the wooden decking of a 55-year-old single-family house is leaking. Rainwater drains through seepage pipes inside the house. I suspect the cause of the leak is at one of these seepage pipes, as discoloration can be seen on the ceiling of the upper floor about 1.5m (5 feet) away from it. This issue was already addressed by professionals eight years ago, with stones removed locally and a membrane installed.

To avoid another limited repair, I would prefer to take decisive action and add a low pitched roof on top, similar to the neighboring house. This would direct rainwater to the outside, which is an advantage, as the downpipes can also show leaks at the basement access covers.

The carpenter has already visited and estimated the costs at €40,000 plus VAT (house dimensions 7 x 13m (23 x 43 feet)). The earliest possible execution date he can offer is August. A meeting at the local building authority is planned soon to discuss the project roughly, for example regarding a gable roof, shed roof, roof overhang, etc. Afterwards, a concrete quote could be prepared and an architect (recommended by the carpenter) involved. The building authority will likely prefer a gable roof, since that is visible on the neighboring house.

I am concerned about the house’s structural stability since a gable roof would add load to the center of the building, while the main load-bearing wall along the longitudinal axis is located about 1.5m (5 feet) off-center. This will need to be calculated by a structural engineer and likely cannot be ideal.

Another issue is that after adding a roof truss, the current flat roof assembly will probably become difficult to access if repairs are needed in the future.

Am I heading in the right direction? Is adding a roof truss really a worry-free solution, as the carpenter claims?

Perhaps I should just carry out a comprehensive renovation of the flat roof, probably at about half the cost of the carpenter’s proposal, but then it will likely leak again at some point.

Which solution should be aimed for? What else should I keep in mind?

Firstly, 40,000 EUR seems quite high to me for a relatively small roof. Besides that, I don’t see why reroofing shouldn’t provide a long-term solution—at, in my expectation, a significantly lower price. Additionally, a pitched roof would of course completely change the appearance of the house, which is something you need to want in the first place.

A flat roof renovation — even if it ultimately involves converting it into a pitched roof — is, in my opinion, better discussed with the roofer rather than the carpenter. How much the roofer will need to involve a carpenter will become clear during the process. I don’t see a traditional timber roof frame as necessary here; trusses would be more suitable. So neither purlins nor rafters, and also lighter with free-span construction.

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11ant schrieb:

A flat roof renovation - even if it ultimately involves converting it into a pitched roof - is, in my opinion, better discussed with a roofer rather than a carpenter. How much the roofer will need to involve a carpenter will become clear as the process goes. I don’t see a traditional rafter roof structure as necessary here; trusses are more suitable. So, neither purlins nor rafters, and also lighter and self-supporting.

Good idea, and since trying is better than theorizing, I will contact a roofer!

Admittedly, I haven’t fully understood the concept of “trusses” yet, but a roofer will surely know how to work with them.

Are there any typical disadvantages associated with a solution using trusses?

Yes, with the poor to almost nonexistent usability of the attic, but that is not an option for you anyway. 🙂

A carpenter will certainly be familiar with terms like roof trusses / nail plate trusses, etc. But a cutting system also needs to be kept busy 😉 The thin battens for nail plate trusses hardly seem worthwhile.
You can also find something similar, for example in riding arenas, where the roof is open from below or has no ceiling 😎.

The roofer comes after the carpenter has done their work and makes the roof watertight...

A flat roof nowadays usually lasts well over 20 years; if you make it a green roof with extensive planting, you can add another 20 years on top.

A tiled roof (concrete roof tiles) can also require replacement after 30–40 years.
Classic clay roof tiles are said to have a lifespan of up to 100 years.

I don’t see a $20,000 budget for flat roof renovation here, but I have often underestimated such prices before.