Flat garage roof replacement

Replacing a Flat Garage Roof: Understanding Your Options

When the covering on your garage starts to fail, you are usually choosing between two practical routes: a quick, budget-conscious single-layer repair or a more durable two-layer system built for long-term protection. The decision is not only about price; it is about knowing what materials are being installed and what level of performance you are paying for. On a straightforward structure, the work is usually simple to measure and plan, although common garage covering issues can still affect the final scope if the timber deck has deteriorated.

Why Old Garage Roofs Fail

Most coverings I inspect on garages around Worthing and Brighton fail for the same reason: the old felt has become hard, brittle, and unable to move with temperature changes. You can see similar patterns in small outbuilding waterproofing failures, where UV exposure gradually breaks down the bitumen until cracks form at seams, edges, and upstands.

Over time, water sits inside these splits, freezes during colder weather, and expands the damage. This cracking pattern, often called alligatoring, is a clear sign that the membrane has reached the end of its useful life. Many older garage covering quote requests start with exactly this type of visible deterioration.

The Two Main Systems: A Technical Comparison

When I carry out a replacement, I first remove the failed covering back to the timber decking so I can check for rot, soft boards, or hidden moisture damage. If active water ingress has already affected the structure, a priority water-ingress repair assessment may be needed before a full new system is installed. Once the deck is sound, I install one of two high-performance torch-on felt systems.

1. The Professional Two-Layer System (BS 8747 Standard)

This is the proper belt-and-braces approach and the option I recommend when long-term durability matters most. It involves:

• Vented Underlay: A base layer of SBS-modified bitumen felt, torched onto the deck. This creates the first waterproof barrier and helps trapped vapour escape, reducing the risk of blistering.
• Mineral Cap Sheet: The second, thicker layer is a high-performance felt with a dense mineral granule finish, usually grey or green. The mineral surface is not only decorative; it is the main defence against UV radiation and helps slow the ageing of the bitumen beneath.

The key advantage is redundancy. With two fully bonded layers, the finished membrane is stronger, more stable, and better able to cope with thermal movement while providing many years of reliable protection.

2. The Economy Single-Layer System

For a detached garage where maximum lifespan is less critical, a single-layer system can be an effective way to keep costs down while making the structure watertight again. I use one heavy-duty layer of mineral-finished SBS torch-on felt. Although it does not offer the backup protection of a two-layer installation, it is still a major improvement over old, cracked felt when the detailing is done correctly.

With this system, the perimeter work is especially important. Edges, drips, corners, and upstands must be sealed properly because there is no second layer beneath the cap sheet to compensate for weak detailing.

Get a Fixed Price to Compare Quotes

I believe pricing should be clear before work begins. For homeowners in Worthing and the surrounding areas, it is sensible to compare several quotes, but the details can be difficult to judge if each contractor describes the system differently. That is why I built an online calculator for an instant garage covering cost estimate, so you can enter your measurements and compare the fixed price for both the professional two-layer system and the economy single-layer option.

Because most garage structures are relatively standard, the price shown is the price you can expect to pay, provided the measurements are accurate and the deck does not need unexpected structural repairs. You can then use that figure as a reliable benchmark when comparing other quotes and deciding which specification offers the best value.