When deciding to improve your flat roof you can either decide on re-roof or an overlay of the existing roof. However, when considering an overlay many factors need to be consider such as is your current roof sound.

This blog explores how to overlay a flat roof and what needs to be taken into consideration when deciding on an overlay and the process of an overlay.

Overlay – Things to Consider

When considering overlaying a flat roof many aspects have to be taken into consideration, the fact that the rood looks to be in good condition should not be the basis of installing a roof covering over it. Before an overlay an investigation of the current waterproofing and roof deck need to be undertaken, there has been many instances of new materials being laid on old only to find that the roof deck has a problem which requires the whole system to be ripped up and re-roofed.

Unfortunately, most people only respond to a roofing issue when they see water entering the building, which could lead to cheap repairs which holds the problem off for a short time until either the repair fails or further issues of the roof arise elsewhere. Roofs are the one of the most important aspects of any building, investing in it will save you costs elsewhere.

Re-roofing specifications will always follow the same principles as that of new work, but there are a range of complications that can arise, such as the need to adapt, reuse or overlay existing materials. Things to consider when renewing an existing roof is whether or not the thermal performance of the roof should be improved and the impact that that may have on detail work.

Inspecting Flat Roofs

When considering overlaying an existing roof, the first thing required would be to carry out an inspection, this helps to determine limiting factors which could influence the design and selection of materials. If you had to thermally upgrade the roof, then a new layer of insulation would be required, which impacts any details, especially abutments. Before assessing the roof, a question of whether it is safe to do so needs to be asked, a visual assessment can be carried out from looking at a roof from a vantage point, if the roof is deemed sound then the access can be given.

Depending on the type of existing roofing system, defects can be well hidden or visible. If the roof has no finishes then it is easy to determine and find problems with the roof such as splits, tears, physical damage, blisters, stresses in surface, advanced ageing and opening of seams and laps, any of these factors could contribute to the roofing membrane failing. However if the roofing membrane is covered with mineral chippings or masked by ponding, then further steps will be required to clear these areas for the inspection.

If the visual inspection indicates that the existing roof appears to be in good condition then a survey or robust inspection such as core samples can be carried out, an inspection should always be carried out by a competent person with flat roofing knowledge or experience.

Determining Roof suitability for overlay

On the market today there is a wide range of flat roofing systems with life expectancies between 5-years (Cheap system) to 40years + (More expensive system). The products with a shorter life expectancy would not be suitable for an overlay  especially is adhering to as they breakdown quickly shorter term products break down quickly.

As well as a visual inspection, it would also be advised to carry out an intrusive inspection such as a core sample, this helps to confirm the existing roof waterproofing build-up. The benefit of carry out such an intrusive inspection is that you can confirm a number of important factors; if the roof was insulated from above, and if so, what is the type and thickness of the insulation, and is the insulation also a means to providing falls, the list can go on. It also allows for the opportunity to confirm water ingress or moisture in the existing system. If water or moisture is found in the core samples, then further core samples would be advised to try and establish how widespread the problem may be.

The inspection should also evaluate whether detailing is correctly designed, and the adjacent structures and weatherproofing appear sound.

As well an external inspection of the roof an internal one may also be advised, especially if the existing roof has failed and water ingress is visible. With a Cold Roof Design, problems can develop over long periods due to condensation build up and insufficient ventilation of the roof void. If core sampling determines the structure to be a cold roof design then the existing ventilation provision should be explored and investigated further.

Deciding on a Flat Roof Overlay

If the inspection declares that the roof is in sound condition and that there is no reason as to why the roof cannot be overlaid. The imposed loading of a new layer or layers to the roof must be taken into account and depending on the overall requirements of the proposed system, it would be prudent to consider sourcing advice from a structural engineer.

  • Has poor detail design contributed to ingress problems?
  • Are the internal conditions unbalanced?
  • Is there water ingress or moisture in the system?
  • What is the roof deck made of?
  • If a cold roof design, is it poorly ventilated?
  • Any of the above issues will need further investigation and possibly more in depth works such as corrective works and, most likely, to strip and replace the existing roof system.

If the issue has been pinpointed then, when required, repairs to the system and/or structure can be isolated and dealt with before an overlay can be carried out.

For more information regarding our systems, please email info@optitec.group or call 01268 944054 to get in touch with our technical team.