Flat Roof Replacement Guide

When Does A Flat Roof Need Replacing

In our Flat Roof Repair Guide, we outlined the steps necessary to repair portions of your flat roof. If you have tried these with limited success or have discovered significant damage on your roof that is beyond a simple repair job, you may need a whole roof replacement. Some of the common signs your flat roof needs replacing are as follows:

Your roof is causing a significant leak
Too much water is pooling on your roof
There are large tears, splits and cracks in the roof material
Adhesion failure is causing widespread blistering and bubbling
Organic growth is starting to appear

While some of these issues can be temporarily fixed, if your roof is extensively damaged then a complete roof replacement will be the more cost-effective solution. In this guide, you will find detailed information regarding flat roof replacement methods for felt roofs, EPDM roofs and GRP fibreglass roofs.

Things to Consider When Removing Your Old Roof

Once you have removed your existing roofing material, remember to check the building substrate to identify any issues that will need to be addressed before applying your new roof. If the substrate is found to be unsuitable, for example, it is defective, decayed or structurally unsound then it will need to be stripped and removed. The timber joists should also be inspected for defects including any wet/dry rot and made good where required.

Felt Removal & Replacement Process

Roofing felt is one of the most common materials used on flat roofs. As felt membranes are lightweight, they can be easily used for a range of different roofing structures such as garages and sheds as well as more robust structures and buildings. Felt roofs are also considerably cheaper to install, maintain and also replace when damaged.

There are two methods to applying felt membranes, ‘torching on’ or applying the felt using an adhesive. Torching on felt is seen as one of the quickest and most reliable methods of felt application, however, adhesive roofing felt may be easier to apply yourself. Below, we describe how to remove old felt roofing material and reapply using the DIY-friendly adhesive method.

Felt Roof Removal

Using a stripping spade, repeatedly dig into a section of the flat roof to start lifting the felt. You will need to get underneath all layers of the felt and expose the timber decking below. Try approaching from an angle of around 30º - 40º and lever against the timber and the underside of the felt to separate the two layers. If your felt roof is fully bonded to the roof deck this can be more time consuming to remove. Use a claw hammer to get rid of any nails that are still present. Remove all the old felt material and brush the surface of the wooden deck, removing any odd nails left behind.

Quick Tip: By removing the felt you are also removing some of its strength so be careful where you stand while you are stripping the roof.

Adhesive Felt Roof Replacement

Before you start the replacement process, make sure that your roof is clean and dry. Remove any moss or debris from the roof with a stiff-bristled brush and use a scraper to get rid of any dirt that is left behind. You are aiming to achieve a dust-free, dirt-free surface on your roof.
Measure out how much felt you will need and allow for an extra 50mm - 75mm for overlaps at the edges of your roof. Place the first strip of felt on your roof and fix this into position by nailing the top edge of the felt with galvanised clout nails. Slowly and carefully fold the overhanging felt over the edge of the roof and nail it into place. Fold and nail any corners for a neat and professional look.
Apply your felt roofing adhesive to the top edge of your felt. Allow the adhesive to develop a tacky surface before overlaying with felt (approximately 15 minutes to one hour dependent on weather conditions). The lap area should be completely coated and pressed firmly together to expel any trapped air. The lap should then be fixed with galvanised clout nails to ensure a secure bond. Continue this process until your roof is fully coated with felt.

EPDM Removal & Replacement Process

EPDM rubber roofing is one of the most eco-friendly and long-lasting roofing materials available. Cost-effective, easy to install and lightweight, EPDM comes with a range of green benefits. It's completely recyclable, can be used as the foundation for sedum roofs and has a huge 50-year lifespan. If your EPDM has been laid incorrectly or is coming to the end of its viable lifespan that a replacement may be required.

EPDM Removal

To make removing your EPDM roof easier you will want to wait until it is a warm day. If it is too cold then the rubber may become rigid and stiff making it harder to remove. Using a utility knife, cut the material into small sections. While cutting along the seam tape is faster and more efficient this is still going to be a very time-consuming job, especially if you have a large roof. You can use a heat gun to loosen the adhesive and make the material easier to pull away from the roof deck. Use a hammer to remove any nails and staples as you go.

Quick Tip: Be sure to wear heavy-duty gloves to protect your hands.

EPDM Roof Replacement

Ensure your roof is clean, dry and free of any debris or old nails. Allow 30 minutes to relax the EPDM membrane. Simply unpack and position it over the roof deck, allowing it to hang over the roof on all sides.
Expose half of the roof deck by rolling or folding the EPDM membrane and secure using gaffa tape if needed. Apply a contact bonding adhesive with an old paint roller in stages from the ridge to the bottom edges. The adhesive must be wet when the rubber membrane is folded back into position. If you have a large roof then you may need to do this part in stages to prevent the adhesive drying.
Finally, apply a thin bead of silicone sealant to the perimeter and press the membrane onto it. Secure the edges of the membrane using either galvanised clout nails, a treated timber batten or an edge trim. The trim will ensure that the roofing membrane is secure and free from wind uplift. You can then proceed to cut off any excess rubber material from the roof using roofing scissors or a roofers knife.

GRP Fibreglass Removal & Replacement Process

The ideal seamless, extra-tough roofing solution for flat roofs of any size, GRP fibreglass roofs are completely watertight and guaranteed for 25 years when installed correctly. To make installation easy, we sell the F1 GRP Fibreglass Roofing Kit. The F1 Fibreglass Roofing system is durable, stylish, cost-effective, maintenance-free and cold applied. It accommodates normal structural expansion and contraction and can be moulded and shaped into and around the most complex architectures, maintaining integrity in all weathers.

GRP Fibreglass Roof Removal

Begin by removing the existing fibreglass roofing material. This is another time-consuming job that can be sped up by using a circular saw to cut up the material. Wear an adequate dust mask to stop yourself from breathing in any fine dust that is generated.

GRP Fibreglass Roof Replacement

Remove the old decking back to the joist and check for any traces of rot. Tongue and grove conditioned water-resistant sterling board is the preferred decking but exterior plywood can be used as an alternative. The boards should be fixed to the timber joists with corrosion-resistant ring shank nails or countersunk screws. Leave 3mm - 5mm between the boards to allow expansion. Always stagger the boards to help reduce stress in the roof.
Now your deck is laid and fixed in place, it is essential that all joints are taped using 25mm masking tape. This offers protection against resin drainage and creates a small expansion joint along the deck at the point of the tape. All masking tape joints should then be over-laminated with a glass fibre bandage tape.
Now you can fit your edge trims and battens. We offer a wide range of GRP trims to suit most applications and roof configurations. Once the trims are fitted, the roof is ready to be laminated.
Quick Tip: The roof at this stage should have trims fitted to each edge so that the area to laminate is edged by the horizontal flanges of the rims around the perimeter.
Ensure that the deck is clean, dry and free from any surface contamination. Choose a starting point on the roof and work back towards the exit point. Roll out the glass so that it is cut correctly to overlap the trims by about 50mm. Roll up each cut length allowing 50mm overlap between each roll.
Quick Tip: The glass should not overlap the fascias.
Apply the resin to the deck with a roller. Roll out the glass matting ensuring that there are no folds or kinks and that the trims are correctly overlapped. Apply more resin over the top of the glass. If there are any dry spots, simply apply more resin until all of the matting is translucent. Allow a few minutes for the glass to soak up the resin, then go over the whole area with a consolidating roller. Now leave the roof to cure, this will take between 30 - 40 minutes depending on weather conditions. The colder the day the longer the resin will take to cure.
Quick Tip: You must remove all air so that the glass fibres disappear and the grain of the timber becomes apparent as the laminate becomes transparent.
Apply a final topcoat to complete the roof. To check if it is cured enough to apply the Top Coat, using a gloved finger apply light pressure to the base coat – if the fibres within the matting are set and not easily disturbed then preparation for top coating can begin. Mix the Top Coat thoroughly and apply to the whole roof laminate including the edging trims. Brush or roller on vigorously to achieve an even finish and a good bond.
Quick Tip: Always apply the topcoat within 24 hours of laminating the roof.