Key points
- A roof is layers: structural slab, falls for rain drainage, waterproofing, thermal insulation and a protective layer; their order determines how long the waterproofing lasts.
- In an inverted roof the XPS insulation sits above the membrane, protecting it from UV, temperature swings and mechanical damage.
- Roof loads, spans and any new opening are a structural engineer's decision; do not cut into an existing slab without their opinion.
- Widening or diagonal cracks in slabs and beams, or a sagging ceiling, call for an engineer.
The roof takes more sun and rain than any other part of the house, and is the most common source of leak and heat complaints. Knowing its layers helps you understand contractor quotes and ask the right questions.
1) Roof layers from the bottom up
- Structural slab: the reinforced concrete that carries the loads. See concrete basics and rebar.
- Screed to falls: slopes the roof towards the outlets so water does not pond (practical advice: test by spraying water before and after waterproofing).
- Waterproofing: bituminous, polymer or liquid membranes1. See roof waterproofing.
- Thermal insulation: insulation boards that reduce solar heat. See roof insulation.
- Protection and finish: tiles or gravel protecting what lies beneath.
2) Conventional or inverted roof?
In a conventional roof the membrane sits above the insulation, exposed to the sun. In an inverted roof the membrane goes on the slab, then XPS boards, then a protective layer, so the insulation protects the membrane from mechanical damage, extreme temperature swings and UV2. See also sun protection.
3) Cracks and warning signs
- Fine surface cracks are common in concrete and have many causes; see concrete cracks for the types.
- Call an engineer for: cracks that widen over time, diagonal cracks in beams, visible ceiling sag, or visible rusting steel with crumbling concrete. See rebar corrosion.
- Repeated roof leaks eventually reach the steel, so the fix is not cosmetic. See leak detection.
4) Access for maintenance
Practical advice:
- Provide a safe route onto the roof to check gutters, tanks and outdoor units.
- Do not fix shade posts or equipment bases through the waterproofing without a detail that keeps it continuous.
- Every roof penetration (pipes, cables, bases) is a weak point; ask for a waterproofing detail around it.
- In recent buildings, waterproofing of external roofs is one of the main basic covers of hidden defects insurance3.
5) Questions for the designer and contractor
- Which roof system (conventional or inverted), and which waterproofing and insulation materials?
- Where are the outlets and what are the falls?
- Will the waterproofing be flood-tested before the protective layer? See construction quality checks.
- What is the waterproofing warranty and its duration? See warranties.
- Does the design allow a future roof room or tank? (a structural decision)
Sources
The number next to a statement points to its source in this list. Sources back the general information; anything marked “practical advice” is editorial experience, not a quoted source. Anything that affects safety, structural design or electrical installations needs a licensed professional, and in Saudi Arabia the binding reference is the Saudi Building Code.
- 1Types of Roof Membranes for Flat or Low-Slope Roofs — Archtoolbox
- 25 Reasons to Choose an Inverted Roof (Protected Membrane Roof with XPS) — DuPont Performance Building Solutions
- 3Mandatory hidden defects insurance — Insurance Authority (Saudi Arabia)